Paddy cares for the children and forbids them by "law" from going to the other side of the island, as he found remains from bloody human sacrifices on an altar. He also warns them against eating a deadly scarlet berry.
Paddy dies after a drunken binge. Now alone, the children go to another part of the island and rebuild their home. Years pass and they grow into tall, strong teenagers. They live in their hut, spending their days fishing, swimming and diving for pearls. This is stressful for them because of their lack of education on human sexuality. Emmeline is frightened by her first menstrual period ; she refuses to allow Richard to inspect her for what he imagines is a wound.
Richard becomes physically attracted to Emmeline but she does not reciprocate his feelings, inciting Richard to go off alone and masturbate. A ship appears for the first time in years, but Emmeline does not light the signal fire. As a result, the ship passes by without noticing them.
When Richard angrily confronts Emmeline about this, she tells him that the island is their home now and they should stay, to Richard's disbelief. Emmeline secretly ventures to the forbidden side of the island, and sees the altar.
She associates the blood on the altar with the blood of Christ's crucifixion. She comes to the conclusion that the altar is God, and tries to persuade Richard to go to the other side of the island to pray with her. Richard is shocked at the idea of breaking the Law. They insult each other and Emmeline reveals she knows about his masturbating, and threatens to tell her Uncle Arthur about it.
They throw coconuts at each other, and she hits him on the head. Following the fight, Richard kicks Emmeline out of their hut. Emmeline accidentally steps on a stonefish. Sick and weak, she pleads with Richard to "take her to God. Emmeline recovers and Richard admits to his fear of losing her. After Emmeline regains her ability to walk, they go skinny dipping in the lagoon and then swim to shore.
Still naked, Richard and Emmeline discover sexual intercourse and passionate love. They regularly make love from then on. Richard and Emmeline are stunned when they feel the baby move inside her and assume it is her stomach causing the movements. Emmeline gives birth to a baby boy, whom they name Paddy. Emmeline holds him and learns how to feed him as the baby instinctively starts suckling. The young parents teach Paddy how to swim, fish and build things.
A ship led by Richard's father Arthur William Daniels approaches the island and sees the family playing on the shore. When they notice the ship, they walk away instead of signaling for help, content with their lives. As they are covered in mud, their appearance is difficult to determine, and Arthur assumes they are natives. One day, the family takes the lifeboat to visit their original homesite.
Richard goes off and finds bananas for them, leaving Emmeline and Paddy at the boat. Emmeline does not notice when Paddy brings a branch of the scarlet berries into the boat. Emmeline and Paddy slowly drift away, and Paddy tosses one of the oars out. Unable to reach the oar, Emmeline shouts to Richard and he swims to her, followed closely by a shark. Emmeline throws the other oar at the shark, striking it and giving Richard time to get into the boat.
They are unable to retrieve the oars without risking a shark attack. They paddle with their hands to no avail; the boat is caught in the current and drifts out to sea. After drifting for days, Richard and Emmeline awake to find Paddy eating the berries he picked. Hopeless, Richard and Emmeline eat the berries as well, lying down to await death.
A few hours later, Arthur's ship finds them. Arthur asks, "Are they dead?