Penny's slutty friend, Chrissy (Brooke D'Orsay) from Nebraska, is in town and is seduced by Wolowitz.  The pair ends up taking over Penny's apartment, forcing her to sleep on Leonard and Sheldon's couch.  With Wolowitz gone, Penny joins their Halo team, causing Sheldon much distress when she ends up being good at the game. 

Chrissy moves in with Wolowitz and Sheldon becomes disturbs at their odd numbered group.  When Sheldon asks Penny to join another Halo night and she rejects in favor of going out with her friend, he is even more distraught.  Believing their group is falling apart, Sheldon heads over to Wolowitz's to get him back only to find Chrissy and Wolowitz's mother fighting and she leaving.  Wolowitz, again single, simply asks, "So, Halo night?" and the group is saved.