Surf’s Up : THE BEACH.<i> By Alex Garland</i> .<i> Riverhead: 384 pp., $23.95</i>

“The Beach” begins as a Robert Louis Stevenson adventure story. A dying stranger gives the narrator a map; he, with two companions, follows its directions to a tropical island on an embattled search for its promised treasure.

Before Alex Garland’s novel is over, “Treasure Island” has become “Lord of the Flies” or “John Dollar.” Adventure turns into a gruesome parable of original sin. Where William Golding’s and Marianne Wiggins’ masterpieces used marooned children to portray evil growing out of innocence, Garland’s book--arresting though no masterpiece--uses a band of second-generation flower children.

The author infuses baleful myth into a contemporary phenomenon. “The Beach” is set among a ‘90s generation of backpacking young wanderers--Western eco-travelers tripping through Southeast Asia in search of whiter beaches, lusher settings, ample supplies of dope and an escape from the commercial tourist culture.

What they are after is something between utopia and nirvana; what they get is a paradox. Renouncing the world of work and money, they phone home for funds. Pursuing the phantom of some farther, unspoiled bit of archipelago, they arrive and spoil it. They drift, pair off and split up, exchanging notes and rumors about the Malabar Coast, remote corners of Thailand and Malaysia, some lesser-known scrap of Indonesia, and what about Vietnam?

Richard, the narrator, recalls playing with his Atari and hearing his baby sitter--these are rich kids--compare the different Thailand islands and rave about Koh Samui. Later, when he graduated, “my friends and I scattered ourselves around the globe. The next August we started coming back, and I learned that my baby sitter’s paradise was yesterday’s news. Koh Phangan, the next island along, was Thailand’s new Mecca.

“A few years later, as I checked my passport and confirmed my flight to Bangkok, a friend telephoned with advice. ‘Give Koh Phangan a miss, Rich,’ she said. ‘Hat Rin’s [another rumored paradise] a long way past its sell-by date. They do printed flyers for the full-moon parties. Koh Tao. That’s where it’s at.’ ”

“The Beach” begins with Richard--English, like the author--on the backpacker-crowded Ko Sanh Road in Bangkok, looking for a place to stay and the latest word on where to go. In his guest house he hears the man in the next cubicle ranting about “the beach.” Next morning the man is discovered dead, having slit his wrists. On Richard’s door he has left an envelope with a map in it.

Before long Richard is teamed up with a young French couple, Etienne and Francoise, who have heard rumors of an island community blissfully inhabited by a select few. Using Daffy’s map--the stranger had registered at the guest house as Daffy Duck--they bribe a boatman to take them not to the island but to one adjacent to it, so as to preserve their “secret.” Both islands are part of a marine preserve, partly or wholly forbidden to visitors.

After the boatman leaves, they swim across to their destination, hike across the hills, plunge down a waterfall and reach the Beach, the name for both the paradisal place and the 30 or so young people who have established a hidden community there. It is Shangri-La, the “treasure” of Richard’s quest.

Provisionally accepted by the community and its leader, Sal--she, her lover, Bugs, and Daffy were the three pioneers--Richard and his two companions settle into a utopian but highly organized existence. There are work parties for spear-fishing, gardening, cooking and carpentry. There are languorous afternoons and evenings swimming and smoking pot. Jed, the community’s scout and lookout, steals the pot from a clandestine marijuana plantation worked and patrolled by Thai armed guards in the hills above the beach. Guards and settlers observe a modus vivendi of mutual avoidance; the pot-pilfering is tolerated.

Up to this point, Garland has written a taut, finely detailed account of his wanderers. He knows the scene, infusing it with a hint of crisp skepticism. There is suspense, both in the adventure itself and in the undercurrents of attraction between Richard and Francoise and of Etienne’s jealousy. There is mystery and a hint of the ominous in Daffy and his suicide. There is a skillfully managed note of unsoundness, both in the trio’s search for eco-utopia and in the seeming order and harmony they encounter. Update Chekhov’s decree that the introduction of a gun early in the play means it must go off before the end: In modern fiction, the apple guarantees the worm.

Where the book deteriorates, and where Garland’s grip becomes forced and spasmodic, is in his effort to grow the worm into a dragon and transform unease into horror. Daffy keeps reappearing to Richard as a ghost, unstoppably loquacious and with a joviality that turns increasingly sinister. His suicide and his deliberate betrayal of the Beach’s seclusion by giving Richard the map are a foreboding message; it is a fairly clumsy kind of signaling, even while we are eager to know what it portends.

A second rough device is the growing confusion in Richard’s mind between his utopian island and Vietnam. As he joins Jed in the pot-stealing patrols, he begins to imagine himself in one of the films he has seen about the Vietnam War. A broken branch is a mine; the armed guards are the Viet Cong. Hiding from them at first, he begins to long more and more for a violent encounter.

Violence grows in him, in fact; he conceives a virulent hatred of Bugs, Sal’s lover and the boss of the carpenter work party. Violence spreads in other forms. A rotten fish contaminates the communal stew; Garland paints a Dantesque scene of the entire community racked with cramps, diarrhea and vomiting. Two of three Swedes in the fishing party are attacked by sharks; one is dismembered, the other receives a fatal internal injury. The third goes berserk.

There is an “Animal Farm”-like degradation; Richard finds himself being assigned by Sal to a murderous control mission to maintain order. When five young people are spotted approaching the island by raft, he follows instructions to let them blunder into the Thai guards; they are savagely massacred. The violence escalates into a climax of grotesque warfare.

Garland’s message is complex and acute. The self-indulgence of a generation of young Westerners--seeking isolated and well-funded paradises and ignoring the miseries and needs around them--can itself breed monsters. There is more than one kind of Vietnam: Invading the Third World to indulge a spoiled fantasy is not altogether unconnected to earlier incursions.

The trouble is that the author lacks the ability of his exemplars--a Golding, a Wiggins--to allow horror to emerge gradually and inescapably from the essential natures of his characters. He applies it, he injects it. The reader’s suspense, in this intelligently conceived and often effective novel, may consist more in wondering what the author will do than in what his characters will do. They illustrate his dark vision--they do not quite become it.