A Hindu, a Muslim and a Christian are trapped on a lifeboat for 227 days with a 450-pound Bengal tiger. It sounds suspiciously like the setup of a joke, something you might hear at a tavern from the guy who's been downing gimlets all night. But ''Life of Pi,'' the Canadian writer Yann Martel's extraordinary novel based on this very premise, is hardly your average barroom gag. Granted, it may not qualify as ''a story that will make you believe in God,'' as one character describes it. But it could renew your faith in the ability of novelists to invest even the most outrageous scenario with plausible life -- although sticklers for literal realism, poor souls, will find much to carp at.
For one thing, the Hindu, the Muslim and the Christian are all the same person -- Pi Patel, an amiable Indian teenager who sees no reason why he can't practice three religions at once. He's also something of an expert on animal behavior. As the son of a zoo owner in the South Indian city of Pondicherry, he grew up on familiar terms with howler monkeys, one-wattled cassowaries and American bison. As a result, he's attuned to the intricacies of interspecies cohabitation. ''A good zoo is a place of carefully worked-out coincidence,'' he explains. ''Exactly where an animal says to us, 'Stay out!' with its urine or other secretion, we say to it, 'Stay in!' with our barriers. Under such conditions of diplomatic peace, all animals are content and we can relax and have a look at each other.''
This zoological savvy proves indispensable to Pi when he and his family decide to escape the political instability of 1970's India and move -- lock, stock and menagerie -- to Canada. Like latter-day Noahs, they load their animals onto a Japanese cargo ship named the Tsimtsum and set sail for the New World. But ''midway to Midway,'' something inexplicable happens. For reasons that will forever elude the maritime authorities, the Tsimtsum sinks -- suddenly and violently -- just before dawn on its fourth day out of Manila. Only five survivors are able to reach the single lifeboat that doesn't go down with the ship: Pi himself, an injured zebra, a prize Borneo orangutan, one very nervous hyena and a tiger who (thanks to a clerical error that confused the names of the animal and its captor) is called Richard Parker.
They make for a rather volatile crew. The politics of the animal kingdom being what they are, the zebra, the orangutan and the hyena are quickly dispatched, leaving boy and tiger alone on the 26-foot craft. But thanks to a territory-defining tarpaulin and the general bewilderment of two traumatized and seasick creatures, the obvious does not immediately occur. Pi remains uneaten long enough to reach an important insight about his boatmate: ''I had to tame him,'' he realizes. ''It was not a question of him or me, but of him and me. We were, literally and figuratively, in the same boat.'' As paradoxical as it may seem, Pi understands that his own survival depends on keeping his ferocious opponent alive and well -- ''because if he died I would be left alone with despair, a foe even more formidable than a tiger. If I still had the will to live, it was thanks to Richard Parker.''
Although ''Life of Pi'' works remarkably well on the pure adrenaline-and-testosterone level of a high-seas adventure tale, it's apparent that Martel is not interested in simply retelling the classic lifeboat-survival story (with a Bengal tiger playing the prickly Tallulah Bankhead role). Pi, after all, is a practitioner of three major religions who also happens to have a strong background in science; with such a broad resume, his story inevitably takes on the quality of a parable. In fact, although the book reverberates with echoes from sources as disparate as ''Robinson Crusoe'' and Aesop's fables, the work it most strongly recalls is Ernest Hemingway's own foray into existentialist parable, ''The Old Man and the Sea.'' But while Hemingway depicted the defining struggle of his archetypal man as one of sheer endurance and determination, Pi's battle is more subtle. The boy must finesse his demon, not overcome it, and do so by means of a kind of psychological jujitsu. He comes to realize that survival involves knowing when to assert himself and when to hold back, when to take the upper hand and when to yield to a power greater than himself. He discovers, in other words, that living with a tiger ultimately requires acts of both will and faith.
There are times when Martel pushes the didactic agenda of his story too hard. One episode involving a bizarre ''Gandhian'' island of passively carnivorous seaweed -- populated by an enormous herd of South African meerkats -- struck me as a little too baldly allegorical, however magical its imagery. But Martel is usually able to keep his feet on the ground by focusing on the physical and logistical details of his hero's predicament.
He writes with a playful and discursive casualness, but that doesn't prevent him from delivering some arresting descriptions. In one of the more cinematic moments in the novel, Pi catches a bioluminescent dorado and must pummel it to death with the dull side of a hatchet: ''The dorado did a most extraordinary thing as it died: it began to flash all kinds of colors in rapid succession. Blue, green, red, gold and violet flickered and shimmered neonlike on its surface as it struggled. I felt I was beating a rainbow to death.''
Moreover, in the book's final chapters, just when many novels are winding down to their foregone conclusions, Martel gives ''Life of Pi'' an intriguing twist. After the lifeboat comes safely to shore in Mexico (and Richard Parker disappears without ceremony into the jungle), Pi finds that his wild narrative is not believed by the officials sent to debrief him. And he knows exactly why: ''You want a story that won't surprise you. That will confirm what you already know. That won't make you see higher or further or differently.''
Urged to provide a more credible explanation for his survival, Pi placates the officials with a story that contains just the kind of ''dry, yeastless factuality'' they're looking for. But is this more straightforward (and tigerless) version of events actually closer to the deeper truth of his adventure? It's a testimony to Martel's achievement that few readers will be tempted to think so.
Gary Krist's new novel, ''Extravagance,'' will be published later this year.