In the future, the Earth is ruled by a race of highly-evolved simians who supplanted humans after a nuclear holocaust. But when the ape-led Earth is threatened with total annihilation, a trio of scientists journey backwards through time in order to prevent the catastrophe. Once in the human-led world, however, they find themselves hunted by a suspicious and fearful government desirous of making them the subjects of an experiment that could alter the course of human and ape events...