Paul Morse (Jason Lee) is a good guy. When his friends throw him a wild bachelor party, he just wants to keep his conscience clean - which is why he's shocked when he wakes up in bed with a beautiful girl named Becky (Julia Stiles) and can't remember the night before. Worse than that, she also happens to be his fianc�e's cousin.
Desperate to keep his fianc�e, Karen (Selma Blair), from finding out what may or may not be the truth, he tells her a teensy lie. Which turns into a bigger lie. From the bachelor party to the wedding, soon his lies are spiraling out of control and his entire life is a series of comical misunderstandings. It's amazing how much can change because of one little thing - A Guy Thing.