Dennis (Simon Pegg) has a problem with committing to anything. So much that he leaves his wife-to-be, Libby (Thandie Newton), on the day of her marriage while she is [obviously] pregnant with their child. Five years pass and Dennis and Libby remain in contact for the sake of the child, Jake (Matthew Fenton). Libby has become involved with a new man, Whit (Hank Azaria), and the relationship seems very serious. In order to win Libby back Dennis decides to show that he is finally ready to commit fully to something by running a marathon alongside Whit, or against him depending on how you see it.