But the narrator is also a mole, a Communist undercover agent assigned to keep tabs on the general and Special Branch’s activities. His closest friend is Bon, an assassin with the C.I.A.’s Phoenix program, “a genuine patriot” who volunteered to fight after Communists murdered his father for the crime of being a village chief. The narrator’s North Vietnamese handler, Man, is also an old chum. Indeed, the narrator, Bon and Man were high school classmates, who in their youth melodramatically swore allegiance to one another by becoming blood brothers. This complex relationship, with the narrator in the tenuous middle, riven by conflicting loyalties, is a recipe for tragic betrayals, and those come, one after the other.

Image Viet Thanh Nguyen Credit... BeBe Jacobs

Working through a C.I.A. spook named Claude, the narrator dispenses liberal bribes to engineer an air evacuation to the United States for the general, the general’s wife and their huge extended family. Bon is also to be lifted out with his wife and child. The narrator wants to stay and take his place in a reunified Vietnam, but Man, convinced that the general and his cohort will plot a counterrevolution from abroad, gives him a new mission that is an extension of his old one: “Your general isn’t the only one planning to keep on fighting,” he explains. “The war’s been going on too long for them to simply stop. We need someone to keep an eye on them.”

Nguyen presents a gripping picture of the fall of Saigon, its confusion, chaos and terror, as the narrator flees with the others under a storm of shellfire from his Viet Cong and North Vietnamese comrades. Bon’s wife and child are killed before their plane takes off, giving him two more deaths to avenge.

This rich narrative stew is assembled in the novel’s first 50 pages, then set on a low simmer. From that brief, intense beginning we proceed to a picaresque account of the narrator’s experiences as a ­refugee-cum-spy in Los Angeles. He lands a clerical job with his former professor, has an affair with an older ­Japanese-American woman and sends messages to Man (written in invisible ink) via an intermediary in Paris. Here the novel becomes both thriller and social satire. If you like your humor written in charcoal, this is the funniest part of the book, though it’s occasionally spoiled by zingers that belong on “The Daily Show” more than they do in a serious novel.

The narrator’s espionage activities lead him to make a foray into the movie business. He is hired by a director, “the auteur” (who bears a resemblance to Francis Ford Coppola), to round up Vietnamese in a Philippine refugee camp to work as extras in his film (which bears a resemblance to “Apocalypse Now”). Nguyen adroitly handles the shifting tones of these episodes, now hilarious, now sad, as the narrator tries to do what Nguyen has done: de-Americanize the portrayal of the war. But, unlike Nguyen, he fails.

Thereafter, the book’s mood darkens. The narrator falls into a web of deceit and treachery spun by his dual role and the schisms in his soul. Man’s suspicions prove accurate: The general and some other die-hards, guilt-ridden for not fighting to the death, bored with their mediocre lives in the States (the general has become owner of a liquor store), plot a counter­revolutionary invasion with the help of a right-wing congressman.

The narrator assists in the planning, while sending reports to Man. However, to avoid having his cover blown, he is compelled to take part in two assassinations. One victim is an ex-Special Branch officer, “the crapulent major,” the other is a Vietnamese journalist at a California newspaper. The descriptions of the murders are tense, psychologically complex, riveting. The narrator’s conscience becomes as torn as the rest of him. “Remorse over the crapulent major’s death was ringing me up a few times a day, tenacious as a debt collector,” he thinks.