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Leonard is disappointed when Penny develops an interest in an attractive physicist at the university. Meanwhile, Sheldon struggles to find an appropriate Christmas gift for Penny.
The Bath Item Gift Hypothesis
Leonard tells Penny about how he and Sheldon became roommates and what happened to the elevator.
The Staircase Implementation
Leonard, Howard and Raj have to decide who will take their extra Star Wars movie ticket, while Sheldon plans something special for Amy's birthday.
The Opening Night Excitation
Sheldon and Amy await big news.
The Change Constant
Bernadette and Wolowitz leave their kids for the first time; Penny and Leonard try to keep a secret; Sheldon and Amy stick together; and Koothrappali makes a new friend, as the gang travels together into an uncharted future.