After years of imprisonment and travel bans in his native Iran, Jafar Panahi returns to Cannes with a furious but funny revenge thriller that takes aim at oppressive regimes and could scoop the Palme d'Or.

The film opens with a long, unbroken, deceptively charming shot of a genial man (Ebrahim Azizi) and his happy, pregnant wife driving in the countryside one evening, with their playful daughter in the back seat. When the car breaks down, the husband persuades a mechanic to tinker with it, but then the mechanic's rumpled colleague Vahid (Vahid Mobasseri) recognises a chilling combination of sounds: the uneven footsteps of someone with a limp, and the squeaks of an artificial leg.

One of the themes running through the competition films at this year's Cannes Film Festival is how hard it can be to battle your way to justice when the state is standing in your way. In Two Prosecutors, the bureaucracy in Stalin's USSR grinds truth to dust. In Eagles of the Republic, an Egyptian actor finds himself being directed by slimy officials, both at work and at home. Dossier 137 is set in today's France, but even there, a police investigation is obstructed by systems that protect some kinds of wrongdoers more than others.

The most immediate and personal of these films is It Was Just an Accident, written and directed by Jafar Panahi. Panahi has repeatedly been imprisoned and banned from film-making in his native Iran, and has been subject to so many travel bans that he hasn't been in Cannes since 2003 (although his films have), so it's hardly surprising that his latest film is so frank about life under an oppressive regime. What may be more surprising is that It Was Just an Accident balances fury with warmth, humour and sympathy for its characters, even when taking on the grimmest possible subject matter.

These sounds, which have haunted Vahid's nightmares for years, recall someone he calls Peg Leg, a sadistic interrogator who tortured him while he was incarcerated on trumped-up sedition charges. On impulse, Vahid knocks the man out with a shovel, and stuffs him in a box in the back of his van. He plans to bury Peg Leg alive in the desert – and........

© BBC