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 fiance’s cousin.

Desperate to keep his fiance, 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.