After landing on the 13th moon of Jupiter, the men from Earth debark from their ship to find a forested area containing the last remnant of lost Atlantis: an old man named Prossus, a bevy of nubile young women eager for husbands, and — The Creature. “The beast with the head of a man,” laments Prossus. “It must be destroyed — yet it is indestructible!”
We open with the take-off of a Lockheed Super Constellation four-engine prop plane. Flying under the corporate livery of TWA it departs Los Alamos, New Mexico. Credits roll. Inside the cabin we are told in voice over that the U.S. and Great Britain have been working on a secret project, designated "Thirteen". The top nuclear scientist will take command of the project–an expedition into outer space. Shuffling through papers is Luther Blair (Anthony Dexter); he lights a cigarette. In London, England, Blair is met by his colleague Dr. Higgins (Sydney Tafler). They drive to the observatory. Higgins explains to Blair that the thirteenth moon of Jupiter appears to be very earthlike. They will leave in a week for the shortest trajectory to Jupiter's thirteenth moon. Total travel time is three weeks. Higgins speculates that there could be humans on the planet. He calls his secretary to send a message to Captain Larson to have the ship ready in a week.The rocket is launched from the American-British Astronomical Station, Missile Section, Houndspoint, Surrey, England. Ground control monitors progress. They are a few minutes into flight when they encounter a meteor shower. Blair orders Anderson (Rodney Diak) to report the situation, while Captain Larson (Paul Carpenter) maneuvers the ship. Once the danger passes they resume course. Three weeks later they reach their destination and begin descent. They pick up a transmission on their radio, "calling spaceship, calling spaceship, report destination immediately, you are under our space control." The ship stops moving. Blair contacts the voice and tells him theirs is a friendly scientific expedition. He is told to proceed and given landing instructions. They land in what appears to be a forest. Blair orders Stanhope (Harry Fowler) to check the atmosphere outside, and Anderson to contact control back on Earth. The atmosphere is found to be earthlike, as predicted. They exit the ship to begin exploration. Beyond a clearing they see a flashing beacon that is sending a message in Morse code. They run to it. They reach the beacon and find it is on top of a nude female figure made of bronze. On their return to the ship they hear a woman scream. They hear it again and head in that direction. They spot a tall, lean, and dark lizard-like humanoid creature accosting a female human dressed in ancient Greek garb. They shoot at the creature (Richard Walter) and it runs off. The men gather and the woman thanks Blair by touching his face. She speaks not a word. We learn later that her name is Hestia (Susan Shaw) and she can speak. She walks away and the men follow her. She goes through an opening in a wall and Blair and Larson follow, the other three remain outside the wall.Hestia leads Blair and Larson into a palace and to a room covered in draperies. Blair observes the decoration is Minoan, but a voice corrects him by identifying the décor as "late-Atlantis". A man wearing a robe welcomes them to "New Atlantis" and introduces himself as Prasus (Owen Berry). He invites them to sit. They recognize his as the voice they heard for landing instructions. He tells them he has been monitoring their progress and was glad the meteor shower did not imperil their journey. He directs them to stay until they can devise a way to destroy the creature. He provides them food and drink. He introduces his "daughter" Hestia. Blair is told Hestia now belongs to him because he saved her life. About a dozen beautiful women bring in food and drink and serve Blair and Larson. They drink not realizing the wine has been drugged. The women, dressed like cocktail waitresses at Caesar's Palace, dance to the tune of Borodin's Polovtsian dances from Prince Igor.Anderson, Stanhope and Higgins remain outside the wall and are getting anxious. They decide to return to the ship. The creature lurks outside. Anderson reports to control and tells them everything is going according to plan. Hestia goes to Blair's room from the garden side, opens a force-field screen and walks in. He is still sleeping off his drugged wine. She offers him more wine but he tells her, "Oh no, once was enough, sister." She drinks some to show him it is safe to consume. He gets up off his day bed and asks her, "Don't you talk?" She only smiles at him. She directs him outside. Once in the relative safety of the garden she tells Blair that the rooms are bugged. She tells him the women are all being kept prisoner by Prasus, her "father". She kisses Blair then departs. Larson is in his room and is awakened by four women. They give him wine. He notices his gun is missing. When he inquires about Blair he is told Prasus is meeting with him now. To taunt the women, he tells them the creature is out in the garden. They recoil in fear.Stanhope, Anderson and Higgins arm themselves, then leave the ship. The creature is lurking in the underbrush near the ship. As the men head back to the compound, the creature follows them. Meanwhile, Blair is talking to Prasus. Hestia pours wine, but when Blair goes to drink it, she signals him not to swallow. The wine is drugged. They switch goblets and Prasus drinks the wine and passes out. Blair and Hestia exit. When the three astronauts return to the wall they notice the entrance is now sealed. The monster appears and they shoot it, but the bullets have no effect. Higgins lobs a gas grenade and it leaves the area. Blair tells Hestia they will find Larson and leave, but their conversation is overheard by Duessa (Jacqueline Curtis). Hestia leaves and Blair returns to his room and calls out to Larson. Larson hears him in an adjoining room through the wall. The three astronauts try to find a way inside the compound. Stanhope climbs a tree and spots some of the fire maidens. They decide to cut down a tree and lean it against the wall and use it as a ladder. It explodes into flames. They decide to tunnel under the wall to gain entrance into the garden.Hestia is walking through the palace when she is ambushed and tied up by the other fire maidens. They lead her down to the sacrifice chamber. She is placed on an Alter with a large fire burning behind her. Apparently she violated protocol by getting engaged before an older fire maiden, and for that she must be sacrificed in the fire pit. Duessa performs a ritual dance. Anderson, Higgins and Stanhope were captured when they entered the garden and are brought down to the sacrifice chamber. They are bound and are told they will be witnesses to the sacrifice of Hestia. Blair finally figures out how to open the door to his room–slide the chair to the side. He tells Larson to do the same and the two are reunited. They find the meeting room and discover Prasus still unconscious. Blair finds their guns behind a tapestry. They overhear the women and learn their three colleagues are in the palace. They follow the women to the sacrifice room.The monster finds the entrance through the wall and enters the garden area. Prasus wakes up and exits the meeting room. In the sacrifice chamber Duessa continues her ritual dance. Prasus is walking through the garden and is attacked and killed by the creature. Blair and Larson hear the ritual and are drawn to it, as is the creature. The creature enters the chamber roaring; the fire maidens scatter screaming. It approaches Hestia as Blair and Larson enter the chamber. They fire their guns at it. Blair lobs a gas grenade, it explodes, and the creature falls into the fire. Blair releases Hestia from her ropes while Larson releases his three comrades. Blair and Hestia kiss.Now outside, they walk through the garden. Blair announces that Prasus is dead. The fire maidens proclaim Hestia as their new ruler, "Queen of Atlantis". Blair tells the women the earth will be sending more expeditions. Hestia intends to return to the earth with Blair. She climbs aboard the rocket. We zoom in on Blair and Hestia in each other's arms, then watch the rocket launch for its return to Earth.