Minutes before his wedding, Dennis Doyle (Simon Pegg) leaves his pregnant fiancé, Libby (Thandie Newton), at the altar. Five years later, he realizes he made the biggest mistake of his life and wants her back. Unfortunately, Libby has moved on and is dating Whit (Hank Azaria), a super-successful financial manager with a great personality and even better abs. Dennis, on the other hand, is overweight and employed as a security guard at a London mall. In order to prove his worth to Libby, Dennis decides to enter the same London Marathon that Whit has been training forthe big difference is that Dennis has only three weeks to prepare for the grueling 26-mile race.