Five years ago Dennis (Simon Pegg) was at the altar about to marry Libby (Thandie Newton), his pregnant fiancé. 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.