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The gang is concerned when Sheldon's former admirer, Dr. Ramona Nowitzki, resurfaces while Amy is away at Princeton.
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.
Leonard isn't thrilled when Penny considers taking a new job offered by her ex-boyfriend, Zack.
Howard and Bernadette have a hard time leaving Halley in day care when Bernadette returns to work. Also, Bert introduces the gang to his new girlfriend, Rebecca.
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.
Leonard, Penny and Raj adjust to their new living arrangement, and Sheldon takes an interest in Amy's work.
When Raj moves out of his apartment, Leonard and Penny offer him Sheldon's old 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 and Amy have their first fight since moving in together, and Koothrappali is shocked at the realities of financial independence when he stops receiving support from his father.
Leonard and Wolowitz try to distract Sheldon when he slows the progress of their guidance system. Also, Penny and Amy take Bernadette out for the night, leaving Raj and Stuart to care for baby Halley.
Sheldon beta tests an MIT invention after hearing it can help him read people's emotions. Also, Leonard can't hide his feelings when Penny announces her brother is coming to town and Raj invites his ex-girlfriends over to give him dating advice.
Leonard and Penny ask for Sheldon's and Amy's help on putting the romance back in their marriage. Howard and Raj go to extreme lengths to stop the floor in Halley's bedroom from making noise.
Sheldon and Amy visit Leonard and Penny for the first time since the holidays and share details about their terrible trip to Texas.
Sheldon and Amy attempt to have an intimate moment together to celebrate Amy's birthday; Bernadette and Wolowitz welcome their newborn baby to the world.
Sheldon and Leonard must divide up their possessions and remove Penny's things from 4B.
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.
After a successful experiment combining their genes, Sheldon will stop at nothing to convince Amy they should procreate. Also, Koothrappali is embarrassed to tell the gang what the new woman he's dating does for a living.
In order to continue living with Sheldon, Amy lies to him about repairs being made to her apartment and they host an "on location" episode of "Fun With Flags" in Penny's apartment. Also, Leonard lets Penny decorate Sheldon's old room after he learns that she has been secretly moving Leonard's collectibles into storage.
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.
Leonard and Penny must separate a quarreling Sheldon and Amy when their cohabitation does not go as planned, and Sheldon threatens to break off their relationship when Amy refuses to adhere to the bathroom schedule. Also, Howard and Bernadette find unexpected guests at their house when they decide to stay home from a planned vacation.
When Amy's apartment floods, she proposes a "cohabitation experiment" with Sheldon. Also, Howard and Bernadette are upset when Koothrappali learns the gender of the baby before they do.
Tensions rise when the boys struggle to complete their government project on time and Sheldon tries an energy drink to stay awake. Also, Penny and Amy go to a "party" at Bert the geologist's house, and Kooothrappali learns Bernadette's true feelings about her pregnancy when they clean out the future baby room.
Colonel Williams, an Air Force representative from the Department of Material Command sits down to talk with Wolowitz.
After Sheldon's mother and Leonard's father share an evening together, everyone deals with an awkward morning the following day. Also, Penny's family arrives for the ceremony, including her anxiety-ridden mother, Susan, and her drug dealing brother, Randall.
Sheldon and Amy's relationship takes a step forward, Penny makes big decisions in her professional and person life, Raj tries to prosper in the dating world with his newfound ability to talk to women, and Howard treats Bernadette to a very special surprise for the anniversary of their first date.
Howard return from space and adjust to normal, married life on earth. Sheldon and Amy's unique relationship continues to progress, though at a slower pace than Amy wants. Leonard and Penny's relationship is business as usual, with Leonard once again proposing to Penny. Meanwhile, Raj finally meets a girl and enters a (somewhat) stable relationship, the demise of which leads to unexpected progress for Raj.
Penny regrets her sleeping with Raj and has become very good friends with Leonard, Leonard breaks up with Priya, Amy pushes Sheldon in their relationship eventually becoming boyfriend and girlfriend, Penny and the other women become a clique of their own, Howard prepares for his wedding and going into space to the International Space Station and Leonard asks Penny out as they begin "Leonard and Penny 2.0".
Sheldon begins a platonic relationship with Amy, Howard reconnects with Bernadette eventually asking her to marry him, Leonard begins to date Raj's sister Priya, Penny becomes friends with Amy and expresses regret at breaking up with Leonard.
Penny throws herself at Leonard after he returns from three months at the North Pole and they begin a relationship, Penny and Sheldon start a quirky friendship though she can still annoy him, Howard begins to date Bernadette Rostenkowski and Sheldon meets Amy Farrah Fowler.
Leonard and Penny's relationship takes a turn for the worse when they break up, but is quickly resolved. Soon their relationship progresses into a stronger friendship than before, while Sheldon and Penny's friendship slowly begins.
The lives of physicists Dr. Leonard Hofstadter and Dr. Sheldon Cooper are shaken up when an attractive young aspiring actress from Omaha, Nebraska named Penny moves into the apartment across the hall from theirs. Leonard begins to become hopelessly enamored of Penny, while she feels only platonic affection for him; as a result, she finds herself putting up with his closest and ever-present friends: his roommate, Sheldon, who appears borderline obsessive-compulsive; engineering whiz Howard Wolowitz, a desperately sex-crazed mama's boy who thinks that his 1960s-style clothing and hairstyle give him an air of cool hipness; and Dr. Rajesh "Raj" Koothrappali, who will not speak to her as he is too shy to talk to women (a case of selective mutism), unless intoxicated in which case he becomes a smooth talking yet highly overly aggressive womanizing flirt.
