A well-done introduction gives us nervous Nellie Dennis (Simon Pegg) on his wedding day, completely unsure of what he's getting into. His solution to the dilemma is to run away ... literally ... right out of the church and down the London sidewalk, leaving his very pregnant bride-to-be, Libby (Thandie Newton), yelling after him.
Five years later, he's still running, though now it's part of his dead-end job as a small-time security guard, chasing after shoplifters. He'll never catch anyone, though, because he eats all the wrong foods and gets no exercise.
"I'm not fat, I'm unfit," he protests when a pal points out that Dennis has one big tummy.
He's also developed a kind of loser attitude, one that perfectly complements his current spell of bad luck. But at least he gets to see his son Jake (cute, but not too cute Matthew Benton), who lives with Mom. Dennis and Jake are tight; he's actually a pretty good dad. But delusional Dennis and steady Libby have nothing between them. She hasn't forgiven him, and he still doesn't realize the depth of his idiocy in running away from her. He still harbors dreams of their getting back together, even though she's now seeing rich, handsome, physically fit Whit (Hank Azaria).
It's charming but kinda smarmy Whit who, upon being introduced to Dennis, tells him he's running a marathon -- which leads Dennis and others to wonder aloud why anyone would put themselves through the hell of running a marathon.
But wait, maybe if Dennis convinces Libby that he can change, that he can be mature, considerate, responsible, that he'll even run a marathon for her, maybe it's not too late. Maybe then she'd take him back.
That's it. That's the very thin, rather shaky foundation that's supposed to keep the story going. Umm, it's been five years. Dennis has never explained his wedding-day actions. What on earth does running a marathon have to do with being accepted back by a woman he literally left in the dust?
There are other stories bobbing up and down in the background. Whit might not be the nice guy he appears to be. Dennis' pal Gordon (Dylan Moran) is a gambler who lays down a load of cash (that he doesn't have) on the results of Dennis' marathon run. Dennis' landlord Mr. Ghoshdashtidar (Harish Patel) becomes his tenant's trainer.
But it's too much of a slapdash affair. There's a good sight gag of Dennis in a skimpy training outfit, but writers Pegg and Michael Ian Black surround it with too many sentimental and sophomoric scenes.
Schwimmer still gets some good performances out of his cast. Pegg makes excellent use of his pliable face, convincingly registering physical pain, comic shock, utter exhaustion, grim determination, and even a rare smile or two. Moran displays a dry and funny sense of humor. Patel exaggerates his Indian accent, but his big, round face anchors a great physical presence.
The root of the film's problems, meanwhile, center on the character of Dennis. Is he a nice guy or is he kind of jerky? Are we supposed to root for him or feel sorry for him? We don't really ever know. There are also easier questions that involve the film's bigger picture: Can a loser win? Will the bad guys (yes, there are bad guys) get their comeuppance?
Everything comes to a head during the big race on the banks of the Thames. Believe it or not, that's where the film really comes alive, nearly making up for what's come beforehand. Both the writing and the direction shine here, nicely presenting a few complex plot twists that stretch from the shooting of the starter's gun to the far-away (in space and time) finish line.
Too bad that the bulk of Fat Boy is running on empty.