A young boy and his sister visit their uncle's laboratory, where scientists are involved with ground-breaking experiments in the field of cryogenics. They have devised a system in which monkeys, and conceivably humans, can be flash-frozen for years and resuscitated later without any ill effects. While enjoying the grand tour with their uncle's assistant Catherine, a massive earthquake erupts and the three of them take refuge in the experimental cryogenic pods. A falling chunk of concrete hits the controls and begins the freezing process. When the trio awakens, they find themselves in a strange world where apes walk and talk like men and wield complete control of their own civilization. Soon they are running for their lives, pursued by an army of primates that has marked them for exterminat