Brilliant physicist roommates Leonard and Sheldon meet their new neighbor Penny, who begins by showing them that as much as they know about science, they know little about actual living.

Leonard asks a woman out after he finds out that Penny is seeing someone.

Penny hosts a Hallowe'en party, putting Leonard, Sheldon, Wolowitz, and Koothrappali through a new test of their social skills.

Penny's break-up spurs Leonard to summon the courage to ask her out. Both go to Sheldon for advice on their upcoming date.

Sheldon creates a monster when he introduces Penny to online gaming.

Penny starts dating Leonard's handsome colleague; Leonard and his friends are tortured by Sheldon's obsession with gift-giving etiquette.

The guys' plan to work over the summer at the North Pole makes Penny revaluate her feelings for Leonard.

When Sheldon learns that the guys tampered with his expedition data he got from the arctic, he leaves to Texas in disgrace. This results in the guys following him, which threatens Leonard's hope for some romantic time with Penny and the guys' friendship with Sheldon.

Leonard and Penny struggle to recover from an awkward first hookup while Sheldon and Howard stake their best comic books on a bet to determine the species of a cricket.

After overhearing that Wil Wheaton is entering a trading-card competition, Sheldon decides to enter it himself. Meanwhile, Penny sets up one of her friends on a date with Wolowitz.

With Leonard, Howard and Raj away camping in the desert, an injured Penny has only Sheldon to rely on.

After he's humiliated on National Public Radio, Sheldon vows to destroy Kripke, while Wolowitz tries not to destroy his new relationship with Bernadette.

A Christmas visit from his mother has Leonard discovering she's kept a lot of news from him that she shares with Sheldon and worrying if she'll approve of his romance with Penny.

A fight between Leonard and Penny threatens their relationship, while Sheldon battles Wil Wheaton in bowling.

Leonard tells Penny about how he met Sheldon for the first time...and what happened to the elevator.

While Penny worries that dating Leonard has ruined her for normal guys, Wolowitz and Koothrappali search for Sheldon's perfect match online.

Penny tags along on Sheldon's first ever date; Wolowitz finds a new use for a robotic arm.

An embarrassing secret of Wolowitz's comes to light, while Sheldon and Raj have a small war at work.

Amy finds she has sexual feelings for Penny's ex, while Koothrappali and Wolowitz try to prove who would be the better superhero.

The guys find a "super" use for Penny's new boyfriend, Zack, when they enter a costume contest as the Justice League.

While Sheldon is in Texas, the rest of the gang realizes how much he has changed their lives as they decorate the apartment Christmas tree.

A bad day causes Penny to reevaluate her life choices – including Leonard. Meanwhile, Howard and Bernadette struggle to care for Mrs. Wolowitz, and Raj gets relationship advice from Sheldon.

Love is in the air when the gang recreates a prom night.

Amy forces Sheldon to celebrate a Victorian Christmas.

The guys finally agree to do something the girls like.

Everyone awaits the reopening of Stuart's comic bookstore.

Sheldon pushes Leonard and Penny to set a date.

After driving to Vegas to get married, Penny struggles with Leonard’s confession that he kissed another girl. Also, Sheldon doesn't know how to act after Amy pushes pause on their relationship.

Sheldon learns of an impending change in his living arrangements and decides to revert back to 2003, a simpler time before he met Leonard and Penny.

Sheldon and Amy try hanging out as friends when they spend Thanksgiving together at the aquarium. Also, Wolowitz reluctantly agrees to volunteer at the soup kitchen with Bernadette, Raj and Emily.

Sheldon has a revelation when he realizes why a song was stuck in his head. Also, Amy invites Dave over for dinner despite their previously awkward date.

After more than five years of dating, Sheldon and Amy spend their first night together. Also, Leonard, Wolowitz, and Koothrappali must decide who will take their extra Star Wars movie ticket.

Sheldon and Amy host a live Valentine's Day episode of Fun with Flags, Leonard and Penny grapple with getting older, Howard and Bernadette find a surprise in their hot tub and Raj is torn between Emily and the cute girl he met at the Comic Bookstore.

When the gang finds out about Bernadette’s pregnancy, they celebrate by singing karaoke.

After Sheldon's mother and Leonard's father share an evening together, everyone deals with an awkward "morning after the night before" in the tenth season premiere.

When Amy's apartment floods, she proposes a "cohabitation experiment" with Sheldon. Meanwhile, Raj learns the gender of Howard and Bernadette's baby….before they do.

