When Leonard and Sheldon encounter a young woman crying on the curb, Leonard invites her to dinner. She is abrasive, sexy, hard, and sarcastic, but she softens a little when she discovers that the guys aren't trying to get something from her.

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. Meanwhile, Sheldon decides to start a YouTube series entitled "Sheldon Cooper presents Fun with Flags" with the help of Amy.

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

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

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

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

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

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

Leonard has to deal with the fallout of disproving Sheldon's new scientific discovery. Raj spends a week at Howard and Bernadette's apartment.

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. Also, Amy receives an unwanted attention from a co-worker.

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.

As the gang celebrates Star Wars Day, Sheldon has difficulty dealing with the recent death of Arthur "Professor Proton" Jeffries.

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 caretaker for his mother. With his entire world changing around him, Sheldon considers a major move.

Wolowitz feels nervous when he is asked to throw the first pitch at a Los Angeles Angels game. Penny and Leonard want to prove Sheldon wrong after he says he and Amy are the better couple.

The girls decide to re-enact their high school proms with the guys. While Sheldon feels under pressure to commit to his relationship with Amy, Howard worries about who Stuart will bring as his date.

Leonard and Howard turn to Raj for help in dealing with a work emergency when they nearly come to blows after blaming each other for a potential disaster at the university. Amy throws an authentic Victorian Christmas party, while Sheldon seeks revenge on her for making him celebrate the holidays.

Leonard and Sheldon publish a paper which is widely accepted except for comments from an internet troll. Meanwhile, the girls reminisce about embarrassing incidents from their past.

When Leonard and Sheldon are invited to speak at UC Berkeley, they take a detour to try to meet an idol. Also, when Bernadette and Howard battle over the fate of his Doctor Who TARDIS, Raj and Amy offer to settle the dispute with a game of ping pong.

Personalities collide when Sheldon and Leonard’s mothers finally meet. Meanwhile, Howard finally decides to “man up” and do his fair share of the housework – but not without a little help from his friends.

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. Also, Howard and Raj form a band to play at the Comic Book Store.

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.

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

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

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.

After Sheldon’s mother and Leonard's father share an evening together, everyone deals with an awkward morning the next day. Also, Penny’s family arrives for the wedding ceremony, including her anxiety-ridden mother, Susan, and her drug dealing brother, Randall.

Penny is surprised to learn she has fans of her performance in "Serial Ape-ist" when she attends Van Nuys Comic Con with Leonard. Also, Amy hosts a Sheldon-style brunch at Penny’s apartment, and Wolowitz makes an impulsive purchase when he starts stressing out about the baby on the way.

When Bert, a Caltech geologist, wins the MacArthur Genius fellowship, Sheldon is overcome with jealousy. Also, Wolowitz finds an old remote control Stephen Hawking action figure he invented.

When Penny feels that Leonard is taking her for granted, she decides to go on a spa weekend with Amy. Also, Wolowitz and Koothrappali try to fix a squeaky floorboard in the baby’s room.

With Koothrappali cut off from his father’s fortune, he appoints Sheldon to help him get out of debt. Also, Penny considers joining Leonard at Comic-Con.

Sheldon pushes himself to the limit when he collaborates on projects with Leonard and Wolowitz as well as Amy on the same day. Also, Bernadette is nearing the end of her maternity leave and must return to work.

After Leonard, Sheldon and Wolowitz celebrate the completion of the top secret air force project, they are met with an unpleasant surprise. Also, Amy is offered a summer position as a visiting researcher.

In the eleventh season opener, Amy gives Sheldon an answer to his proposal while Howard and Bernadette struggle with some unexpected news.

Sheldon and Amy struggle with picking a wedding date, which stresses out Sheldon and causes him talking in his sleep. Meanwhile, Raj and Stuart fight over the attention of one of Bernadette's co-workers.

Sheldon and Raj confide in Bernadette when they can't handle Amy and Howard working together on a science project. Also, Penny and Leonard learn new tactics for how to deal with Sheldon from an unlikely source.

Sheldon and Bert collaborate on a research project, despite Sheldon’s embarrassment. Also, Penny and Bernadette coach Raj on how to navigate his relationship with Ruchi.

Sheldon tries to teach the guys a lesson after they cut him out of a potentially valuable Bitcoin investment. Also, a seven-year-old video reveals a secret about Leonard and Penny's relationship.

Raj and Howard's feud continues as the gang plan Halley's first birthday party. Sheldon's surprise for Amy's birthday goes awry when they get food poisoning. Also, Leonard gets a Christmas letter from his brother that causes him to spiral out about his own lack of accomplishments.

When Sheldon kicks Amy out to work solo, she and Leonard bond during a series of science experiments. Also, Bert the geologist replaces Wolowitz in the band Footprints on the Moon, and Sheldon finds Penny a surprising source of scientific inspiration.

Raj finds himself in the middle of domestic drama when he learns the woman he's dating, has a very upset husband. Also, when Sheldon rents his old room back for a quiet place to work, he drives Leonard crazy by being a model tenant.

When Bernadette won't go into labor, the gang tries different tactics to get things started. Also, Howard confronts Bernadette after Amy accidentally reveals she's already chosen their son's name.

Bill Gates visits Penny's work, and the gang schemes to meet him. Hilarity ensues. Meanwhile, Sheldon is convinced that he's a target of an April's Fools prank.

A brilliant but reclusive scientist, Doctor Wolcott, invites Sheldon to his cabin in the middle of nowhere, and Leonard, Raj and Howard go along for the trip. Also, Penny and Bernadette improvise after Amy doesn’t like the tame bachelorette party they planned for her.

Sheldon goes to Vegas to win money for science. Also, Penny and Bernadette take Amy wedding dress shopping, but her terrible choice entangles them in a web of lies.

When Sheldon learns that his mother won’t attend his wedding unless he invites his brother, Georgie, he and Leonard travel to Texas to end a family feud. Also, the Wolowitz kids inadvertently infest Amy, Bernadette, Howard and Raj with pinkeye.