The Big Bang Theory Season 2 Episode 1
The Bad Fish Paradigm
Penny and Leonard return from their date from the end of season one and Leonard finds Howard and Raj spying on them via a webcam. They analyze how the end of the date went when Penny wanted to slow things down a bit.
We find out that it's because Penny feels insecure about never having completed community college and lying to Leonard about it. She confesses everything to Sheldon and makes him keep her secret. Sheldon, unable to lie to Leonard, moves out of the apartment to avoid him.
Sheldon goes to crash at Raj's and Wolowitz's where he is kicked out of both homes. Wolowitz takes Sheldon back to Leonard's loaded up with his mother's Valium. The drugs make Sheldon come clean about Penny.
Leonard then heads over to Penny's thinking he would help by presenting her pamphlets from local community colleges. She finds it insulting and slams the door in his face.