Cornelius and Zira, the simian ape couple from the first two ape films, fled from their doomed planet (just before the events in Beneath the Planet of the Apes) in the same spaceship used by Taylor in the first film, and traveled back through the same space/time portal and landed on late 20th Century Earth where they are received with fascination and fear from the people they meet. As military personnel and civilians struggle to understand how apes managed to commandeer Taylor's spaceship, Cornelius, Zira, and Dr. Milo (Sal Mineo) attempt to understand how they came to Earth’s past. The reason given for such leaps in scientific capability is Dr Milo, a genius ahead of his time who somehow rescued and repaired Taylor's craft from the Forbidden Zone. While the three chimps are held captive i