While Sheldon and Amy celebrate Amy's birthday, Howard and Bernadette welcome their new Baby Wolowitz.

Sheldon and Amy visit Leonard and Penny for the first time since the holidays and share details about their terrible trip to Texas. Laurie Metcalf returns as Sheldon's mother.

On the tenth season finale, the gang is concerned when Sheldon's former admirer, Dr. Ramona Nowitzki resurfaces while Amy is away at Princeton.

Amy answers Sheldon's proposal.

Bernadette gives birth with the gang's help.

Amy's parents and Sheldon's family arrive for the wedding.

In the series finale, the gang travels together into an uncharted future.

Hoping it will make him a better teacher, Sheldon has Penny give him acting lessons. Raj has fantasies about a friend's girlfriend.

A wealthy donor to the university makes Leonard consider how far he's willing to go for the sake of science.

As Wolowitz contemplates taking things with Bernadette to the next level, Leonard rekindles his relationship with Raj's sister.

Sheldon becomes concerned when everybody begins to hang out at Raj's apartment and discovers Leonard is the center of the group.

Sheldon is baffled by Wolowitz's magic trick. Meanwhile, Leonard realizes dating Priya may make it impossible to continue being friends with Penny.

Sheldon's World of Warcraft online game account is hacked and he is determined to find out who did it. Also Penny shares some girl talk with her friends about Leonard and his new girlfriend.

Sheldon and Amy decide to start several rumors about the others as a test. Meanwhile, Howard takes his relationship with Bernadette to even higher level.

When Leonard spends too much time in the shower with Priya, Sheldon charges Leonard with two violations of the roommate agreement.

Raj attempts to cure his social anxiety disorder while Penny uses Bernadette to spy on Leonard and his girlfriend.

When Howard finally breaks the news to his mom that he's engaged to Bernadette, it lands her in the hospital.

When Bernadette receives her PhD, Wolowitz feels emasculated. Raj and Leonard swap places and Penny does something she regrets.

Penny is worried with how damaged her friendship with the guys will be in the aftermath of her sleeping with Raj while Sheldon becomes the leader of the paintball team.

A fight between Sheldon and Penny leaves Amy caught in the middle, while Leonard tries to spice up his long-distance relationship with Priya.

Leonard and Amy get to know each other better, while Howard and Bernadette spend the weekend with Mrs. Wolowitz.

Penny plays Matchmaker for Raj, which results in him finding a girl he is comfortable talking to.

Sheldon and Leonard get invited to a party at Wil Wheaton's house, and Howard gets an out-of-this-world opportunity.

Sheldon's mother, Mary Cooper, comes to visit, but Sheldon is annoyed that she spends time with the rest of the gang instead of catering to his every demand.

A cute comic book artist puts Leonard and Priya’s relationship to the test, while Sheldon tries to scare the guys for Halloween.

Amy becomes heartbroken when she discovers that Penny and Bernadette went wedding dress shopping without her.

Leonard and Penny try hanging out as friends on a 'non-date', but end up purposely ruining each others chances with perspective dates. Meanwhile, Sheldon enlists the help of Bernadette and Amy to overcome his fear of birds.

Sheldon becomes jealous when Stuart asks Amy for a date.

Leonard must face his fears after being contacted by his high school tormentor.

Sheldon gets in trouble with Amy when he isn't impressed by her recent accomplishment, and Howard must come to grips with Bernadette's dislike of children.

Leonard, having broken up with Priya, asks Penny out to a spur-of the-moment romantic dinner for two.

Leonard and Penny deal with the idea of starting a relationship again and Raj finally finds a 'woman' he can talk to without being drunk, in the form of Siri on his new phone.

Sheldon's selfish demands force Leonard to reconsider their friendship. Meanwhile, Wolowitz tries to pick his astronaut nickname.

When Sheldon is forced to take his vacation, he goes to work with Amy in her neurobiology lab.

Penny is horrified when Amy gives her a gift: a huge, quite horrible painting of the two of them. With no good way to avoid displaying it, she only hangs it up when Amy is over.

Sheldon becomes agitated when his regular barber is out sick. Astronaut training causes Howard to reconsider wanting to go into space.

Sheldon must choose between joining Amy at her aunt's birthday party or playing videogames all weekend with the guys.

