In 1970, Nicholas Garrigan (James McAvoy) celebrates his graduation from medical school in Scotland. Faced with unexciting prospects in his homeland, he embarks on a journey of adventure by accepting a position at a Ugandan missionary clinic managed by Dr. David Merrit (Adam Kotz) and his wife, Sarah. Garrigan quickly finds himself attracted to Sarah, who, despite relishing the attention, resists his advances, asserting her commitment to her marriage. During this time, the political landscape shifts as General Idi Amin (Forest Whitaker) stages a coup and topples the then president, Milton Obote.
Garrigan's first encounter with Amin occurs at a public rally in the village where he serves. He is initially taken in by Amin’s enthusiastic vision for Uganda—a sentiment not shared by Sarah, who cautions him about the perils posed by dictators. When Garrigan is summoned to treat Amin after a minor car accident, his quick thinking in euthanizing a mortally injured cow earns him Amin's admiration. Delighted to learn that Garrigan is Scottish, Amin, who views Scotland as a symbol of strength, offers Garrigan the position of his personal physician, tasking him with overhauling Uganda’s healthcare system.
As Garrigan becomes more entrenched in Amin’s inner circle, he finds himself grappling not only with medical duties but also with state matters. He acknowledges the violence surrounding him but is swayed by Amin’s claims that authoritarian measures are necessary for peace. Garrigan learns about Amin's troubled polygamous household, particularly the ostracization of the youngest wife, Kay (Kerry Washington), due to the birth of their epileptic son, Mackenzie (Apollo Okwenje Omamo). Their professional interaction leads to an affair, with Kay urging Garrigan to leave Uganda for his safety.
As Garrigan acquires power, he is contacted by the British Foreign Office, who hint at Amin's initial support by the British government. Tensions rise when Garrigan saves Amin from an assassination attempt, solidifying his status as Amin's most trusted confidant. However, when he witnesses Amin's increasing paranoia and violent purges, his allegiance begins to falter. After his British passport is exchanged for a Ugandan one to prevent his escape, Garrigan turns to Stone (Simon McBurney), the local British office representative, for assistance.
Garrigan is informed that the British would only help him if he agrees to assassinate Amin, an offer he firmly declines. The situation escalates when Kay reveals she is pregnant with Garrigan's child, fearing for her life should Amin discover her infidelity. Time slips away from Garrigan, and unable to meet Kay for a crucial appointment, she resorts to a dangerous abortion which leads to her capture and brutal death at the hands of Amin's regime. Upon discovering her mutilated body, Garrigan is forced to confront the disturbing realities of Amin's tyranny, culminating in a resolve to take matters to a deadly conclusion.
A dramatic sequence unfolds when pro-Palestinian hijackers seize an Air France plane, forcing Amin to intervene, with Garrigan in tow. There, his plot to poison Amin is unveiled by a bodyguard, leading to severe punishment as Garrigan is tortured. However, with the help of Dr. Junju (David Oyelowo), he escapes amidst the chaos of the passengers' release. Junju encourages Garrigan to speak out against Amin's brutal regime, banking on his white identity to gain attention. Dressed in Junju's jacket, Garrigan successfully blends in and boards the plane just as chaos ensues with Amin realizing his betrayal too late.
The film closes on a poignant note, featuring actual footage of Amin and the repercussions of his rule, revealing the grim truth of the 300,000 lives lost during his reign. Lastly, it brings to light Amin's eventual downfall in 1979 and his death in exile in 2003.
