A Separation (PG)

Ryan Gilbey admires a perfectly choreographed story of suspense.

There are no car chases in A Separation, no international espionage. Only a single punch is thrown in the course of its two hours and no one leaps daringly across rooftops in a hail of bullets. Yet this Iranian picture, which won the top prize at this year's Berlin Film Festival, has some claim on being the year's most explosive action movie and its most suspenseful thriller.

Simin (Leila Hatami) and Nader (Peyman Moaadi) have reached an impasse. She wants to move abroad with their ten-year-old daughter, Termeh (Sarina Farhadi), but he insists on staying put in Tehran with his father (Ali-Asghar Shahbazi), who has Alzheimer's. As the film begins, we are invited literally to pass judgement: the opening five minutes are seen through the eyes of the judge at their divorce hearing, so that the aggrieved couple seem to be appealing directly to us. Not for the last time, the writer-director, Asghar Farhadi, encourages us to compare contrasting aspects of a moral dilemma, competing versions of the truth.

The frame is static during the divorce hearing and the scene is shot in one unbroken take, but don't get used to that. Once Simin and Nader leave the court, the handheld camera wheels around them and the editor cuts more ferociously than a Conservative minister. Back at the couple's apartment, Simin moves out and Nader hires Razieh (Sareh Bayat) as his father's carer. Razieh questions the salary - isn't it a bit paltry, considering that she and her young daughter, Somayeh (Kimia Hosseini), will need to leave their home at five each morning to travel two and a half hours on several buses? Nader won't budge, even though we can see from his spacious home, good job and well-groomed appearance that he is not exactly struggling. Somayeh gazes in awe at the butterflies pinned under glass. A mounted elephant's head could not look more exotic.

Razieh backs down, but decides not to tell her husband that she will be working in the home of a single man. Immediately, there are problems. On her first morning, she arrives to find that the old man is padding around in wet trousers. Would it be a sin for her to change his clothes? "I won't tell Dad," chirps Somayeh, doodling at the kitchen table. Razieh touches the child's face and whispers: "Angel."

The incident sets off alarm bells in the viewer's head, and we dutifully file it away for future reference. In fact, it plays only a small part in the trouble that engulfs the characters a few days later, when Nader decrees Razieh unfit to care for his father and bundles her out of the apartment. It's an understatement to say that all hell breaks loose. All hell breaking loose would be a Buddhist retreat compared to what happens next.

As Nader goes to war against Razieh and her husband, Hodjat (Shahab Hosseini), and their grievance spills out into the courts and the local school, the camera keeps noting the witnesses on the edge of the hubbub. Somayeh is one of three people in the film shown looking on silently as moral predicaments on which they have no influence play out around them. Similarly stranded are Nader's father, whose entire perspective is locked away in his fading mind (it is one of the story's unstressed ironies that, were he able to express himself, he could forestall the catastrophe) and the wise Termeh, who becomes the conscience of the film.

If I have revealed little about the crisis at the centre of A Separation it's because I want you to experience this film - which resembles an Iranian Winslow Boy - exactly as I did; that is, with hand raised to mouth to stifle repeated gasps of "Oh God!" or "No!" The free camerawork and effortlessly natural performances do not prepare you for how meticulously choreographed the film is, or how elegantly it incorporates thriller conventions; what starts out as a domestic drama moves on to eyewitness statements, the trajectories of bodies and so forth. Its eye is forensic, from the moment when Nader and Termeh start to familiarise themselves with the washing machine in Simin's absence. Surely it should be set to cycle number four, reasons Termeh, because that is the one which has faded through use. The kid is some detective. A Separation turns out to be her greatest case.