American millionaire Harold Gern has fled to Puerto Rico to escape an indictment for his shady business practices. There, while watching a cockfight with his desperately bored wife Evelyn, Harold’s young lawyer, Martin Joyce, urges the inveterate playboy to discuss his case. Harold instead insists on gambling at the casino and flirting with women, frustrating Martin further. That night, Harold sends Martin to his hotel room to retrieve some paperwork. There, Evelyn is drinking and tries to seduce Martin, to no avail. She lures him onto the balcony, where she balances precariously on the railing and announces that her unhappy marriage has left her with a death wish. She later finds Harold in the bar and urges him to pay more attention to her, and he responds that he has “business” that evening but will take her boating the following day. She is chagrined, however, to discover that Martin is joining them on the boat to discuss business. At sea, Harold is explaining to Martin that his money will protect him from prosecution, when Evelyn suggests the trio go diving. They enjoy their swim until, about twenty minutes later, Martin is stung by a manta ray. He is unharmed, but when they surface, they find the air unbreathable and so return to the boat with their oxygen masks intact. There, they discover that the captain is dead on the deck, and the radio is no longer receiving signals. When they are unable to fire the engine, they take the remaining oxygen tanks and row to shore. There, while cutting through the jungle to reach town, their oxygen supply runs out, and they fearfully remove their masks. Still unsure if they will survive, Harold lights a cigarette, and Martin points out gleefully that the lit match signifies that the air now has enough oxygen. They continue on to town, but there discover that everyone has died, their bodies littering the streets. Horrified, they take a car and return to the hotel, but can reach no one over the phone lines. In the bar, as they all drink to steady their nerves, Martin deduces that either a nuclear bomb has detonated or a natural disaster has occurred, killing most, if not all, of the human race. Realizing that they must leave town before the bodies begin to decompose, Harold drives them to his work partner’s vacation house at the tip of the island. There, after a night of rest, they discern that they have enough canned food to last for years. Although Martin is cynical and Evelyn near hysteria, Harold insists they formulate a survival plan and stay calm. Later, when he sees a live insect and speculates that various bugs will soon thrive on the decaying bodies, bringing with them disease, Harold suggests that they plan to move farther north. In a colder climate, he reasons, they will be better able to store food and avoid disease. To that end, over the next weeks they teach themselves sailing, fishing and navigation. Harold is discomfited to note that Martin and Evelyn are growing closer, but he is so focused on his plan that he has no time to intervene. Meanwhile, Martin is becoming more and more resentful of Harold’s gruff officiousness, as well as his claim to Evelyn. When Harold tries to control the tension between them by urging Martin to work harder, Martin chafes further. Evelyn also pulls away from Harold, whom she feels still refuse to allow her to be independent of him. One day, Martin wanders the beach, bereft, and when Harold and Evelyn find him, he taunts Harold, later admitting to them that if he can goad Harold into attacking him, he will at least know he is alive. Harold later goes out fishing, and Martin invites Evelyn to go to the beach with him, where the two kiss. Back home, Harold walks in just as Evelyn collapses into Martin’s arms. The next day, the two men are on the boat when Martin announces that he will no longer take orders from Harold, and the two begin to fight. They are still fighting when they reach the shore, where Martin mocks Harold about Evelyn, prompting Harold to hit him in the face with a rock, grazing his eye. When Martin bitterly asks if Harold plans to “wipe out one third of the human race,” Harold responds that he and Evelyn are leaving the island without Martin. At home, Evelyn pleads with Harold to change his mind, stating that she and Martin cannot comply with his demands. After Harold dismisses her, Evelyn approaches Martin and asks to leave with him. He tries to resist, knowing Harold will perish alone, but then relents and plans their escape. Later, Harold gives Martin the keys to the car, then is shocked to see Martin steal the keys to the other vehicle, a van, before driving away with Evelyn. As Harold hotwires the van to follow them, Martin and Evelyn discuss their escape plan, but when Martin’s eye begins to throb, he loses control of the car and crashes it. They are unhurt, and run to the harbor just ahead of Harold. There, Evelyn asks Martin if they will have a child, but he declares that the human race is over and all that is left is to live without pain. He leaves her in a church while he searches for Harold, who is on the pier with a rifle. The two fight, and Harold once again delivers a head wound to Martin. After running through the El Morro fortress, Martin stumbles into the church, his vision failing. Harold follows, and inside, Martin declares that there is no God, then collapses and dies. Harold, horrified at what he has done, asks Evelyn to help him find a new life with her.