Caleb Sinclaire (Adam Scott) is an insomniac construction worker who is still hurting over an ex-girlfriend that dumped him, making him believe that "all women are whores." His brother Peter (Alex Frost) is coming home for Thanksgiving, bringing his new girlfriend Emma (Brittany Snow) with him to meet Caleb and his father Donald (J.K. Simmons). Throughout the time home, Caleb tries to tell his brother that Emma is a whore that will hurt him and continuously threatening Emma not to break his brother's heart. However, in Caleb's attempts to sleep with her (thus proving she is a whore) so his brother will break up with her, Caleb starts to become infatuated with her. This is due in part to the fact that she reminds him of his ex.
Late one night, after an aborted attempt by Emma and Peter to have sex for the first time, Caleb confronts Emma they wind up having sex. Caleb says it felt like it was her first time and Emma cries and laughs nervously. Caleb realizes in shock that he took her virginity and that A)She wasn't a whore and B)He stole an experience that was meant for his brother. Caleb tries to leave quickly, but his father finds him and quickly realizes what happened. They are forced to hide when Peter comes to find Emma. Caleb blackmails his father with silence, threatening to tell Peter that their mother didn't cheat on him, it was the other way around. Peter, meanwhile, tries again with Emma and loses his virginity to her, none the wiser on what happened.
Peter, Emma, and Donald go to the train station so Emma and Peter can return to school. Donald tells his son he wasn't the perfect father and he's sorry for that, and that sometimes people do bad things because doing the right thing is so hard. On the train, Peter tells Emma he loves her, and she breaks down crying, unable to deal with the guilt she feels for sleeping with Caleb. It is unknown if she tells him the truth or not (and if they would stay together if she did). Caleb goes to see his father at home, and his dad lets him in, ready to rebuild their relationship.