Avenging Angel in an Airless World : THE LAST THING HE WANTED.<i> By Joan Didion (Alfred A. Knopf: $23, 227 pp.)</i>

Age 12 is young to bear a child, but if Joan Didion’s new novel is not the offspring of her 1984 “Democracy,” it is a niece or nephew. In both books an American woman feels an alienating oppression within the ruling class to which she belongs. Didion makes her unease the symptom of a corrupting American hegemony in the Third World. The woman immolates herself, one way or the other, as an act of protest.

“Democracy” was set in the era of the Vietnam War. “The Last Thing He Wanted” has to do with another domestically divisive venture: covert American aid to the Nicaraguan contras in the 1980s. Vietnam led to a national political upheaval at home. The Nicaraguan affair, with its contaminating sideshows in El Salvador, Honduras and Costa Rica, led to a smaller domestic storm: the Iran-Contra scandal. Each led to a Didion novel: “Democracy,” perhaps her masterpiece, and “The Last Thing,” smaller, more contrived, not as fresh, yet still remarkable.

Twelve years was almost long enough to let us forget the exhilarating velocity of Didion’s writing. There have been other books in between--notably “Miami,” a searching portrait of the Cuban American community in Florida--but this is the first novel. If Didion’s reportorial writing cuts lethally fine, her fiction adds the fume of poppies.

She is one of our true stylists. Her sentences have the wicked precision of a Wodehouse or a Waugh though she uses them for a different purpose: a cold keening for the times we live in. If she were at breakfast in the state of Denmark, her way of announcing that something was rotten would be to make it into a perfect omelet. We would eat it entranced, then fall ill.

I cannot really tell this story, her narrator says, in effect, as she tells the story of Elena McMahon, a waif fatally caught in a 1984 right-wing plot to escalate covert American moves against the Sandinistas into full-scale war. The nameless narrator, a writer much influenced by Didion, recounts evil in attenuated tones that exclude all recourse, even the recourse of anger.

She speaks in a removed conditional mode, not out of detachment but because she cannot break out of the confinement of her class and her generational disorder: an excessive ability to discriminate and a shrunken ability to choose. Thrillers--this is a moral thriller on the order of one of Graham Greene’s entertainments--traditionally located their moral center in the whiskey voice of a disenchanted but honest tough guy. Didion puts hers in the cocaine voice of a shaky sophisticate.

The narrator diligently tracks down the truth about her friend’s death but--a child of postmodern times--she has no faith in truth. Journalism was called history’s rough draft, she remarks, “when we still believed that history merited a second look.”

She is an avenging angel whose temperature is too low to metabolize vengeance and whose wings provide no lift in an airless world. Her flat voice is the heart of the book: Nothing much that happens, either in the public or private world of comfortable America, holds any comfort.

The voice has its drawbacks. By the end, Didion exerts a stylistic and thematic control that is close to suffocating, though elegant. On the other hand, if you speak low while speaking thrillingly, the thrill is doubled. Her tone irksomely low and undeniably thrilling, the narrator struggles through a maze of secrets, lies, concealments and partial accounts told in complex shifts of time and focus.

The confusions do not put us off--this is one of the book’s masteries--they pull us in. Didion has created a menacing world where the reader is held hostage by shadows and is led, in a kind of literary Stockholm syndrome, to depend helplessly on the author.

Elena, the protagonist, is hardly a heroine. She is flighty and self-centered yet almost without a self. A journalist, she meets and marries Wynn Janklow, scion of a huge oil fortune and a player in Los Angeles’ business and entertainment world. After 12 years, she flees the emptiness of her cosseted life, landing a job, through political connections, with the Washington Post. She regrets her lost luxuries and, after an abrasive day on a presidential campaign, she quits once more and goes to visit her ailing father in Florida.

Dick McMahon is an old gun-runner, a selfish but genial scoundrel who has dipped into and from every Caribbean intrigue over the previous 30 years. Now he raves about one final arms flight that would make him a million except that he is too ill to go. After her husband’s odorless and abstract $100-million deals, Elena finds her father’s smaller larcenies have the stink of humanity. For the first time in a life of evasion, she makes a commitment--disastrous, of course.

Soon, dressed in a black Bergdorf frock, she is standing on a steamy new-built runway in the Costa Rican hinterland. The arms she had brought are trucked away without payment. The plane she came on leaves, despite her entreaties, and she is stranded.

The layered creeping evil that takes over, her suddenly resolute and utterly vain resistance, and the horror that follows are brilliantly assembled. It is a double plot. First there are the clandestine preparations by U.S. government agencies to set up a secret base on a Caribbean island for use against Nicaragua, if and when such an action is decided on.

Piggybacking on this plot is another by American and Central American extremists, along with rogue U.S. agents, to precipitate the decision. Dick McMahon was to be lured, through his arms delivery, to be the scapegoat in the assassination of an American official. Dick would die in the gunfire; it would be announced that he was working for the Sandinistas. Now that Elena has come in his place, she will take his place.

A vividly drawn cast of characters gathers around her. The spectacularly scary villains include the man her father entrusted to organize the arms flight, her father’s partner, a murderous Salvadoran colonel posing as a businessman and the sinister aide to an American senator based on Jesse Helms. More ambiguously, there is Treat Morrison, a top U.S. official--foreign ministers take his calls--who has organized “special situations” around the world for 30 years.

Treat is pragmatic, forceful and ruthless; on the other hand, there is an inkling of humanity in him. He manages plot No. 1; he tries desperately to stop plot No. 2. For one thing, he and Elena, who finds in him some of her father’s buccaneer spirit, become lovers. It is hasty and sentimental: The author works her characters too hard for them to take time off to love convincingly.

In any case, Didion in no way lets him off. The point of her book is that there is no clear line between “respectable” reasons of state and what the vicious murderers do. Treat walks honorably and trails slime; the hapless Elena is lured by the honor and destroyed by the slime.