Penny and Leonard have a huge fight after Penny tells him that a male musician friend of hers, who she dated for a short time, is coming to stay with her while he is in town. Leonard is enormously bothered by this and can't understand why Penny thinks it is fine. They begin to argue and we find out that Sheldon cannot take hearing people argue. Apparently Sheldon heard his parents argue all the time and it traumatized him. Koothrappali and Wolowitz then take sides and begin to argue about who is right in the fight between Penny and Leonard. Sheldon can't handle anymore fighting so he takes refuge in the comic book store where Penny and Leonard have to convince him that they are done fighting in order to get Sheldon to come home.