During World War II, a troop transport ship stops in a dangerous part of the South Pacific while emergency surgery is performed on an important officer. After the successful operation, Dr. Snyder compliments nurse Elizabeth Smythe on her exceptional skills and desire to become a doctor. On deck, meanwhile, accomplished but youthful Marine Michael J. "Chicken" Dugan is mercilessly teased about his age. Suddenly, a bomb hits the ship and bursts into flames. Mike's heroic efforts to save his fellow Marines go to waste, and eventually he is forced to jump ship into a life raft. In the water, he hears Elizabeth calling for help, and picks her up, informing her that the man she has been holding onto has already died. As they drift during the next four days, Mike remains cheerful about the likelihood of spotting an island, while Elizabeth grows sick and fearful. Elizabeth makes an innocent comment about Mike's youth, and, despite her superior rank, he rudely reminds her that he is the expert sailor and therefore rules their boat. They finally see an island, but when Mike tries to guide the boat ashore, Elizabeth makes a mistake that costs them the boat. Immediately, Mike is delighted with the beautiful island, but Elizabeth, despondent at the idea of harming her career plans, remains concerned only about making a signal fire and avoiding a gila monster. Soon, they discover a wrecked ship containing the thirteen-year-old skeleton of a man named Grimshaw. They read his sad tale in his diary: Grimshaw washes ashore during a hurricane, and all the natives avoid him out of the fear that he controls the weather, except for one woman, Tukuo. Grimshaw and Tukuo soon fall in love, but after another storm hits, the natives sacrifice Tukuo to their gods and flee the island. Within months, Grimshaw is crazed with loneliness, and shoots himself after hallucinating about a monstrous lizard. Elizabeth is terrified at this image, but Mike sets out making the island livable, building huts and a signal fire. Elizabeth softens somewhat, especially after domesticating a wild pig, Barbecue, but insists that their hut have a wall to separate them. Soon, she comes to admire Mike's extreme capability. He teaches her to fish, wield a spear and make clothing from palm fronds. The more time they spend together, however, the more deeply Mike falls in love with Elizabeth, and he eventually lashes out when she does not respond in kind. One day, he spies a shark in the water while she is swimming, and attacks it with a knife. Afraid the blood in the water is Mike's, Elizabeth cries, and although she embraces him when he swims ashore, she pushes him away when he holds her for too long. For days, Mike refuses to speak to her, and one day she finds him building a boat with which to sail for help. She begs him not to leave her alone, and he responds that he is going crazy with unrequited love. Dazed that the young man would love "an old maid," Elizabeth finally breaks down and kisses him. One year later, the happy couple watches a British Royal Air Force plane crash in the nearby jungle. They rescue pilot William Peck, whose arm has been torn off. Elizabeth performs surgery and nurses him back to health, and within weeks, the worldly, sophisticated Bill has fallen in love with her. Mike notices Bill's attentions and jealously urges Elizabeth to reveal their relationship to Bill, but, conflicted about her affection for both men, she does not. One day, Mike eavesdrops as Bill grabs Elizabeth and declares his love. She explains that she owes her life to Mike, and unable to bear hurting him, cannot leave him. When Bill replies that he will fight for her, Mike leaps out and starts a fistfight with Bill, holding one hand behind his back. While they are fighting, however, a Marine boat appears on the horizon to rescue them. They are taken on board, where the captain announces that Mike has been declared a hero and is wanted in Washington, D.C. Realizing that he must leave Elizabeth, Mike sadly informs Bill that he has been wrong to try to keep her, and has no claim on her. Before he disembarks, Mike asks the captain to marry Elizabeth and Bill. Elizabeth weeps to see him leave, but relaxes into Bill's embrace, knowing she is with the right man.