Nicholas Garrigan (James McAvoy), a young Scotsman, graduates from med school in 1970 and at his celebratory dinner his father congratulates him, despite the fact that Nicholas's degree isn't as fine as his own, and then welcomes him into the family practice. Nicholas's brashness always strains against his bourgeois mannerliness, and so, although his father makes him want to scream, he can let loose only once he's alone in his room (where he still hides his cigarette smoking). Nicholas suddenly sits up and spins a globe, vowing to venture wherever his finger lands. Canada is his unexotic first hit, so he spins again and comes up with Uganda, the East African nation that gained its independence from the United Kingdom (which had created the modern political entity in 1914) in 1962.

Nicholas goes to Uganda to work with Dr. Merrit and his wife Sarah (Adam Kotz and Gillian Anderson) tending to backwards and impoverished villagers, 80% of whom, however, prefer the ministrations of the witch doctor. A chance roadside encounter with Idi Amin Dada (Forest Whitaker), recently come to power as "president" in the coup d'état of 25 January 1971, ends with Nicholas installed as Amin's personal physician and, depending on the tyrant's mood, his closest advisor.

At first it seems that Nicholas's abandonment of the Merrits' clinic will save him from a "dangerous" flirtation with Sarah, who can't live up to her husband's example any more than Nicholas can live up to his father's. In addition, and to Sarah's disbelief, Nicholas becomes a cheerleader for Amin and can't remain civil when speaking to pragmatic British foreign office field veterans. He laughs at one man's diplomatic phrasing with open contempt, but also with adolescent certitude that an opinion that he finds offensive must also be groundless. (He also has a Scottish resentment against Englishmen who assume he's one of them.) Nicholas later answers what he takes to be the man's smug skepticism towards the "president" with counterculture self-righteousness, hailing Amin as a black leader for a post-colonial Africa and even justifying Amin's violence against his political opposition. Thus, what starts as a vaguely vocational adventure takes on the air of a mission, while providing Nicholas access to unregulated power and the women and luxury goods that attend it.

Then Nicholas makes a string of missteps which bring home to him Amin's bloody ruthlessness. In one case, Nicholas informs Amin about a meeting between a European and one of Nicholas's fellow "ministers," who subsequently disappears. In another, Nicholas has a truly dangerous affair with one of the dictator's cast-off wives, whom he gets pregnant. After she is butchered in retribution (based on the fate of Amin's second wife Kay Adroa, though her affair was apparently with a Ugandan doctor), Nicholas clumsily attempts to poison Amin. It requires the sacrifice of a local doctor to get Nicholas out of the country alive — during the 1976 Entebbe Incident, no less.