Seth Winnick (FRENCH STEWART) is a successful TV writer who's just met a beautiful woman, Chelsea Turner (BRIDGETTE WILSON), at a wedding for Seth's writing partner, Larry Garnett (BILL BELLAMY), and his wife, Holly (TYRA BANKS). Although they've only known each other for less than a day, Seth and Chelsea quickly hit it off and instantly become an item with her moving into his house not long after that.
With advice from Holly about how to manipulate a man into proposing, Chelsea, who's obsessed with getting married, sets out to snare Seth. The only problem is, he's not keen about tying the knot, no matter how much he loves her or enjoys the sex. That soon creates a problem between the two, but they finally agree that if things are still going well after a one-year test period, he'll propose to her.
When that time's up, however, he doesn't pop the question, thus evoking her wrath as she sets out to ruin his life for breaking his promise. Soon, the two are at each other's throats, with her refusing to move out of his house and even trying to date his agent, Marty (STEVE HYTNER). Meanwhile, he plans to go out with Tawny (SHANNA MOAKLER), a blonde bimbo he's hired to appear on his TV sitcom, "Ronnie & Juliet," where the actors, Jesse (JASON BATEMAN) and Rebecca (TIFFANI-AMBER THIESSEN), act out Seth and Chelsea's life, from his perspective.
From that point on, Seth tries to contend with Chelsea's various tactics of ruining his life by countering her moves with those of his own, and soon a full scale domestic war ensues.