Penny buys Leonard and Sheldon “Star Trek” collectibles as a thank-you, leading Sheldon to be haunted by Mr. Spock. Meanwhile, Raj decides he has met the 'Future Mrs. Koothrappali' after his parents set him up on a date.

When Wolowitz gets to work with Stephen Hawking, Sheldon is willing to do anything to meet his hero.

After the guys throw Howard a bachelor party, Bernadette reconsiders marrying him after learning about his sexual history.

When NASA moves up Howard's mission, he must face his fears and Bernadette's dad, thus putting the wedding plans in jeopardy.

The fifth season ends with Howard and Bernadette deciding to get married before his NASA mission, causing the gang to scurry into overdrive in an effort to stage the wedding.

With his friends on dates, Raj must face his loneliness. Also, Wolowitz is caught in an argument between Bernadette and his mom, even while he’s in space.

When Sheldon learns that Penny is thinking about breaking up with Leonard, he tries to intervene. Meanwhile, in space, the other astronauts pick on Howard.

Amy feels threatened when Sheldon hires a young female assistant, and the pressure of being in space takes its toll on Wolowitz.

When Wolowitz returns from space, he doesn’t get the hero’s welcome he expected. Meanwhile, Game Night turns into a battle of the sexes.

The gang celebrates Halloween at Stuart’s comic book store. Meanwhile, Leonard seduces Penny with science.

Sheldon’s relationship with Stephen Hawking is threatened over a game of ‘Words with Friends,’ and Penny secretly enrolls in a class at the local college.

Wolowitz struggles with moving out of his mother’s house. Meanwhile, Sheldon is caught in the middle of an argument between Amy and Wil Wheaton.

Raj and Howard try to to figure out why Sheldon disappears at 2:45 every afternoon. Meanwhile, jealousy arises in Penny and Leonard's relationship.

When the University reassigns Sheldon’s parking spot to Wolowitz, their fight affects the whole gang.

When Amy gets sick, Sheldon tries to nurse her back to health; Wolowitz and his father-in-law plan a fishing trip.

During a game, Sheldon revisits Christmas memories. Amy, Penny and Bernadette look for a date for Koothrappali.

When Sheldon is accused of sexual harassment, he ends up getting Raj, Leonard and Wolowitz in trouble.

As the guys don costumes and go on a road trip to a comic book convention, the ladies decide to stay home and try to interpret a comic book.

Sheldon faces a crisis of confidence when he is forced to work with his nemesis, Barry Kripke; Raj and Howard buy expensive action figures of themselves.

A fight between Sheldon and Leonard impacts Amy and Penny's living arrangements. while Howard is away, Raj helps Mrs. Wolowitz out.

The guys search for a perfect Valentine's Day gift for their significant others and Stuart and Raj host a "lonely people" party at the comic book store.

After a bad date, Koothrappali decides to never leave his apartment. Penny's acting skills impress Sheldon.

Leonard, Sheldon and Wolowitz talk to junior high girls about science-related careers and Raj plans a date with Lucy.

Howard considers whether or not he should open a letter from his father; Penny and Leonard host a 'grown-up' cocktail party.

The girls get involved when Leonard, Sheldon and Raj compete for tenure at the University.

Amy tries to help Sheldon get over his compulsive need for closure when one of his favorite TV shows is canceled. Raj discovers a secret about Lucy.

Leonard and Sheldon hire the host (Bob Newhart) of their favorite childhood TV show to perform. meanwhile, Bernadette and Howard run into trouble while dog-sitting for Raj's pet.

Amy and Sheldon's relationship takes a surprising turn during a game of 'Dungeons & Dragons'. Lucy and Koothrappali have an awkward date.

When Leonard receives an exciting opportunity to work overseas, Penny and Sheldon are thrown for a loop.

With Leonard away, Sheldon and Penny spend quality time together sharing intimate secrets. Also, Wolowitz helps Raj get over his ex-girlfriend.

Leonard and Sheldon's friendship is tested; Wolowitz's relationship with his mom causes a threat to his masculinity.

When the entire gang competes in a scavenger hunt designed by Raj, it brings out the best - and worst - in everyone.

Beverly Hofstadter writes a psychology book about Leonard which Penny buys and reads. Sheldon wants to ruin something for Amy after she ruins an Indiana Jones movie for him and Raj and Stuart try online dating.

Amy starts working at Caltech which causes friction with Sheldon. Howard agrees with Sheldon who mentions this to Bernadette causing a big fight for the Wolowitzes.

