He smells of rosewater, and on good days he brings gifts of cakes and apricots. On bad days he dispenses with the sweets and the sweet talk and storms in with bunched fists.

This is the modus operandi of the interrogator who keeps Maziar Bahari (Gael Garcia Bernal) company during the 118 days he spends in Tehran's infamous Evin prison. Bahari, an Iranian-born journalist based in London, was arrested in 2009 while covering the Iranian elections and accused of being a spy. The most bizarre piece of evidence used against him was a comedy sketch he had filmed for The Daily Show with Jon Stewart. Bahari's interrogator, whom he nicknamed Rosewater, and his boss chose to take the sketch seriously.

In Evin in 2009, humour was no defence.

But humour of the most absurdist kind does find its way into the film, which was directed by Stewart from the memoir written by Bahari and a collaborator, Aimee Molloy. At one point, Bahari is reduced to hysterical laughter by his interrogator's mysterious fixation on New Jersey. For some reason, he is convinced it's the most sinful spot on earth - a sinkhole of sexual depravity unique in the Western world. And when Bahari impishly makes up a story about having been there and sampled its delights, he can't believe his luck.

By then, Bahari has virtually given up hope and is finding its surrender oddly invigorating. It's the final irony in the film's exploration of power and the way it's so often exercised solely for its own sake. Rosewater  (Danish actor Kim Bodnia) and the office he serves have no practical reason for keeping Bahari captive. They are not trying to extract names from him. They already have the anti-government protesters he was interviewing before his arrest. They just want him to confess to being a spy. As the embodiment of Western decadence, he's an eminently suitable candidate for ritual humiliation.

Bahari has spoken and written of the Stockholm Syndrome-type symptoms he experienced while in prison - how he came to look forward to his sessions with Rosewater because they provided his only relief from the boredom of his isolation. What's more, the two men shared a bond - both their fathers had been imprisoned and tortured during the Shah of Iran's regime.

The film underplays this connection. Rosewater is feeling too bowed down by the pressure being exerted from above to have any time for empathy. At times, he produces a show of compassion, and for a moment or two you can almost believe him. Bodnia played the sympathetic detective in the Scandinavian TV series, The Bridge, and the residual whiff of benevolence bestowed by the role keeps you on the lookout for a soft side. It's one mark of the film's subtlety.

Another is Bernal's performance, with its complete lack of macho heroics. Boyish and slightly built, Bahari can see it's futile to think of lashing out against the bullying of his burly interrogator. All he really wants is to return to his pregnant fiancee in London.

At the same time, he can't help but comparing his own attempts at resistance with the conspicuous courage shown by those close to him. There is the dignity and stoicism of his mother (a fiercely  intelligent Shohreh Aghdashloo). And he thinks continually of his older and much loved sister, Maryam (Golshifteh Farahani) - seen in flashback - who died after serving six years in prison. He's also intimidated as well as sustained by memories of his father (Haluk Bilginer), with whom he holds imaginary conversations. Then there are the young activists he met before his imprisonment. They proudly showed him the satellite dishes they had smuggled on to the rooftop of their block of flats, dazzling him with their buoyant faith in the future of their country.

It is Stewart's debut as a director. He says guilt played a significant part in his decision to make the film since his program was partly responsible for Bahari's arrest. Whatever his motive, it has paid off, for he's made a film for our times. Political prisoners, like refugees, have become figures emblematic of so much that is going on in the world. It's a story which translates into many languages.