Leonard fails to stop Sheldon from overreacting after a past slight comes to light, and as punishment Leonard must walk a mile in Sheldon's shoes. Meanwhile, Penny has a run-in with Raj's former girlfriend Lucy.
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When Sheldon's accidental scientific discovery is disproved, Amy and Leonard try to help him deal with the shame and the disappointment. Meanwhile, Howard and Bernadette open their home to Raj for a week.
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With Sheldon in Texas, the gang gathers to decorate for Christmas, and they contemplate if each would have had a wonderful life had they never met him.
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Penny turns to Leonard for comfort after her guest appearance on NCIS does not turn out well. She ends up putting Leonard on the spot regarding their relationship. Sheldon works on his sense of humor and Raj practices talking to girls.
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There's impactful news on the work front as Penny makes a big step toward pursuing a full-time acting career, and Sheldon is forced to take a vacation.
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Leonard's purchase of a dining-room table for the apartment doesn't sit well with Sheldon, so much so that it forces him to reevaluate his life. Meanwhile, Bernadette isn't quite over the moon about Howard taking another trip to space.
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Things turn rocky between Sheldon and Wolowitz after a barrage of insults, and Bernadette steps in to mediate a peace. Meanwhile, Penny is offered a movie role that may prove embarrassing.
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While Sheldon and Howard continue their bonding, Sheldon's visit to see his mommy forces him to start addressing a new reality. The gang are subjected to a murder mystery party that leads them to speculate where they might be in 20 years.
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Raj wrestles with dating two women at the same time; Penny is offered a part in a trashy movie; and Sheldon can't decide between two gaming systems.
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Leonard reinstates Anything Can Happen Thursday, and it leads the regimented Sheldon to attempt something spontaneous, which creates friction among Penny, Amy and Bernadette. Raj requests help from Howard in prepping for a date with Emily.
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Sheldon receives counseling from his childhood TV idol, Professor Proton, while Penny and Leonard reach a relationship milestone and turn it into a competition and Bernadette and Amy turn Star Wars Day into a special event for the guys.
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There's trouble in paradise when Amy discovers Sheldon signed up for a one-way mission to colonize Mars, and the two get into a dogfight at a pet store. Meanwhile, Leonard makes a very "adult" purchase for Penny.
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The guys surprise Leonard with a belated bachelor-party weekend in Mexico, but their plans hit a bump in the road when they get a flat tire on the way. Meanwhile, the girls lean on Penny to tell her family that she and Leonard eloped.
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