Howard, Raj, Sheldon, and Leonard are deeply engrossed in Halo, when Penny comes over to seek refuge from her friend, Christy, who has come from Omaha to stay with her for a while. Penny is put off by Christy as she thinks Christy is a whore. This intrigues Howard, who immediately goes over to make a move on Christy. When Penny goes back to her apartment, she sees Howard and Christy having sex and asks the guys if she can stay over at their apartment for the night. Leonard willingly obliges though Sheldon has his qualms. Moreover, Penny plays Halo with the guys, since they need one more player and she turns out to be surprisingly good.
The next morning, Sheldon finds Penny sleeping on the couch and gripes to Leonard how she is disrupting his Saturday morning routine (which is to basically have cereal while watching an episode of Doctor Who). Leonard tells Sheldon to adjust, but Sheldon is not willing to do so. This commotion wakes up Penny, and when Howard comes in (after sleeping with Christy), Penny finds out that not only did he use all of her personal items, he is still with Christy, who is obviously just a gold-digger. Though she warns Howard about this, Howard ignores her and is just happy to spend so much time with Christy.
When Sheldon, Leonard, and Raj go out for lunch at a Chinese restaurant, they realize that since there are only three of them (since Howard is spending so much time with Christy), complications will arise when having proportions of the food. Sheldon becomes picky about the proportions of food each person receives, and this results in them just going home.
On Wednesday night, the three guys invite Penny over for "Halo Night" since Howard isn't available. However, Penny declines so that she can go dancing, and the three guys start to quarrel. Sheldon decides to ask Howard to come back. When they go over to Howard's house (who lives with both Christy and his mother), they hear Christy and Howard's mother arguing, and when Christy finds out that Howard doesn't have that much cash, and isn't worth it, she storms out. Howard then returns to the three guys, and they all have Halo Night.
The episode ends with Penny and her friends sardonicly offering to have sex with the guys, who are too engrossed in Halo to pay attention.