The legendary lost city of Atlantis is run by an advanced race of aliens. Over the years, a giant octopus, guardian to the entrance of Atlantis, has snatched boatloads of sailors and their families from the sea. The slaves are then put to work repairing damage, and protecting the city. On a diving expedition to find Atlantis, Greg (Doug McClure) and Charles (Peter Gilmore) are swept up by the huge tentacles of the octopus, together with the crew of the expedition ship, and deposited on a deserted beach in the centre of Atlantis. Determined not to become slaves Greg. Charles and the rest of the crew have to race against time to escape from the city and return to the ship.