In Run Fatboy Run, Dennis (Simon Pegg) leaves his fiancée, Libby (Thandie Newton), at the altar. He spends the next five years getting fatter while she gets even fitter. By the time Dennis realizes he's made a big mistake, Libby's already attached to an American suit named Whit (Hank Azaria) who, in addition to being the perfect boyfriend and an able provider for Libby and her son, is also an impassioned marathon runner. In one of those it's-so-crazy-it-might-work plot twists, Dennis decides to train for a London marathon in the hopes of winning Libby back.
There are some genuinely funny bits here, but Run FatBoy Run is mostly a by-the-numbers redemption tale that careens from sentimentality to stomach-turning sight gags. And Pegg is hard to sympathize with, starting out a jerk and upgrading only to a sad sack.