Season 1: Episode 2 - The Big Bran Hypothesis

The Big Bran Hypothesis: Sheldon and Leonard go over to Penny's apartment to drop off a package that came for her. Sheldon is deeply disturbed when he sees how messy and disorganized her apartment appears. Later that night, when Leonard is sleeping, Sheldon sneaks into Penny's apartment to clean and organize it. Leonard catches him and tries to stop him, but Sheldon is obsessed. When Penny wakes to find what Sheldon has done she is furious.