Dennis (Simon Pegg) is an ordinary guy. He's a little overweight and he smokes, but he's nice, charming and unassuming. However, five years ago, Dennis did something that wasn't so nice. On his wedding day to his pregnant fiancée Libby (Thandie Newton), Dennis did a runner. He just couldn't go through with it. He didn't think he was good enough for Libby, so instead of ruining her life, he ruined her big day.
Now, five years later, Dennis realizes he made a big mistake. He's still in love with Libby. He still sees her all the time because of Jake (Matthew Fenton), their five-year-old son, but all romantic ties have been severed. Libby's now seeing Whit (Hank Azaria), a charming professional from the City, and Dennis takes an immediate dislike to him. Whit is casual, easy-going, successful and good with Jake; much to Dennis's annoyance. How can Dennis compete with him?
In direct contrast to Whit's sleek lifestyle in the City, Dennis is a mere security guard for a ladies retail store. He doesn't earn much money and he's well behind on his rent, something which his landlord, Mr. Ghoshdashtidar (Harish Patel), doesn't take lightly. Mr G's daughter Maya (India de Beaufort) is particularly intent on evicting Dennis and replacing him with a much more reliable tenant. Compared to Whit's effortless abilities, everything Dennis attempts seems to end in disaster. Even when he wants to take Jake to see "Lord of the Rings: The Musical," he finds himself in trouble
with the police for trying to buy from a ticket tout. There is no help forthcoming from Dennis' feckless best friend Gordon (Dylan Moran), whose life is just as much of a mismatch. Gordon's main sources of interest are the poker games held at the back of a piano store with a bunch of misfits: Grover, Claudine and Mickey, overseen by
the imperious Vincent (Simon Day).
Dennis feels demoralized and overwhelmed by Whit's long list of achievements. In a last-ditch effort to prove to Libby that he's changed his ways, Dennis impulsively announces at her birthday party that he will run in the same London Marathon that Whit has been training for. But does Dennis have the stuff to trade his cold feet for running shoes?