THAI HORSE By William Diehl Villard Books. 451 pp. $18.95

William Diehl, who wrote "Chameleon" and "Sharky's Machine," has turned all the tools of the adventure-writing trade loose in his newest novel, "Thai Horse," and for a couple of hundred pages the pace matches that of "Raiders of the Lost Ark," the movie that taught Hollywood that two hours' worth of film can have more than 120 minutes' worth of adventure crammed into it.

Hot stuff, this "Thai Horse." It's got a hero who's a mixture of Ollie North, James Bond, Lee Marvin, Clint Eastwood and Charles Bronson, with a plot that takes us from Central America to Southeast Asia, Hong Kong, New York and, briefly, to an island off Georgia, and combines elements of "The Maltese Falcon," "The Dirty Dozen," Sax Rohmer's Fu Manchu books and "The Wizard of Oz."

Getting there is half the fun, Cunard used to tell us. And in "Thai Horse," getting there, that is, reading the first two-thirds of the book, is a lot of fun. But in the end, after I was barely able to put this number down on several nights, "Thai Horse" falls apart, with a denouement that shrieks "so what!" even after a deus ex machina appears to tie up the loose threads.

The plot is really nice, well conceived, with solid connections and synapses.

Murph Cody, a Navy flier and Annapolis graduate, is downed in Vietnam in 1972. He is listed as missing in action and presumed dead, and if he were not the son of "Buffalo Bill" Cody, the general in charge of Vietnam operations for the United States in that war, that would be the end of it.

The story moves quickly to today, and to a prison in Central America, where Special Forces veteran Christian Hatcher, our hero, is winding up three years of hard, silent time in Los Boxes, a prison in the jungles of Madrango, a country that could be Nicaragua, or Panama, or El Salvador, or ... well, you get the picture.

The reason Hatcher is doing time is Harry Sloan, his personal Captain Bligh, mentor, alter ego, boss, inspiration and devil, and the civilian leader of the Shadow Brigade, a covert operations group committing international acts of espionage, terrorism, assassination and general troublemaking.

Sloan, who got Hatcher into Los Boxes in the first place, gets him out for a private mission: Find Murph Cody, so that "Buffalo Bill" Cody, the Army general daddy who, we learn, is dying of cancer, can see his boy one more time.

Murph Cody, we are led to believe, is Thai Horse, or is connected to whatever Thai Horse is. It could be heroin, from the opium fields of Bangkok. It could be somebody who deals in dope, or dope racketeering. It could be connected with a statue, which is where "The Maltese Falcon" is evoked.

In one scene, China Cohen, a Westerner turned river trader and pirate in China, leads Hatcher to a small jade statue of a horse by Cohen's bed in his palatial Hong Kong home. "It's a real treasure. Authentic Thai Horse, about 3rd century B.C. Been kicking around for a long time."

As explained by Diehl, the Thai Horse is a mythical ghost horse that carried Thai heroes of antiquity to Heaven after the great wars. And it becomes the grail of this quest book when Hatcher, seeking clues about Cody's whereabouts, discovers that some unknown person has left a note at the Vietnam Veterans Memorial wall, beneath Cody's name, saying, in part, "Thank God for Thai Horse."

Well, it turns out that Thai Horse is something other than a jade statue.

It's the mission that counts here. The reader gets glimpses of Hatcher's talents as a human fly, rock climber and cat burglar, and gets to see him arm himself the way Ian Fleming let us observe James Bond's preparations. Perhaps one eventually wearies of rows of bullets stitching the chests/foreheads of victims.

And we get a collection of characters that might have been assembled by Lee Marvin for one unlikely mission or another: China Cohen, with a Harvard MBA and a shy streak that was erased only by the dangerous life of a brown-water trader; Daphne Chien, who wears men's suits when she trades with pirates and takes them off to make love to Christian Hatcher; Wonderboy, first a rock star, then a Marine with most of the skin burned off his face, which he now paints half black, half white, divided from hairline to chin vertically; and a cast of other assorted characters who inhabit Bangkok's "Tombstone" section of Wild West bars -- a milieu that would make the Intergalactic Cantina in "Star Wars" seem sort of normal.

It is a paradox that Diehl brings these people to life so that the most minor character is developed into a multidimensional figure, yet there is almost no depth to the two main figures, Hatcher and his soul-companion Sloan, and none whatsoever to the women, who appear and disappear in clouds of passionless sex.

One of them, I am really disappointed to have to report, is tortured to death by Tollie Fong, a Chinese who is so inscrutable that Diehl at one point calls him ... inscrutable. Here I must add that Diehl is an expert in keeping his characters sorted out, occidental or Oriental, so that by inflection, characterization or name we have been prepared for the reemergence of each. And Tollie Fong, villainous one, reappears to the point that I was reminded of how I never wanted to leave the theater with the Frankenstein monster not accounted for.

Perhaps the most exciting and interesting part of this novel, which comes halfway toward being of grand dimensions, is that Diehl has used his obvious experience in the Orient to present the Third World as something more than a place for aging remittance men to smoke out their lives with little black balls of opium: It is the last frontier of planet Earth, perhaps, where men are men and women are there for men and the only law is the law of the six-gun.

The reviewer is an assistant news editor of The Washington Post.