The episode starts with Leonard and Sheldon hanging out in the living room of their apartment. Leonard gets upset when Sheldon gives away a key plot point in the Harry Potter book he’s reading. An argument escalates from there, resulting in Sheldon accusing his friend of being too sensitive due to consuming an abundance of soy milk and Leonard throwing the roommate agreement in the trash can.
He then announced he didn’t want to live there anymore and was moving out. He went across the hall to Penny’s apartment to give her the good news that he was moving in.
Penny was uncomfortable with this, but Leonard missed the clues in his excitement. She later spilled the beans to Sheldon who used it against her when Amy Farrah Fowler wanted to move in with him.