Sheldon's latest scientific breakthrough leaves him feeling like a fraud and a romantic gesture between Howard and Bernadette causes Penny to up her game with Leonard.

When Professor Proton asks Leonard for help with an experiment, Sheldon retaliates by befriending Proton's rival, Bill Nye.

When a mistake comes to light, Sheldon makes Leonard walk a mile in his shoes; Penny decides to confront Raj's ex.

Sheldon protests when the gang drags him to Mrs. Wolowitz’s house for Thanksgiving. Meanwhile, Penny and Leonard must deal with a mistake from her past.

Leonard and Amy help Sheldon cope with the shame he’s feeling after his accidental contribution to science is disproved. Meanwhile, Raj must stay with Howard and Bernadette for a week.

When Penny’s big acting break on NCIS is a bust, Leonard struggles to help her, which results in Penny asking Leonard a bold relationship question. Meanwhile, Sheldon tries to learn how to be funny.

Sheldon tries to relax after he is forced to take a vacation. Meanwhile, Leonard struggles to be supportive of Penny after she quits her job, and Bernadette seeks Stuart’s help in replacing one of Howard’s comic books.

After the guys can’t get Comic-Con tickets, Sheldon tries to hold his own convention and winds up spending a wild night with James Earl Jones.

Sheldon and Amy go on a trip to wine country with Howard and Bernadette; Penny and Leonard rush Raj's dog to the vet.

Leonard buys a dining room table and it causes Sheldon to reevaluate the changes in his life. Meanwhile, Wolowitz is offered a chance to go back to space and Bernadette struggles with whether or not to encourage him.

Bernadette tries to negotiate peace between Sheldon and Wolowitz; Penny receives an embarrassing offer and Raj wants Amy to write to a woman on his behalf.

When Sheldon visits his Mom in Houston, he is forced to confront a new reality. Meanwhile, Raj’s ‘Murder Mystery’ party starts some fights within the gang.

While Raj is wracked with guilt when he tries to date two women at once, Penny wrestles with whether to take a role in a cheesy movie and Sheldon is torn trying to choose between two gaming systems.

Sheldon faces a personal crisis after deciding he’s wasting his time with string theory. Meanwhile, a double date between Raj, Emily and the Wolowitzes ends in embarrassment for Howard.

To prepare for a date with Emily, Howard acclimatizes Raj to horror movies. Penny and Sheldon are upset when they discover Amy and Bernadette lied to avoid hanging out with them.

As the gang celebrates Star Wars Day, Sheldon discovers he is having difficulty dealing with the death of Professor Proton.

Sheldon get upset about his career, the destruction of the comic book store and Leonard and Penny's future living arrangements. Howard and Bernadette can't keep hold of a carer for his mother.

Leonard and Amy take a road trip.

Howard takes a class that Sheldon's teaching.

Howard is invited by NASA to throw the first pitch at a game.

Penny wonders why Raj's girlfriend doesn't like her.

The guys unsuccessfully try to invent "the next big thing".

Sheldon and Raj venture underground in the name of science.

Penny's flirtatious sales techniques work a little too well.

Leonard's minor surgery leads to Sheldon's major reaction.

Sheldon records the final instalment of "Fun with Flags".

Sheldon asks his friends to challenge him.

Leonard and Sheldon defend themselves from an online bully.

Sheldon and Penny try an experiment in falling in love.

Sheldon applies for a one-way mission to colonise Mars.

A final family dinner is held for Mrs. Wolowitz.

Sheldon and Leonard try to sneak onto Skywalker Ranch.

Wolowitz meets the half-brother he never knew he had.

Penny contemplates a return to acting.

Raj's father cuts him off from the family fortune.

Sparks fly when Leonard and Sheldon's mothers finally meet.

Leonard confronts the woman he kissed on the North Sea in an attempt to set Penny's mind at ease. Also, Sheldon films a special episode of 'Fun with Flags' after his breakup with Amy

The guys' science skills are put to the test when they get a flat tire during Leonard's bachelor party weekend in Mexico. Also, the girls force Penny to finally tell her family that she eloped with Leonard.

The guys take a fencing lesson from Barry Kripke and Sheldon finds himself in an awkward position when Kripke expresses interest in Amy.

In the midst of a nation-wide helium shortage, Sheldon and Leonard take desperate measures to get the supplies they need. Also, Penny and Bernadette download a dating app on Amy's phone to try and find her a new man.

