First Night: Four Lions, Sundance Film Festival

The Comedian Chris Morris made his name on radio and television by courting controversy, and his incendiary humour is paraffin-fuelled in his directorial feature film debut about a terrorist cell situated in Sheffield.

The protagonists are loosely based on the 7/7 bombers, but rather than being hardline fundamentalists, the bombers in this movie could easily have stepped out of the television comedies Citizen Smith or Dad's Army.

There is something quite apt about depicting suicide bombers as bumbling fools, nonetheless Morris saves his best jokes and most cutting criticism for the police. He calls the boys in blue to task for their poor surveillance techniques and woeful communication skills, and the funniest skit of the movie is a brilliant send-up of the Jean Charles de Menezes Tube shooting.

The tone of the comedy is set in the opening scene, which sees an attempt to make a homemade video claiming responsibility for terrorist activity. Immediately the inadequacies of the cell are highlighted. Especially foolhardy is the Islamic convert Barry (Nigel Lindsay), a character seemingly based on the shoe-bomber Robbie Reid. His cohort Fessal (Adhil Akhtar) is shown with a box on his head claiming that it is haraam (against Islamic law) to put his image on camera. While Waj (Kayvan Novac) just has several screws loose.

Morris has in the past criticised both Martin Amis and Abu Hamza for taking Islamic writings out of context and using them to incite hatred, and this trait is a feature of all his characters. Indeed Morris does a grand job of treading the fine line of criticising the bombers without being offensive to Islam.

The leader of the group, Omar (Riz Ahmed, who made his own droll contribution to sending up the terror threat with his song "Post 9/11 Blues"), disillusioned by his amateur comrades, goes to Pakistan for a training camp run by his cousin. He takes Waj along for the ride, and so ridiculous are the pair that they're soon ostracised at the training camp. In an attempt to prove themselves worthy of being in the company they try to shoot down a US drone, and only succeed in killing a terrorist leader. Morris always ensures that comic accidents push these Sheffield lads into terrorism rather than any great link with al-Qa'ida.

On their speedy return to England they discover that Barry has recruited another member to their unit, Hassan (Asher Ali), who is first seen making a scene singing a rap before pretending to explode a bomb at a community meeting discussing the terror issue with the local MP.

But this moment signals a rather lacklustre second act of the film, in which polemics replace humour. The introduction of Omar's traditional brother trying to be the voice of reason falls flat, as does Omar's bedtime stories to his son, teaching him that suicide bombing is good. It's when the comedy is designed to offend rather than as commentary that it doesn't work.

Just as the action seems to be heading into a cul-de-sac, Fessal dies in a training accident when he trips over a sheep. His death forces the remaining four lions into action as they decide to wear fancy dress and blow themselves up during the London marathon.

Morris does a great job of balancing poignancy and comedy in a hard-hitting finale that shows a humanity and observational brilliance that surpasses everything else in this movie. In doing so he exposes a myth of terrorist bombers being trained assassins but instead exposes them as being confused young men.