The Last King of Scotland (15)

It's 1971, and a newly qualified doctor moves from Scotland to Uganda because he wants to help people, he says, and because he wants to have an adventure. He could have added that he also wants a father figure, some validation, a break from his dour home, and plenty of no-strings sex. In The Last King of Scotland, no one's motives are cut and dried - not even those of a mass-murdering dictator.

The film is adapted from Giles Foden's novel, a Whitbread-winning mix of fact and fiction. Foden invented the character of the doctor, Nicholas Garrigan, but slotted him into real events from Ugandan history, so it's fitting that the film-makers are experts at melding journalism and drama. Kevin Macdonald, the director, proved with One Day In September and Touching The Void that documentaries could fill the big screen. And the screenplay was co-written by Peter Morgan, who established with The Queen and Frost/Nixon that he's the go-to guy if you need to put made-up dialogue into the mouth of a 20th-century head of state. Together they've made a magnificent film - dense with ideas, rivetingly plotted and dripping with atmosphere.

James McAvoy stars as the naive and impulsive Garrigan, who soon learns that there's not much fun to be had in an understaffed medical station in the African bush. It's no wonder he's swept off his feet by the country's charismatic new president, played by Forest Whitaker. Idi Amin is considered by the British to be "one of us", and hailed as a saviour by the Ugandans. He's also a lover of all things Scottish, so he offers Garrigan the job of personal physician. Garrigan can build Uganda's national health service, he's told, and there might be a Mercedes in it for him, too. It takes some time for the doctor, like the rest of the world, to appreciate how monstrous his new best friend can be.

In the less showy of the two lead roles, McAvoy keeps hold of our sympathies, despite his character's cavernous flaws. Whitaker, meanwhile, puts in the performance of a lifetime as Idi Amin, a bear-like joker who changes into a paranoid executioner as easily as the film changes from cheeky comedy to febrile thriller.

n.barber@independent.co.uk