Wil Wheaton interviews Sheldon for a Star Trek documentary where he reveals the ring and his plan to propose to Amy. Bernadette wants to start renovating the house and the topic of starting a family together with Howard is brought up.

Sheldon, Wolowitz and Koothrappali post an online advertisement to find Sheldon a new girlfriend. Also, when Amy is coy with details about a guy she is dating, Penny and Bernadette decide to spy.

Penny gets more than she bargained for when Leonard agrees to meet with a psychiatrist on her behalf. Also, Sheldon and Koothrappali collaborate on an astronomical discovery.

After having to deal with a sick Sheldon, Leonard, Penny and the gang try to treat themselves to a Sheldon-free weekend.

Meemaw meets Amy, but it isn't all roses. Meemaw seems to have an axe to grind with her. Sheldon comes to Amy's defense, but not before Meemaw lets a little secret of his slip.

After more than nine years together, the gang finally celebrates Sheldon’s birthday, surprising him with a special guest.

Leonard, Sheldon, and Wolowitz run into problems when they file for a patent for their infinite persistence gyroscope. Also, Penny, Amy and Bernadette give Koothrappali dating advice when Emily reaches out to him after their breakup.

Amy is shocked at a revelation from Sheldon after she buys him a new laptop. Also, Koothrappali sells out Leonard and Wolowitz after they lie to their wives in order to attend an early screening of a movie.

Sheldon spills a secret of Leonard’s when the two take a weekend cabin trip with Penny and Amy. Also, when Koothrappali goes overboard with pregnancy gifts, Wolowitz and Bernadette must draw the line.

Everyone must choose sides when a small argument between Leonard and Sheldon erupts into a heated fight during a group get-together.

The gang goes to a wine bar and runs into Penny’s old boyfriend, Zack. Also, Bernadette gets stuck spending the evening with Sheldon when she can’t go wine tasting with the group.

Sheldon hires Stuart to spend the day with Amy when he’d rather go to a movie screening. Also, Leonard’s mother, Beverly, comes to town and Penny struggles to make a connection with her.

Chaos ensues when Leonard’s recently divorced parents, Alfred and Beverly, and Sheldon’s devoutly religious mother, Mary, come to town.

Colonel Williams, an Air Force representative from the Department of Material Command, sits down for a talk with Howard.

Tensions rise when the guys struggle to complete their government project on time - and Sheldon tries an energy drink to stay awake. Meanwhile, Penny and Amy go to a geologist's "party".

Sheldon threatens to break off their relationship when Amy refuses to adhere to his "bathroom schedule", putting their cohabitation experiment in jeopardy.

Penny learns there are fans of her performance in "Serial Ape-ist" when she and Leonard attend a Comic Con event; Amy hosts a brunch at Penny's apartment; and Wolowitz makes an impulsive purchase.

Amy lies about repairs being made to her apartment so she can continue living with Sheldon; Leonard lets Penny decorate Sheldon's old room.

After a successful experiment combining their genes, Sheldon is determined to procreate with Amy. Meanwhile, Raj is embarrassed with his new date's occupation.

Sheldon struggles to contain his jealousy when CalTech geologist Bert wins the MacArthur Genius Fellowship. Stephen Hawking returns for a guest appearance.

A battle ensues when Leonard and Sheldon argue over who will get custody of their shared belongings. Christopher Lloyd guest stars.

When Leonard and Penny fight, Sheldon plays the unlikely peacemaker. Meanwhile, Howard and Raj try to fix a squeaky floorboard in the baby's room

While Raj gets dating advice from his ex-girlfriends, Sheldon gets help reading people's feelings when he beta tests an emotion-detection invention.

Leonard and Howard try to distract Sheldon by giving him tickets to a historic railway, while Penny and Amy take Bernadette out for the night, leaving Raj and Stuart with the baby.

Amy is furious when she learns Sheldon has shared intimate details of their relationship with people at the university, prompting their first fight since moving in together.

When Raj is cut off from his father's fortune, he appoints Sheldon in charge of his finances to help him get out of debt. Meanwhile, Penny considers joining Leonard at Comic-Con.

When Raj moves out of his apartment, Leonard and Penny offer him Sheldon's old room. Christine Baranski returns as Leonard's mother, Beverly.

Leonard, Penny and Raj adjust to their new living arrangement, while Sheldon takes an interest in Amy's work.

