WHEN FIRST we meet Army Staff Sergeant Joe Pena, jazz pianist, Pueblo Indian, and once the eighth- ranked heavyweight boxer in the world, the year is 1943 and he is hallucinating in a 4-by-8 foot Leavenworth jail cell. His hallucinatory companions are a minotaur, a mountain lion and a beautiful girl with the body of a swallow.

A certain Captain Augustino of U.S. Army Intelligence has also come to call -- in the flesh, fortunately -- and hopes to recruit Sergeant Pena for a special mission. But before there is even time for the experienced reader to fret over the prospect of the mission turning out to be one of those impossible derring-do affairs (perhaps the assassination of either Hitler or Hirohito), Martin Cruz Smith draws us unresisting into his splendid tae of intrigue and espionage at Los Alamos, New Mexico, that features, among others, J. Robert Oppenheimer, Klaus Fuchs, Harry Gold, Brigadier General Leslie Groves and, of course, the first atomic bomb.

The sinister and politically paranoid Captain Augustino, we learn, has the power to spring Sergeant Pena from the army prison at Leavenworth, where he is doing a stretch for having bedded an officer's wife. Augustino also has the power to clap him right back into jail unless the sergeant does the captain's bidding, which is to spy on Oppenheimer while serving the brilliant physicist as driver, bodyguard and confidant.

Pena is particularly suited for the combined bodyguard-informer role, in Augustino's mind anyway, not only because of his boxing skills, but also because he and Oppenheimer first met years ago when the parents of the young genius dispatched their 16-year-old son to the New Mexico mountains to regain his health.

Oppenheimer's guide and mentor then was 10-year-old Joe Pena who taught the eastern tenderfoot -- among other things -- how to shelter under the belly of a horse when it rained. Smith also reminds us that it was largely Oppenheimer's enchantment with New Mexico that led to the first atomic bomb being assembled and exploded there on the site that the brooding Oppenheimer himself named Trinity. Before that the natives called it Stallion Gate.

Once out of jail, Pena renews his acquaintance with Oppenheimer, plays the piano at staff parties, and promptly seduces Captain Augustino's compliant wife. He also gives carefully non-committal answers to Augustino's deadly serious but rather nutty questions such as: "Sergeant, what would you say if I told you that Oppenheimer is an agent of the Soviet Union, intent on developing an atomic weapon here only so that he can deliver the finished plans to his Soviet friends?"

Smith skillfully mixes his real characters in with his fictional ones. Sergeant Pena especially is a major accomplishment since it's no mean feat to turn a soldier, boxer and be-bop pioneer into a sympathetic and even sensitive protagonist.

But if Smith succeeds admirably with Pena, he does equally well with Dr. Anna Weiss, the strange young beautiful German-Jew mathematician who was spirited out of Nazi Germany by a soft-hearted communist and has now landed high on Captain Augustino's list of prime suspects. The love affair between Anna Weiss and Sergeant Pena is as inevitable as is its end.

Stallion Gate is crammed with facts about the customs of the various Indian tribes that dwell in New Mexico, and of another tribe, this one composed of scientists, which lives nearby on its own reservation, tinkering away at a gadget that could end the war and possibly the world. It is to his credit that Smith seems comfortably at home with all tribes.

Smith has employed a dry, controlled, almost laconic style to tell his tale of treachery, obsession and betrayal. For although this is a novel ostensibly about espionage, it equally concerns the clash of cultures. In one scene, General Groves, the man who built the Pentagon and ran the Manhattan Project, is confronted by a pair of armed Indians and nervously sends Sergeant Joe Pena over to placate them.

"Joe passed out cigarettes. Apaches were Chinese to Joe. Navajos were thieves. Likewise, Apaches and Navajos thought all Pueblos were women. The Navajo moved close enough to take a smoke and stepped back. Flakes drifted down. The storm was resting, not leaving. . . ."

Throughout the novel there is this constant clash of culture and race, of ideals and ideologies. But looming over it all is the bomb that sometimes seems almost enshrouded by the doubts of those who are creating it -- doubts that it should be dropped on Japan; doubts as to whether it even should have been invented; and, most of all, doubts about whether the thing will actually go off.

It all ends, or maybe just begins, on July 16, 1945 ith a chilling confrontation during the bomb's countdown, thus providing an eminently satisfying conclusion to an extraordinarily good novel.