It's been five years since Dennis (SIMON PEGG) , suffering from cold feet and a fear of long-term commitment, ran away from his pregnant fiancée, Libby (THANDIE NEWTON), on their wedding day. He's now employed as a security guard at a lingerie store, while she works at a bakery when not raising their son, Jake (MATTHEW FENTON). Since she allows him to see their boy, Dennis harbors hopes that Libby might forgive and let him back into her life.
Yet, she's now dating hedge fund manager Whit (HANK AZARIA) and seems quite happy with him. Besides, Dennis is late with his rent to Maya (INDIA DE BEAUFORT), the adult daughter of his landlord, Mr. Ghoshdashtidar (HARISH PATEL), while his best friend, Gordon (DYLAN MORAN), is a gambling addict, none of which would likely impress Libby.
Instead, she likes that Whit follows up and completes anything he commits to, such as running a marathon for charity. Learning of this, Dennis decides he'll do the same. Unfortunately for him, Dennis has never undertaken such a quest, particularly since he's anything but fit. Nevertheless, he starts training, hoping that Libby will take notice and have a change of heart.