The Last King Of Scotland, London film festival opening gala

Forest Whitaker gives a barnstorming performance in a bitter, juicy plum of a role, which allows him effortlessly to steal the scene in the London film festival's opening movie. He plays Uganda's brutal dictator Idi Amin, who terrorised his country throughout the 1970s, and with mercurial panto-villain charisma mesmerised the rest of the world.

Director Kevin Macdonald, who made the prizewinning documentaries One Day In September and Touching The Void, now presents his debut feature, based on Giles Foden's novel, and as a movie it becomes a rivetingly enjoyable, if more simplistic piece of red-blooded storytelling.

James McAvoy plays Nicholas Garrigan, a callow young Scottish doctor who comes out to Uganda to help with a medical mission, but yearns for greater adventures. Idi Amin takes a shine to the young man, and with a dazzling mixture of flattery, charm and implied threat, induces him to take a job as his "personal physician", instantly and bizarrely entrusting him with confidences of state. It all goes to Garrigan's head, and he sees too late how compromised he has become by Amin's savagery. Kerry Washington plays Amin's beautiful third wife, and Simon McBurney is the feline, cynical British diplomat who offers Garrigan a Faustian bargain in exchange for a safe passage out of Africa.

This is McAvoy's best performance yet: his Garrigan is well-intentioned, but also weak and arrogant, dangerously excited by Amin's demagoguery, and with an answering streak of violent craziness of his own: he impresses the president on their first meeting by shooting dead an injured cow. It is Whitaker, though, who pinches the movie, playing Amin as the Caligula of 20th-century Africa. Enraged by press criticism for expelling the Ugandan Asians, Amin blames Garrigan for not advising him against it. When Garrigan protests that he did exactly that, Amin declares smilingly: "Ah, but you did not persuade me, Nicholas!"

It is another feather in the crowded cap of co-writer Peter Morgan, who has a great gift for this kind of face-off drama taken from real life. (What next? A movie about Robert Maxwell and Peter Jay, perhaps?) It's a thoroughly engrossing drama, with never a dull moment.

· The Last King Of Scotland screens tonight at 7pm, Odeon Leicester Square, and Saturday at 1pm, Odeon West End. Booking: 0207 928 3232

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