Sheldon pushes himself to the limit when he collaborates on projects with Leonard and Howard as well as Amy on the same day. Meanwhile, Bernadette nears the end of her maternity leave.

Howard and Bernadette have a hard time leaving Halley in daycare when Bernadette returns to work. Meanwhile, Bert introduces the gang to his new girlfriend, Rebecca.

Leonard isn't thrilled when Penny considers taking a new job offered by her ex-boyfriend, Zack.

After Leonard, Sheldon and Howard celebrate the completion of their top secret Air Force project, they're met with an unpleasant surprise.

Leonard gives an embarrassing radio interview.

Amy tries to convince Sheldon he can be easy-going.

Leonard's mom finds a new best friend in Penny.

Penny figures out the "secret" to Sheldon.

Sheldon auditions to be the new Professor Proton.

Sheldon secretly collaborates with Bert on a project.

Sheldon goes behind Leonard and Howard's backs.

The gang flashes back to 2010.

Sheldon and Amy develop a system for wedding planning.

Sheldon plans a special themed dinner for Amy's birthday.

Sheldon and Amy use science to choose Best Man & Maid of Honour.

Penny helps Sheldon figure out his next scientific move.

Raj discovers he's dating a married woman.

Leonard asks his friends for feedback on his novel.

Barry Kripke interferes with Sheldon and Amy's wedding plans.

Penny gets the opportunity to host Bill Gates at work when his company wants to partner with her pharmaceutical company.

Leonard runs against Sheldon for President of the Tenants Association.

Penny and Bernadette plan a bachelorette party for Amy.

Penny makes an astronomical discovery.

Sheldon goes to Vegas to win millions for science.

Sheldon visits his older brother Georgie in Texas.

Sheldon and Amy's honeymoon runs aground in New York.

Amy and Sheldon can't figure out their wedding gifts.

Penny and Leonard's marriage is put to the test.

Sheldon's childhood friend, Tam, visits.

Sheldon sabotages Amy's career.

Howard dresses up as Sheldon for Halloween.

Leonard regrets his newfound authority at work.

Raj and Anu have trouble making things physical.

Sheldon and Amy's scientific theory hits a wall.

Sheldon seeks advice from his younger self.

The truth comes out on the paintball field.

Penny's ex-boyfriend asks Leonard for a huge favour.

A rival team of physicists tries to take credit for Super Asymmetry.

Sheldon must choose between his friends or following the rules.

Penny tries to divert Leonard's plans to be a sperm donor.

The guys try to crash Wil Wheaton's celebrity D&D game.

Sheldon wants to experiment on Howard's kids.

Amy and Sheldon try to win over Nobel laureates.

The weight of winning a Nobel weighs heavy on Amy.

Penny encourages Leonard to stand up for himself.

Amy and Sheldon try to take the high road for the Nobel Prize.

Leonard's mother is finally nice to him.

Sheldon and Amy await big news.

Leonard volunteers to sign for a package in an attempt to make a good impression on Penny, but when he enlists Sheldon for help, his attempt at chivalry goes terribly awry.

Getting fired forces Sheldon to explore what life has to offer outside physics, leaving Leonard to take drastic action to snap his friend out of his funk.

Sensing that he's wasting his time on Penny, Leonard ends up making a connection with Leslie.

With Wolowitz AWOL with one of her friends, Penny's recruited to fill in for him on the guys' Halo team.

Raj's parents set him up on a blind date, but just when he discovers he can talk to her, she shows interest in Sheldon.

When they're invited to present at a physics conference, Sheldon's reluctance to share the credit leads to a tiff between him and Leonard. Penny's intervention only exacerbates the situation.

Leonard and Sheldon each lie to avoid seeing Penny's concert, but Sheldon's is a bit too complicated for his own good.

With his friends knowing how sour he acts when he’s sick, Sheldon is nursed by Penny, who hasn't got a clue what she's in for.

With Sheldon rattled over meeting Dennis, a 15-year-old physicist who's smarter than he, Leonard helps him look for a way to debunk Dennis's research.

Sheldon finds himself booted from the gang's Physics Bowl team and replaced with Leslie, so he forms his own team hoping to prove he's superior.

When delivery of another collectible leads to Penny's lashing out at Leonard, he decides it's time to give up his habit, much to his friends' protestation.

The rest of the gang fall for Sheldon's twin sister and Leonard's plan to win her heart is derailed when Sheldon declares he'll be the one to decide who can date her.

