The Last King of Scotland (15) <!-- none onestar twostar threestar fourstar fivestar -->

Cinema loves stories about young men who become the protégés of charismatic brutes - they tend to be gangsters, or soldiers, or politicians - and in The Last King of Scotland the genre hits a double-top. Idi Amin, President of Uganda during the 1970s, was a soldier, a politician and, as his protégé belatedly discovers, one of the biggest gangsters of them all, having terrorised his country and killed an estimated 300,000 of its citizens.

He fled Uganda in 1979, having reduced it to tatters, but, in common with Hitler and Stalin before him, his absence is still lamented by a proportion of his countrymen. It seems that barbarity on an enormous scale isn't enough to lose you the native goodwill; would ineptitude do the trick?

Kevin Macdonald, previously acclaimed for his documentaries (One Day in September, Touching the Void), examines the "King" by way of a courtier, a fictional doctor composed of several real-life figures. Young Scots medic Nicholas Garrigan (James McAvoy) has no idea who rules the roost in Uganda when he arrives in 1971, nor does he care. He wants to help a country hospital outside Kampala and, his sap still high from student days, he wouldn't mind getting laid too. (The credits aren't over before he's managed to boff an obliging coach passenger.)

On arrival, he seems to have found himself in a Graham Greene novel - a flyblown outpost where the wife (Gillian Anderson) of the doctor is wilting from sexual neglect - but their half-dalliance is only a feint. Fate throws Garrigan in the way of a road accident, where the country's new leader is nursing a sprained hand after his car hit a cow. Snatching Amin's pistol, the doctor puts the animal out of its misery, and his decisiveness wins him favour - as does his Scottishness: it is one of the dictator's inexplicable fancies to style himself a Caledonian king.

Amin invites the young man to become his personal physician, and Garrigan delightedly accepts. Thus is fulfilled JM Barrie's famous observation: "There are few more impressive sights in the world than a Scotsman on the make." Garrigan's cockiness is nicely caught by McAvoy, his limpid blue eyes always alert to opportunity. Amin, with his low cunning, immediately spots the young man's vanity, and flatters him into the illusion of intimacy.

The part of the dictator needs a massive performance, in all senses, and it gets one from Forest Whitaker. So often the gentle giant, Whitaker changes tack here with an intimidating display of girth and guile, mixed in with the outward signals of untouchable nuttiness. Only take a look at his jacket, festooned with medals and frogging, like a costume run up for a Gilbert and Sullivan opera. He wears it because, like any despot, he has no sense of the ridiculous. When Amin tells Garrigan that he needn't worry overmuch about his medical condition ("I know precisely when I am going to die," he says), the doctor treats it as an amiable eccentricity.

The script, adapted by Peter Morgan and Jeremy Brock from the 1998 novel by Giles Foden, engrossingly charts the way eccentricity becomes something dangerous, and murderous. There is menace even in his generosity. When Garrigan saves an epileptic boy, one of Amin's many children, he's rewarded with a Mercedes. But it is the largesse of a gangster, who believes that by giving to people he somehow owns them.

Too late does Garrigan realise the pact he has made. Promoted from doctor to "closest adviser", he finds himself with more responsibility than he knows what to do with. It is useless in any case: when he does advise Amin he is dismissed as "nothing but a doctor - a nobody". After distant gunfire, ambushes and sudden disappearances, even Garrigan sees the signs that the country is in meltdown.

I wonder if the film-makers realise that, in spite of McAvoy's charm, the safety of this complacent, rather dim man won't concern the audience quite as much as they'd like. Simon McBurney, excellent as a cynical Foreign Office dignitary, has the best line when he coolly replies to Garrigan's plea for help: "'Please'? That's a nice touch."

This seems to me the major miscalculation of the film; that when the protagonist is most in need of rescue we, the audience, feel anything but sympathy for his plight. By this stage, after all, he has betrayed his colleagues and been complicit in the economic mismanagement of the country, and he stands personally accountable for the murder of people who trusted him. The press notes, however, insist that Garrigan "finally finds the courage to make a stand".

Macdonald, aiming for a thriller conclusion, picks up the pace in the last reel, but when the hero seems barely less ignorant at the end than he is at the beginning, you struggle to care. If this had been written by Graham Greene, the doctor would have sacrificed himself out of pity for his crimes, or for love, or for the whole tragic mess of Africa, and then we might have found something to admire in him. Instead, it becomes a botched bid for "redemption", which has not been earned either dramatically or morally.

Perhaps this air of compromise is intended to echo the West's treatment of Africa. As a Ugandan doctor bids Garrigan: "Tell the world about Amin. They will believe you. You're a white man." The Last King of Scotland at least honours that brief.