Penny's break-up spurs Leonard to summon the courage to ask her out, and each goes to Sheldon for advice on their upcoming date.
Penny wants to throw a surprise party for Leonard, but Sheldon's indulgence at an electronics store throws her off-track.
The rest of the gang fall for Sheldon's twin sister, and Leonard's plan to win her heart is derailed when Sheldon declares he'll be the one to decide who can date her.
When the delivery of a full-size prop of 1960's The Time Machine makes Penny miss work, she scolds Leonard about his sci-fi collectibles. He decides it's time to give up such childish pursuits.
Sheldon finds himself booted from the gang's Physics Bowl team and replaced with Leslie, so he forms his own team, hoping to prove he's superior.
With Sheldon rattled and impossible to be around after meeting Dennis, a 15-year-old physicist who's smarter than he, the rest of the guys search for a way to eliminate the threat.
With his friends knowing how sour he acts when sick—and avoiding him as a result—Sheldon is nursed for by Penny, who hasn't a clue what she's in for.
Leonard and Sheldon each lie to avoid hearing Penny sing in a production of Rent. However, Sheldon's lie becomes a bit too complicated for its own good.
When they're invited to present at a physics conference, Sheldon's reluctance to share the credit leads to a tiff between he and Leonard, and Penny's intervention only exacerbates the situation.
Raj's parents set him up on a blind date, and when he discovers he can talk to her, she shows interest in Sheldon.