Pegg plays Dennis, an amiable underachiever who ran out on his fabulous pregnant fiance Libby (Thandie Newton) five years ago and has been running ever since. Dennis is smart enough to know he made a massive mistake, but he's too apathetic to fix it. Instead he mopes around and dreams vaguely about getting Libby back, a project that is suddenly made urgent by the fact she's dating Whit, a go-getter hedge-fund manager from Chicago.

Despite the title, Dennis isn't really fat, just pasty and English. And he does look rather abject in his underpants, of which we get a couple of extended views. Pegg gets a lot of mileage from the simple sight gag of his pale, boneless body in short-shorts, just part of a very funny anti-Rocky montage of Dennis's unorthodox training regimen.