Astronauts Alan Virdon and Peter Burke crash land on a strange planet. After being rescued by a friendly human, Virdon and Burke discover this planet is ruled by intelligent apes. To their horror, they discover that they are back on Earth in the year 3085. Meanwhile ape leader Dr. Zaius learns of the presence of the spacemen and wants them captured and questioned, much to the chagrin of General Urko, who wishes them dead. To protect his interests, Zaius sends his young assistant Galen with Urko to retrieve the humans. The apes capture the humans and they are taken before the council. Zaius orders them confined for questioning, but Urko conspires to free the humans, and then kill them as escaping prisoners. However, Galen learns of this and foils Urko's plan, allowing the humans to escape. Galen is arrested and accused of murdering one of the guards. The humans must rescue Galen before retrieving a magnetic disc, which could hold the key to returning to their time.