CBS

Season 1: Episode 10 - The Loobenfeld Decay

The Loobenfeld Decay: The guys are invited by Penny to see her in a performance of Rent, but they know how horribly she sings, so Leonard lies to get out of it. But Sheldon is so disturbed by the lie, that he makes up a very intricate new lie, which he thinks is better. They tell Penny that Sheldon has a cousin who is a drug addict, and they have to do an intervention in Long Beach. They invite over a fellow physicist, who minored in theater in college -- to be the cousin. Penny comes over, and falls for the whole thing.