Five years ago Dennis (Simon Pegg) was at the altar about to marry Libby (Thandie Newton), his pregnant fianc+e. He got cold feet and ran for the hills and he's been going in circles ever since. When Dennis discovers Libby's hooked up with high-flying-go-getter Whit (Hank Azaria), he realizes it's now or never. He enters a marathon to show he's more than a quitter, but then finds out just how much sweat, strain and tears it takes to run for 26 miles. Nobody gives him a chance, but Dennis knows this is his only hope to be more than a running joke.