Sorry you all know the story right now I dont deserve this treatment on the site deleteing my reviews how dare they so here is a recap.
Sheldon, who has declined to come on the trip to Vegas, is ready to spend the weekend alone. He returns home to find that he has forgotten his key inside his locked apartment. Since the building manager will not arrive until the next morning, and Penny says she left her emergency key in their apartment, he is forced to spend the night with her. He sleeps in Penny's bed because he can't fit on the couch. When Leonard returns at the end of the episode, Sheldon comes out of Penny's apartment, telling Leonard he had dinner with her and spent the night. He also tells him that he has a better understanding of the term "friends with benefits," (which was a question he had the entire episode) before entering the apartment, leaving a bewildered Leonard looking shocked.