The novel opens in early 1945. Peter Marlowe, a young British RAF Flight Lieutenant, has been a P.O.W. since 1942. Marlowe comes to the attention of the "King", an American corporal who has become the most successful trader and black marketeer in Changi, when the King sees him conversing in Malay. Marlowe's languages, intelligence, honesty and winning personality cause the King to befriend him and attempt to involve him in black market deals, which bring Marlowe to the attention of Robin Grey, a British officer and Provost Marshal of the camp, who has developed a Javert-like obsession with the King and hopes to arrest him for violating camp regulations. Grey is attempting to maintain military discipline among the prisoners and sees the King as the antithesis of his beliefs. As the son of a working-class family, Grey follows the rules for their own sake using his position as Provost Marshal to gain a status otherwise unavailable to him in British society.
Despite being an enlisted man and without distinction in civilian life, the King has become a major power in the closed society of the P.O.W. camp through his charisma and intelligence. Trading with Korean guards, local Malay villagers and other prisoners for food, clothing, information and what few luxuries are available, the King keeps himself and his fellow American prisoners alive. Senior officers come to him for help in selling their valuables to buy food and other officers are secretly on his payroll. Marlowe is initially put off by the King's perspective and behavior, which are at odds with the British upper class ideals he has been taught. He turns down a lucrative business partnership with the King because "Marlowes aren't tradesmen. It just isn't done, old boy." Marlowe soon understands that the King is not the thief and con artist that Grey would have him believe. Rather, the King asks for the best of each man and rewards him accordingly, irrespective of class or position.
Through the experiences of Marlowe, the King and other characters, the novel offers a vivid, often disturbing portrayal of men brought to the edge of survival by a brutal environment. The P.O.Ws are given nothing by the Japanese other than filthy huts to live in and the bare minimum of food. Officers from various parts of Britain's Asian empire, accustomed to having native servants provide them with freshly laundered uniforms daily, are reduced to wearing rags and homemade shoes. For most, the chief concern is obtaining enough food to stay alive from day to day and avoiding disease or injury, since almost no medical care is available. Some are degraded and come close to losing their humanity, while others display courage and compassion beyond anything one would expect. Some literally steal food out the mouths of their comrades, while others give away what they have or take terrible risks to help their friends.
Rats are bred for food, and in the end are abandoned in their cages when the camp is liberated. The final scene shows the rats consuming each other one by one, with the final survivor being "king of the rats."
Characters in "King Rat"
- The King: an American corporal; at the end of the novel he is sent back to the United States under an impending investigation for illegal profiteering in a prison camp, and is never seen again by Marlowe
- Peter Marlowe: the protagonist, a young British fighter pilot who later becomes an author; based on James Clavell
- Robin Grey: an older British officer.
Two characters from King Rat also appear in Noble House (published 1981), a novel set in Hong Kong in the early 1960s, when Marlowe is a writer visiting Hong Kong to do research on the great British trading companies there. Grey, embittered by his failure to obtain a commission in the postwar British Army despite his suffering during the war, has become a radical socialist Member of Parliament and is also in Hong Kong on an official visit. Unknown to Marlowe, Grey has become a secret Communist and a Soviet agent who tries to thwart efforts to improve relations between China and the West.