With her biological clock ticking down and no father-figure husband in sight, Kassie (JENNIFER ANISTON) has decided the time is ripe for getting pregnant via artificial insemination, a decision approved by her friend Debbie (JULIETTE LEWIS). Wally (JASON BATEMAN), on the other hand, and Kassie's best friend and neurotic equity analyst, thinks it's an awful idea, although he can't specifically articulate why he has that feeling.
Of course, it could be that he harbors a secret love for her, something he can't even recognize, although it doesn't escape his boss and friend, Leonard (JEFF GOLDBLUM). Even so, Kassie decides to go through with it and chooses married college professor Roland (PATRICK WILSON) -- who needs the money -- as her donor. But at her "I'm Getting Pregnant" insemination party, Wally has too much to drink and accidentally spills the all-important vial of Roland's contribution.
Panicked, and with his judgment clouded by too much booze, Wally replaces that with his own and then unintentionally forgets what he's done. Seven years later, and after having moved to Minnesota to raise her child, Kassie moves back to New York City with 6-year-old Sebastian (THOMAS ROBINSON). He's a bright but neurotic boy, much like his father who slowly begins to see signs of himself in the kid. After piecing together what he did, Wally must then figure out whether to tell Kassie the truth, knowing it might very well end their friendship, especially considering that she's become romantically involved with the now-divorced Roland.