Two brilliant physicists meet their new neighbor Penny, and the balance of nerd and normal is forever changed. Sheldon and Leonard's world just got a lot more interesting.
Leonard and Sheldon sneak into Penny's apartment to organize it while she's asleep. The gesture is sweet in theory but deeply creepy in practice.
Leonard tries to work up the courage to ask out a woman from the coffee shop. Sheldon's idea of helpful advice is characteristically unhelpful.
Sheldon gets fired and discovers his calling might be making luminous fish. His mother Mary flies in from Texas to talk some sense into him.
Leonard goes on a date and the guys monitor his progress from the apartment. The Cheesecake Factory becomes the gang's second home.
Penny throws a Halloween party and the guys show up in full nerd costumes. Leonard meets Penny's intimidating ex-boyfriend Kurt.
Howard's girlfriend disrupts Halo night and takes over the guys' routine. Sheldon is not pleased about the new apartment dynamic.
Raj participates in an experiment to overcome his inability to talk to women. The results are unexpectedly smooth — but temporary.
Leonard and Sheldon are invited to present at a conference but can't agree on anything. Their professional rivalry escalates publicly.
Leonard lies to avoid hanging out with someone, and Sheldon creates an elaborate cover story that spirals beautifully out of control.
Sheldon gets sick and becomes an insufferable patient. Penny discovers firsthand what Leonard has been dealing with all these years.
A teenage prodigy arrives at the university and threatens Sheldon's position as the smartest person in the room. Sheldon handles it poorly.
The guys compete against a rival team in a physics bowl. Sheldon gets kicked off and joins the other side, making it personal.
Leonard buys a full-size time machine prop and Penny questions the guys' spending priorities. The collectible causes building-wide problems.
Sheldon's twin sister Missy visits and all three guys compete for her attention. Sheldon tries to control who gets to talk to her.
The gang throws Leonard a birthday party even though he doesn't want one. Penny's gift triggers a chain of emotional reactions.
Leonard and Penny finally kiss, sparked by an experiment involving cats and quantum theory. The first season ends with real romantic possibility.