Ultra-tempting teen queen Michelle (Nixon) is the object of carnal obsession for two Belfast boys whose raging testosterone threatens to blow a big hole in their trousers.
There's the affable Malachy (Grint), a straight-A, middle-class student whose straitlaced yet loving family smothers him in layers of security he's just dying to break from.
Then there's his buddy Luke (Sheehan), a too-cool-for-school rebel desperate to quit his broken home - a seedy drug-dealing brother and an embarrassing suicidal soak for a dad.
While working a humdrum shift on reception at the ominously-titled Titanic Leisure Centre, Malachy spots Michelle, a precocious vixen - and daughter of sleazy centre manager James Nesbitt - who sees some sport in playing off the smitten ginge against his swaggering chum.
With the ultimate goal of a knee-trembler with the provocative Michelle, the two boys launch themselves into a giddying round of one-upmanship, each trying to top the other's drug-fuelled previous dare.
For those used to watching Grint play second fiddle to a schoolboy wizard, it's a bit of shock seeing him dropping his kegs and indulge in rumpy-pumpy with a girl who doesn't even attend Hogwarts.
However, his performance (generally - not in bed. Filth) is a considerable improvement on Driving Lessons and the whole cast deliver convincing portrayals of idle, Twitter-connected youth even if their lack of moral parameters are a cause for some concern.
Heck, this lot can even concoct a grammatically correct SMS message.