Dennis Doyle (Simon Pegg) is about to marry Libby (Thandie Newton), his Pregnancy Engagement. However, he gets cold feet and runs away on the day of the wedding. Five years later, Dennis discovers that Libby, who has their son, Jake, has started seeing successful but arrogant Whit (Hank Azaria) and realizes that he has truly lost her. He finds out that Whit is running the Nike River Marathon in London, and to prove himself to his uncertain friends and, most importantly, Libby and his son, he decides to run the race himself.