Penny feels left out of all the science talk now that Bernadette is part of the gang. She enlists Sheldon to teach her about physics. In the mean time, Wolowitz gets irrationally jealous when Bernadette shows interest in Leonard's work. Wolowitz lets Leonard know that he better stay away from his "girlfriend." When Bernadette gets wind of Howard's jealousy, she threatens to break up with him. He apologizes and realizes there is no harm done. Back at Sheldon and Leonard's apartment Penny struggles to learn the basics of physics form Sheldon. Penny surprises everyone at the end of the episode when she rattles off a monologue about what Leonard is working on.