Idi Amin, the Ugandan dictator who fancied himself in a kilt, could be gruffly charming and merry one moment and as cruel and brutal as Saddam the next. If you got into his affections, you could be well rewarded. But if you took the liberty of bedding his latest wife, your fate would be considerably worse than a simple death.

These are the bones of Kevin Macdonald's first fictional feature, made after the success of the docudrama Touching the Void and based on Giles Foden's book. It makes a riveting two hours, given the virtuoso central performance of Forest Whitaker as Amin and the way Macdonald and his team have shot the African scenes.

There are, however, doubts. I am not sure that the highly promising James McAvoy carries off his part as Nicholas Garrigan, the young Scots doctor.

Garrigan is flattered into Amin's net to be his personal physician and is then unable to extricate himself even when he sees what the general, who dubbed himself the last king of Scotland, is doing to his people.

Amin, of course, killed thousands of them. As Simon McBurney's British diplomat puts it: "He's got a firm hand. The only thing Africa understands."

No one could call McAvoy's a bad performance, but somehow it doesn't make sense that even such an innocent would dare to seduce the monster's third and prettiest wife, Kay (Kerry Washington).

She is murdered and Garrigan is tortured in a sequence I would rather forget. But the fact remains that this part of the story seems too incredible to be true, even if we are shown, in a brief and rather tepidly dramatised earlier romance with a doctor's wife (Gillian Anderson), that the young man, free from a domineering father at home in Scotland, had an eye for a pretty face.

Yet the film stands up both as a picture of a chaotic African state that is easy prey for an effective rabble rouser, and as a portrait of Amin himself that is all the better for not going over the top and showing him, as legend has it, eating human remains stored in his fridge. This is partly thanks to the more than competent script from Peter Morgan (The Queen), Jeremy Brock and Giles Foden.

Whitaker commands his scenes as effectively as he did in Clint Eastwood's film about Charlie Parker. He is an actor with weight and charisma who has clearly done his homework on the increasingly crazy character he inhabits so strikingly.

Beside him, everything else pales, except perhaps the cinematography of Anthony Dod Mantle, which is exceptional throughout. This is a fine-looking - and despite some flaws, intelligent - film, whose African support players never let it down.