What do you get if you take Mrs Doubtfire and replace Robin Williams's comedy bosoms with Simon Pegg's chaffed testicles? Well, a greatly improved entity to be sure.
Run, Fat Boy, Run, the second film directed by David Schwimmer, is no classic, but it demonstrates that Pegg, even when working on cruise control, is capable of delivering more laughs than, well, a nuisance in a dress.
This romantic comedy began life as an American script with, Schwimmer has confessed, Jack Black's name pencilled hopefully next to the lead. A rewrite from Pegg has left us with an acceptable low-key companion to the economic epics that were Shaun of the Deadand Hot Fuzz. It's messy and obvious, but fun.
Pegg, a slovenly loser, dumps his pregnant fiancee (Thandie Newton), a baker of pink delicacies, at the altar and launches himself into a life of beer and afternoon television. Some years later, he becomes angry when Hank Azaria - the sort of evil businessman whose laptop always displays graphs - begins moving in on his former lover.
Pegg, terminally unfit, decides that the only way to atone for his earlier cowardice and win back the girl is to compete alongside the buff American in an upcoming marathon All he has to do is finish.
Dylan Moran, very funny in the role that normally goes to Nick Frost in Pegg productions, is just one of a dozen or so recognisable comic actors offering agreeable support. David Walliams deserves particular credit for his tiny cameo as a pernickety shopper in search of confectionary rabbits.
There are, it is true, some very clumsy and unsubtle moments in Run, Fat Boy, Run. Azaria, a decent actor with a wide range, does everything but cackle and roll a moustache during his scheming, and the scenes with Pegg's son are cloyingly sentimental.
But, given its peculiar history, this picture is significantly better than we had a right to expect. I suppose the distributors would like me to say run, don't walk, to Fat Boy. That would be overstretching it a tad. But you might like to, at least, jog towards the cinema.
