The Last King Of Scotland,Young newly-qualified Scottish doctor Nicholas Garrigan (James McAvoy) heads to Uganda seeking adventure and escape from the bourgeois life of his father. He works in a small clinic on the countryside with Dr. Merrit (Adam Kontz) and his wife, Sarah (Gillian Anderson). At the time of Garrigan's arrival, General Idi Amin (Forest Whitaker) leads a coup against current president Milton Obote. He is enamored of Amin's rhetoric when Amin comes to the village where they are working, which held a rally for him. Garrigan and Amin meet on a chance encounter on a country road, where he treats Amin's hand. Amin is impressed when Garrigan takes his gun (without asking) and shoots a wounded cow. He likes Garrigan because he is Scottish, and offers Garrigan the job of becoming his personal physician, and, after some hesitation, Garrigan accepts.

Amin welcomes Garrigan into his personal confidences quickly, asking for advice on state matters and presenting Garrigan with a comfortable living and a new Mercedes. Now entrenched in a lifestyle far removed from the impoverished countryside he first came to Uganda to serve, Garrigan's image of Amin turns to idolization. He detests the local British officials for their imperialistic attitude and continues to help Amin on state projects and family matters while working in the main Kampala state funded hospital.

Slowly, however, Garrigan cannot reconcile this positivity with the increasingly brutal and repressive images of violence and oppression enveloping the country around him; at the same time, the audience comes to realize that he is no longer an innocent bystander, since his participation in Amin's household is itself support of Amin's repressive policies. At one crucial moment in the film, Garrigan goes to Amin to ask him to talk to his other closest Ugandan advisor because he suspects him of treason; Amin immediately has the advisor killed. When this crime is revealed to Garrigan by a British official, Garrigan first makes excuses for Amin's government, but, unable to sustain his self-delusion, then confronts Amin. The scene is revealing: Amin says to him, essentially, "You knew exactly what was going to happen when you asked me to 'talk' to the minister—don't act as if you are completely innocent of this."

As a result of this confrontation and this moment of self-awareness, Garrigan decides he must leave Uganda. At first, he asks Amin to let him leave, but when he returns home one day to find his house ransacked, his passport gone, and a new Ugandan passport in an envelope addressed to him, he realizes the dictator will not let him go willingly. He turns for help to the British officials that he has been contemptuous of, and they return the favour: why would they help a man who has allowed Amin to wreak such havoc on Uganda and the world? However, they will allow him to leave on one condition: he must assassinate Amin.

Garrigan, meanwhile, has an affair with one of Amin's wives, Kay (Kerry Washington), and impregnates her. While seeking an abortion in a nearby village, Kay is caught, executed, and mutilated by Amin's forces. Garrigan sees her mutilated cadaver knowing that he has caused her death: his actions (and his initial refusal to provide the abortion himself) left her no choice in her course of action. As a result of this trauma, Garrigan decides to assassinate Amin by poisoning him, under the ruse of giving him pills for a headache.

Soon after, a hijacked Air France aircraft lands at Entebbe International Airport seeking asylum for the Palestinian hijackers on board. Amin, Garrigan, and other state officials rush to the airport to deal with the situation. Here, Garrigan's plot to kill Amin is discovered and Amin reveals he is aware of the affair between Kay and Garrigan. While Amin leans close to Garrigan to tell him how he will die, Garrigan, with his characteristic boldness in addressing Amin, denounces him: "You're a child, that's what makes you so fucking scary."

Amin orders Garrigan beaten and tortured, and he is strung up by meat hooks, while Amin looks on. after Amin and his officials leave, Garrigan is rescued by a Ugandan colleague, Dr. Junju (David Oyelowo), who offers his help in exchange for Garrigan's promise to return to Europe and tell the world the truth about Idi Amin. Again, the ambivalence of Garrigan's moral position is invoked: Garrigan asks Junju, "Why are you doing this [for me]?" and Junju replies, "Frankly, I don't know—you deserve to die, but dead you can do nothing, alive you can redeem yourself." Junju goes on to explain that Garrigan is in a position to go back home and tell the rest of the world what Amin is really like, and what he has done. "They will believe you," he says. "You are a white man."

Garrigan sneaks aboard a plane with a group of released hostages, leaving behind a furious Amin. After his last act of compassion and bravery, Dr. Junju is discovered to have helped Garrigan to escape and is shot dead. Junju's last words were, "My wife," before being shot in the head.

The last scene is of Garrigan in the airplane. With archival footage, the closing coda notes that Amin was deposed and fled Uganda in 1979.