When Penny invites the guys to her Halloween party, they are all overjoyed - except for Leonard, who worries about impressing Penny's friends. After an embarrassing situation in which all the guys try to dress as The Flash, they each finally agree to change. Sheldon dresses as the Doppler Effect, which only worries Leonard more.
At the party, Penny's friends mingle while the guys sit together on the couch. Wolowitz unsuccessfully tries his hand at wooing various women as Leonard and Sheldon try to determine how Leonard can make friends with Penny's friends. Unfortunately, Penny's ex, Kurt, shows up at the party. When there is a confrontation between Kurt and Leonard, Penny attempts to put a stop to it, but Leonard ends up humiliated anyhow. He and Sheldon promptly leave the party.
At the end of the night, Penny comes over to Leonard and Sheldon's place to console Leonard. Instead, she cries on his shoulder and kisses him. In a chivalrous gesture, Leonard brushes Penny off due to her drunk state, but quickly kicks himself for letting an opportunity to sleep with her get away.
Later, Howard tries to find Raj at Leonard and Sheldon's, but Raj is in fact recovering from coitus with a cute girl from Penny's party.