The most shocking thing about the debut feature film from Chris Morris is not the swearing, of which there's a deliciously inventive abundance, nor the extreme tasteless horror, of which there are plenty of hand to mouth, gasp inducing moments.

What's most likely to enrage the same people who steamed with indignation at Morris's infamous Brass Eye Paedophile Special on C4 is that Four Lions shows the human side of extremist Islamic terrorism. (Now that's a pull quote we'd like to see on the poster).

Four Lions, which was written by Morris alongside Peep Show's Jesse Armstrong and Sam Bain and The Thick Of It's Simon Blackwell, is a strangely beautiful film, successfully pairing absurdity, stupidity and gratuitous unpleasantness with a measured and at times quite moving understanding of the reasons why people might choose to blow themselves up in the name of an angry god.

CCTV footage of the 7/7 bombers laughing is often used by the media to emphasise the terrorists' evident evil. Four Lions suggests, with their own band of DIY jihadists that perhaps the reason for their smiling faces is that they believe they're doing the right thing; that they're actually happy and in the company of people they care about. Some may well be disturbed loners but others are loving, family men who just happen to believe an abhorrent act is justified by a heavenly reward. This will be the hardest element of the movie to swallow, for those who'd like a simple explanation for the mentality of would-be suicide bombers. But it's also what makes Four Lions so much more than an expletive-ridden shock-fest. It'll stay with you for days.

And if all that makes it seem somewhat sobering then rest assured that Four Lions is as blackly funny as you want it to be. Imagine if Monty Python's Life of Brian was remade as a fly-on-the-wall documentary about the People's Front of Judea, to get some idea of Four Lions' mix of farce and humanity. It's a very special film.