The Big Bang Theory Episode 6.02 The Decoupling Fluctuation

Penny confesses that something is wrong with her relationship with Leonard to the girls, but she's not sure what it is and feels like she might need to break up with him. While at the movies with the group, Amy tells Sheldon that Penny wants to break up with Leonard, but makes him promise not to say anything. Later, Sheldon goes to Penny to privately encourage her not to break up with Leonard. By the end of the episode, Penny has made up her mind to break up with Leonard, but his jubilant reaction to her preface ("You know I love you..") catches her off guard and she can't go through with it.