Also known as Seven Cities to Atlantis, this is a youth-oriented, British-produced adventure fantasy which uses rather ordinary special effects. In Victorian England, an explorer, Greg Collinson (Doug McClure), leads an unlikely expedition to discover the lost continent of Atlantis. Among the team are scientists Professor Aitken (Donald Boisset) and son Charles (Peter Gilmore). The crew members mutiny, setting their sights on the legendary treasures of Atlantis. The ship is attacked by sea monsters and a giant octopus. Sinking to the bottom of the sea, the explorers find that Atlantis is populated by intelligent beings from another planet who enslave shipwrecked sailors. The aliens' goal is to create a fascist state that will rule the world, and they want to recruit Charles to help them. Delphine (Lea Brodie), the daughter of one of the shipwrecked slaves, helps the heroes find a high priestess, Atsil (Cyd Charisse), who holds the key to their chances of escaping.