Penny wants to throw a surprise party for Leonard, but Sheldon's indulgence at an electronics store throws her off-track.

Penny's first date with Leonard goes awry and she finds Sheldon to be an unwilling confidant.

A jealous Leonard reacts to Penny's new guy by rebounding with Leslie.

Raj gets an inflated ego and makes a play for Penny after being referenced in a magazine article.

Leonard and his friends stage an intervention to force Sheldon to face his fear of driving.

Sheldon and the gang are confused when a young grad student is attracted to him.

When Sheldon bans Penny from the apartment for numerous minor infractions, she decides to retaliate.

Leonard inadvertently steals away the new woman Wolowitz thinks is the love of his life.

Sheldon is thrilled when Leonard starts dating a woman who meets Sheldon's stringent standards.

Leonard asks Penny for advice when his new relationship starts moving too fast.

Penny's criticism of Wolowitz's romantic life pushes him into depression and he goes AWOL for a fighting robot battle the rest of guys are depending on him for.

Sheldon creates a scientific procedure for making friends.

Sheldon's solution to Penny's financial difficulty leads to a confrontation between Leonard and Penny's ex-boyfriend.

Leonard and Penny are drawn together after a disastrous visit from Leonard's mother.

A paintball game leads to a fight between Sheldon and Penny and romance for Leslie and Wolowitz.

Leonard, Sheldon and Wolowitz are ecstatic to learn that a famous science-fiction actress (Summer Glau) is on board the train they are taking to San Francisco.

Penny's home business turns into a complete nightmare when Sheldon takes over.

Penny squares off with a new beautiful female neighbor who may become the building's 'new Penny'.

A fellow comic book enthusiast's hitting on Penny leads to Leonard and Howard going to a bar's Ladies' Night.

Leonard and Koothrappali escort a heartbroken Wolowitz to Las Vegas. Sheldon becomes locked out of his apartment so must stay with Penny.

Penny's second date with Stuart sees her making a comment that gives Leonard new hope for a relationship with her. Wolowitz realizes there's a crucial error in a NASA project he's working on.

Howard and Raj visit a Goth nightclub to pick up women while Sheldon attempts to build a better Penny using chocolate-based behavior modification.

Raj must find a new job or be sent back to India, but Sheldon has a solution, which leaves Leonard and Penny dealing with Howard as a third wheel.

Sheldon becomes Leonard's teacher in order to understand football, while an incident with flying a kite threatens Howard and Raj's friendship.

A squabble between Leonard and Penny sees Sheldon playing mediator, though out of his own incovenience from the sitiation rather than altruism.

Penny gets help from Sheldon in order to understand what Leonard's work is, while Wolowitz develops a jealousy when Leonard starts hanging out with his new girlfriend.

Leonard is aghast to find out a secret of Penny's: she believes in psychics. Meanwhile, Sheldon tries to assist Koothrappali in picking up a girl at a university mixer.

When their apartment is robbed, Leonard and Sheldon turn to their friends to create a state-of-the-art security system.

Sheldon's search for the answer to a physics problem leads him to work at the Cheesecake Factory with Penny.

Leonard can only bring one guest on a trip to see CERN's Large Hadron Collider (the world's largest and highest energy particle accelerator) in Switzerland on Valentine's Day.

Sheldon winds up in traffic court when he gets a ticket because of Penny.

When the guys find a ring from 'The Lord of the Rings' at a garage sale, it threatens to tear them apart.

Sheldon's friends come to his aid when his fear of public speaking stands between him and a coveted award.

When Leonard and Penny aren't speaking, Sheldon goes to extremes to keep them both happy.

Leonard and Sheldon compete for the attention of a famous female physicist.

Realizing he will not live long enough to download his consciousness into a robot body, Sheldon tries to extend his life span.

The guys are concerned as Sheldon searches for an alternative to human companionship.

Leonard realizes he’s the only one without a girlfriend, while Sheldon wants to get rid of his.

Leonard keeps an affair a secret from Sheldon, Howard and Raj.

Sheldon's answers during an FBI interview put Wolowitz's security clearance in jeopardy.

While the guys camp in line for a long-awaited movie screening, Penny and Bernadette invite Amy to her first slumber party.

Penny asks Leonard to lie to her father, while Howard, Raj and Bernadette pull an all-nighter at a telescope.

Leonard's idea for a smartphone app derails his friendship with Sheldon.

Bernadette runs into her ex-boyfriend while attending a science event with Wolowitz.