Sheldon and Penny commiserate over their mutual sadness of Leonard's absence. Howard encourages Raj to start looking for another girlfriend and is surprised at who he chooses. Amy & Bernadette are away at a conference and bond/clash when they talk about their relationships.
Leonard arrives home early and chooses not to tell Sheldon. Howard's unusual behavior results from an unlikely source.
When the entire gang competes in a scavenger hunt designed by Raj, mixing up the couples making the guys work with a different significant other. it brings out the best - and worst - in everyone.
Sheldon looks to exact revenge from Amy after she ruins Raiders of The Lost Ark by revealing a plot hole. Leonard uses guilt as leverage over Penny, and Raj and Stuart try the Internet to find a date.
Sheldon must decide how much "Amy time" is too much after she takes a job at his university. Meanwhile, Howard finds himself sleeping on Raj's couch after a fight with Bernadette.
Sheldon is insufferable after making a major scientific discovery, or did he? Howard plans an elaborate romantic celebration for Bernadette and Penny decides she needs to do something for Leonard.
Sheldon feels slighted when Professor Proton seeks advice from Leonard instead of him, and he seeks revenge by befriending a rival science TV host, Bill Nye. Meanwhile, Raj gets jealous when Howard crashes "girls' night".
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.
Howard invites everyone to his mother's house for Thanksgiving where unexpected drama emerges from Penny's past and Howard's father-in-law bonds with an unlikely person.
Sheldon's disgust at his accidental discovery and subsequent fame reach a level that compels Leonard and Amy to try to mollify him. Raj needs a temporary place to stay for a week so Howard invites him to stay with he and Bernadette.
While Sheldon is away in Texas, everyone gathers to decorate the apartment Christmas tree, and they each realize how much he has changed their lives.
Penny finally gets her big break, or does she? Her reaction leads her to make Leonard an offer that is difficult for him to accept. Sheldon tries to quantize humor while Raj and Stuart go to the mall to practice talking to girls.
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. Meanwhile, the girls see if they can act like "grown-ups".
Love is in the air when Sheldon and Amy join Howard and Bernadette for a trip to wine country. Meanwhile, Leonard and Penny must 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.
Howard takes Bernadette's comments seriously about he and Sheldon's bickering and invites Sheldon to fly with him to NASA at Houston. Penny's career luck continues its downward slide while Raj asks Amy to be his wingman on an online dating service.
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.
When Sheldon tries to be spontaneous, it leads to unexpected friction between Penny, Amy and Bernadette. Meanwhile, Raj seeks Howard's help in preparing for a date with Emily.
The loss of Sheldon's idol Professor Proton provokes feelings that Sheldon cannot seem to process. Leonard enjoys torturing Penny about their numerous marriage proposals while Amy and Bernadette decide to make a surprise to help celebrate "Star Wars Day".
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.
Sheldon is forced to confront an uncertain future and considers a major move. Meanwhile, Raj and Emily take it to the next level, and Mrs. Wolowitz's injury is a major headache for Howard and Bernadette.