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.
While Raj is wracked with guilt over dating two women at the same time and Penny is wrestling with whether to take a role in a cheesy movie, Amy must help Sheldon through the even greater hell of choosing between two gaming systems.
When Sheldon visits his mom (recurring guest star Laurie Metcalf) in Houston, he is forced to confront a new reality. Meanwhile, Raj's "Murder Mystery" party starts some fights within the gang.
After the insults fly, Bernadette tries to broker the peace between Howard and Sheldon by having the two travel together to Houston in an attempt to improve their friendship. Meanwhile, Penny is offered an embarrassing movie role and Raj asks Amy to write to a woman on his behalf.
When Leonard buys a dining room table, 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.
Love is in the air when Amy convinces Sheldon to join her (as well as Howard and Bernadette) for a romantic weekend in Napa Valley to celebrate Valentine's Day. Meanwhile, Leonard and Penny must rush Raj's dog to the vet.
After they guys can't get Comic-Con tickets, Sheldon decides to hold his own convention, and winds up spending a wild night with JAMES EARL JONES (guest starring as himself). Meanwhile, the girls see if they can act like "grown-ups." Star Wars icon CARRIE FISHER makes a cameo appearance as well.
When Sheldon is forced to take a vacation, he tries to relax, but instead spends a lot of time with Penny after she quits her job. Meanwhile, Leonard struggles to be supportive of Penny and Bernadette seeks Stuart's help in replacing one of Howard's comic books.
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 and Raj tries to work on his "game" before talking to girls.
While Sheldon is away in Texas, everyone gathers to decorate the apartment Christmas tree, and imagines what their lives would be like if they had never met Sheldon.
Sheldon faces a crisis after Leonard disproves his accidental scientific discovery. Meanwhile, Raj must stay with Howard & Bernadette for a week.
An upset Sheldon protests when the entire gang drags him to Mrs. Wolowitz's house for Thanksgiving. Meanwhile, Penny and Leonard must deal with a mistake from her past.
Leonard tries to keep Sheldon from overreacting when a past mistake comes to light, but Sheldon punishes Leonard by making him "walk a mile in his shoes." Meanwhile, Penny confronts Raj's ex-girlfriend, Lucy (guest star Kate Micucci).
Sheldon feels slighted when Professor Proton (recurring guest star Bob Newhart) seeks advice from Leonard instead of him, and he seeks revenge by befriending a rival science TV host, Bill Nye (guest starring as himself). Meanwhile, Raj gets jealous when Howard crashes "girls' night."
Sheldon's latest scientific breakthrough makes him feel like a fraud – and threatens to haunt him forever. Meanwhile, Howard's romantic gesture to Bernadette causes Penny to step up her game with Leonard.
Sheldon must decide how much "Amy time" is too much after she takes a job at his university, and their relationship is put to the test. Meanwhile, Howard finds himself sleeping on Raj's couch after a fight with Bernadette.
Things begin to look up for Leonard when Penny makes a surprising revelation on her second date with Stuart (guest star Kevin Sussman). Meanwhile, Wolowitz enlists his friends' help when he discovers a critical design flaw in his project for NASA.
Suffering from a case of physicists block, Sheldon's search for the answer to a physics problem leads him to work at the Cheesecake Factory with Penny.
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