A likeable but chubby loser Denis (Simon Pegg) bottles out of marrying his pregnant fiancee, Libby (Thandie Newtoon), and leaves her at the alter.

Five years later, Dennis us still on good terms with Libby because of their five-year-old son. He realises he has made a terrible mistake and just as he works up the courage to ask Libby for another chance, he discovers she's seeing high-flying go-getter Whit (Hank Azaria).

Desperate to prove that he's changed and is more than a quitter, Dennis decides to compete against Whit in a marathon, despite being hideously unfit.