Sheldon is stuck in string theory, per the university. Lenny tells Sheldon they are moving in together and that startles him. The comic book store burns down, he flips. He goes to Amy and she suggests moving in together too. He mocks her like an ass and walks out on her. Then leaves on a train while Lenny tries to stop him but he leaves. Ends with Amy asking Lenny, "How could you let him go?"