Neurotic Wally Mars (Jason Bateman) has one bright spot in his life: his friendship with Kassie (Jennifer Aniston). When Kassie announces her intention to become pregnant using a sperm donor, Wally thinks he will be the lucky guy, but Kassie has someone else in mind. While drunk at Kassie's insemination party, Wally substitutes his sperm for the donor's, then passes out, remembering nothing. Seven years later, Wally meets Kassie's son, who seems oddly familiar to him. Wikipedia