Five years earlier, nervous bridegroom Dennis (Simon Pegg) leaves his pregnant girlfriend, Libby (Thandie Newton), at the altar. Now he realizes he made a big mistake and wants to win her back. Unfortunately, Libby is involved with a handsome, attentive and fit financier (Hank Azaria), who plans to run in a charity marathon. Out-of-shape Dennis enters the race -- hoping to impress Libby -- even though he only has three weeks to prepare.