In 3000 A.D., in a New York City that has been ravaged by atomic war, three tribes struggle to survive--the Norms, healthy humans who live in the remains of the subway; the Upriver Men, ruthless raiders who roam the woods; and the Mutates, village-dwellers scarred by genetic deformities. On the day that he is to marry, Rob, the son of the Norm chief, goes hunting with his friend, Bram, and comes across Gordon, the brutal leader of the Upriver Men, torturing two Mutates. Although he, like all Norms and Upriver Men, is repulsed by the Mutates, Rob asks Gordon to release the surviving Mutate, Riddon. To avoid trouble, Gordon complies, then shows up at Rob's wedding with some of his men, professing friendship and peace. Unaware that Jason, a trusted friend who desires Rob's conniving fiancée Catherine, is conspiring with Gordon to overthrow him, Rob's father welcomes the Upriver Men at Jason's urging. During the pre-nuptial festivities, Gordon orders his men to attack, and the unsuspecting Norms are overwhelmed. Rob and Bram flee to the woods and soon are surrounded by Mutates, led by Riddon. With the Upriver Men close behind, Riddon guides Rob and Bram through the Mutates' secret cave entrance into a subway tunnel that crosses a river. Once safely in the Mutate village, Rob and Bram are put on trial by a three-man council for "crimes against Mutates." Riddon defends Rob and Bram, noting that they saved his life, and protests that killing them would make the Mutates no better than the tribes they revile. The council members, who defend their right to kidnap Norm women for breeding purposes, spare Rob and Bram on condition that they never return to the Norms and reveal the cave's location. In the Norms' underground city, meanwhile, Gordon performs a marriage ceremony for Jason and Catherine, and as he had promised, allows Jason to have the slain chief's chair. As soon as Jason sits in the chair, however, he is strangled, and Gordon becomes the Norms' undisputed leader. Riddon's position as Mutate leader is threatened by the ambitious Carver, who uses Riddon's friendship with Bram and Rob to foment disquiet among the tribe. With the aid of fellow Mutate Sabron, Carver secures a mandate to challenge Riddon in personal combat. After Carver, who will assume leadership if he wins the fight, lays down the staff of challenge outside Riddon's hut, the two men battle each other. Riddon wins, but instead of killing Carver, banishes him. Later, after yet another genetically deformed baby is born to the tribe, Riddon orders a raid on Gordon's captured Norm women, including the beautiful, kind Ruth. The raid is successful, but the revenge-hungry Carver negotiates a deal with Gordon that if he guides Gordon's men safely to the Mutates, he will be named the Mutates' new chief. The Mutate men, meanwhile, lead the captive Norm women toward their village, and along the way, a terrified Ruth escapes. Riddon, whose deformities are mild, re-captures her and, after making an impassioned plea for his people's right to survival, kisses her. Ruth then returns to the Mutates, but the group is soon attacked by Gordon's men, who have been led through the tunnel by Carver. During the melee, Carver is killed and the Norm women are reclaimed by Gordon. Riddon is taken prisoner, but Rob and Bram devise a plan to free him. As they are about to leave, Catherine appears, having escaped from the cruel Gordon. Showing Rob scars left by Gordon's beating as proof of her disaffection, Catherine offers to guide Rob to an unguarded path into the Norm city. After Rob directs the others to set a trap at the river cave, he, Bram and Catherine set off. Just outside the Norm city, however, Rob reveals to Bram that Catherine's scars are painted on, exposing her as a traitor. Leaving behind a bound Catherine, Rob and Bram slip into the city through abandoned sewer pipes. In the city, meanwhile, Ruth sneaks some food to the jailed Riddon and confesses her love. When Rob and Bram show up, Ruth helps them free Riddon, and the four of them head for the river cave. Anticipating their move, Gordon sends his men to the cave, arriving just as the Mutates are knocking out the last of the wall supports. Moments before a wall of water rushes in, Rob, Ruth, Bram and Riddon scramble out with the other Mutates. Gordon's men are drowned in the ensuing flood, thus ending the reign of terror. Later, Ruth offers hope to the Mutates and renews their faith in God by willingly marrying Riddon.