The story is narrated by Thomas Fowler, a British journalist covering the war, and centers on a love triangle between Fowler, a young American named Alden Pyle, and Phuong, Fowler's Vietnamese mistress.
Pyle, an idealistic American aid worker, befriends Fowler and becomes infatuated with Phuong. He wins her away from Fowler with promises of marriage and security in America. Pyle is secretly a CIA operative working to shape the war in America's interests by supporting a corrupt Vietnamese general, Thé, and his militia. Pyle's actions lead to a series of terrorist bombings in Saigon that kill innocent civilians.
When Fowler discovers Pyle's involvement in the bombings, he arranges for Pyle to be assassinated. After Pyle's murder, Phuong returns to Fowler. The novel explores the decline of French colonialism in Vietnam and early American involvement in the Vietnam War through the lens of this love triangle. It questions the ideological foundations of growing American interventionism in the 1950s and foreshadows the disastrous outcome of the Vietnam War.