It is the Victorian era in the South Pacific. Two young children, cousins Richard and Emmeline Lestrange are sailing with Richard's father, Arthur, to San Francisco. When the ship suddenly catches fire, the ship's cook, Paddy Button, gathers the children in a lifeboat and rows away from the wreck. However they are separated from the children's father, who has escaped on a separate dinghy, and drift out to sea. They arrive on the shores of a lush, tropical island. Paddy assumes responsibility for the children and, over an indeterminable amount of time, teaches them how to find food and build shelter, though he can't deter them from taking their shirts off and running about half naked.One day he discovers a stone altar covered in blood and the remains of what appear to be a human sacrifice. He warns the children to stay away from that place, telling them it's 'the law' and that the boogeyman lives there. He also tells them to never eat a certain scarlet berry that Emmeline finds, presumably to keep them from eating anything uncertain. He calls them 'never-wake-up berries'.One day, Paddy discovers a barrel of rum or whiskey washed ashore from the sunken ship. The children wake up in the morning and take the rowboat out to where they see Paddy lying in the sand. Thinking he's merely asleep, they flip him over to see that he's died in a drunken binge. Emmeline begs him to take her away. They pack up everything they have and row around to the other side of the island where they find a new stretch of beach to settle on.Richard and Emmeline mature into strong teenagers (Christopher Atkins and Brooke Shields), spending their days diving for pearls and fish, gathering food, or maintaining their tree house.Richard and Emmeline soon start developing both physically and emotionally in ways they can't explain. Growing up as children, they are generally ignorant to the changes. Emmeline experiences her first menstrual period while swimming and calls out to Richard, afraid at first. When he arrives and asks what's wrong, she yells at him, embarrassed, and tells him to go away. Richard finds himself becoming physically attracted to Emmeline but she, though often fearful of being left alone, doesn't reciprocate. One day, ever-curious Emmeline wanders to the forbidden side of the island and finds a Moai-like stone idol there with the tell-tale stain of blood. Thinking instead that the place is holy, she prays and later tells Richard that she thinks Paddy was wrong and that the 'boogeyman', who bleeds like Jesus, is actually God.They speak often of being rescued and going to 'San Frisco' to see Richard's father, but when a ship passes by the island for the first time in years, Emmeline does not light the signal fire they had set up. Richard emerges from foraging in time to see the ship depart and angrily confronts Emmeline. She tells him that the island is their home now and that they should stay, to Richard's disbelief. They insult each other and Emmeline reveals that she knows what happens when Richard goes off alone which leads him to throw a coconut at her. She throws one back, hitting him on the head.After building a makeshift hut for herself near the water, Emmeline steps on a stone-fish. Richard discovers her deathly ill in her hut and she begs him to take her to God, despite the law. Fearing for her life, Richard does so and places her on the stone altar, reciting what little he remembers of his prayers. Emmeline eventually recovers and, after she regains her ability to walk, goes swimming with Richard in the lagoon. Afterwards, they sit naked together and share some fruit as well as their first kiss. They lie down with each other, expressing their love for each other, and discover sexual intercourse and passionate love. Putting all their past arguing aside, they spend most of every waking moment with each other, playing in the water or making love. However, Emmeline soon becomes pregnant. Though this is clear to the viewer through Emmeline's hunger cravings and her ability to feel the baby, she and Richard are unsure of her physical changes and have no knowledge of childbirth, attributing it all, at first, to Emmeline merely getting fatter.One night, Emmeline goes missing and Richard looks through the jungle for her. Following the sound of drums, he comes upon the altar where 'God' stood. There, he witnesses a native tribe performing a human sacrifice. Richard flees as the victim is killed and hears Emmeline's cries, following them in time to help her give birth to a baby boy whom they name Paddy.They bring the baby back to their home and try to feed him. Frustrated by the baby's crying and that he won't take solid foods or juice, Emmeline takes him in her arms to see that he instinctively suckles at her breast. Richard tells Emmeline what he witnessed the night she gave birth and assures her that if any of the natives come to find them he will stick them with his spear like a fish. But the natives never come to their side of the island. As the baby grows, they teach him how to swim and play. As they all play together in wet mud near the shore, a ship led by Richard's father approaches the island and sees them there. As they are completely covered in mud, Arthur does not recognize them. Paddy points to the ship but when Richard and Emmeline see it, they resign all previous intentions of leaving the island with silent glances and retreat with Paddy into the forest.Richard is seen taking the rowboat alone out to the small island where Paddy Button died. He approaches the beach and sees the old man's skeleton lying in the sand, bleached white from the sun. Richard tentatively touches his ribs, matching them to Paddy's, and looks at the bones in his hands. A morbid understanding comes to him and it's implied that he has a better grasp on his own body as well as death.One morning, the young family takes the rowboat to visit their original home site and to collect food and other supplies. Richard goes to harvest bananas while Emmeline and the baby explore near shore and Emmeline fails to notice young Paddy collecting scarlet berries and putting them in his pocket. The two of them go back to the boat to wait for Richard but the tide takes them out when Emmeline falls asleep. She is awakened by the sound of Paddy pushing one of the oars into the water and tries to fetch it with the other. When she can't, she calls out to Richard who starts to swim to the boat, followed closely by a shark. Just before he reaches the boat, Emmeline throws the last oar at the shark, hitting it and giving Richard enough time to get out of the water. They try to paddle to shore with their hands but the tide is too strong and they don't dare swim for fear of a shark attack. Slowly but surely, the boat is taken out to sea.After a day or two at sea, Richard and Emmeline awake from a nap to find Paddy eating the berries he had collected. Frantic, they try to make him spit them out, but he's already swallowed them. After another day, young Paddy has a hard time keeping his eyes open and falls into a deep sleep. Resolute, Richard splits the remaining berries between Emmeline and himself and they lie down together to await death. A short time later, a large schooner approaches the rowboat. Arthur Lestrange and the captain of the ship (Alan Hopgood) approach in their own dinghy to find the young couple and their child lying in the boat. He asks the captain, 'Are they dead?' The captain replies, 'No sir, they are asleep.'