American nuclear scientist Luther Blair travels from Los Alamos, New Mexico, to London, England, to oversee a secret expedition to outer space. At an observatory in England, Luther meets with British scientist Higgins, who has discovered a thirteenth moon orbiting the planet Jupiter. Their project, dubbed Expedition 13, is to travel to the Jupiter moon aboard a nuclear-powered rocketship. Within a week, the spaceship is ready, and Luther goes over the flight list with his crew, which is comprised of Higgins, Captain Larson, communications specialist Anderson and geologist Sydney Stanhope. After launching, their flight is monitored by the American-British Astronomical Station in Surrey. The spaceship encounters a hail of meteors, but emerges safely, and approximately three weeks later they reach Jupiter’s thirteenth moon. The crew is surprised to be contacted via radio by someone at the surface, who directs them to a landing site. After determining that the atmosphere is safe, the crew exits the ship and discovers an Earth-like terrain. Having located the radio beacon, which is in the form of a statue of a nude woman, the crew decides to return to the ship. However, they are distracted by a woman’s screams, and Luther comes to the aid of Hestia, an inhabitant of the moon, who is being attacked by a humanoid creature. Luther scares off the beast by firing his gun, and Hestia leads them to a cave entrance surrounded by a high electrified wall. Having instructed Higgins, Anderson and Stanhope to return to the ship if they do not return in half an hour, Luther and Larson then follow Hestia inside. They emerge into what appears to be a palace, where they are met by Prasus, the leader of New Atlantis. Prasus explains that he is the sole male survivor of the former Atlantis on Earth, and seeks their help in destroying the creature. Luther and Larson accept Prasus’s invitation to visit, and Prasus tells Luther that Hestia’s life now belongs to him, then introduces his eleven other daughters. The daughters dance for a delighted Luther and Larson, who soon pass out from their drinks. Higgins and the crew, meanwhile, return to the ship and cautiously report that their expedition is proceeding. The next morning, Luther awakens in a room without doors, but Hestia opens a window and takes him outside, where she warns him that they are monitored indoors by Prasus. She then reveals that her ancestors evacuated Earth because they believed all the continents would sink with Atlantis. However, she and her sisters are now held prisoner by the powerful Prasus. When Luther and Larson fail to return to the ship, Higgins and the crew decide to attempt a rescue. Later, Prasus meets with Luther and explains his desire to destroy the creature and repopulate New Atlantis, with Luther and Larson’s help. Unknown to Prasus, Hestia has drugged his drink. After Prasus passes out, Hestia and Luther discuss leaving New Atlantis, unaware that Duessa, another daughter, has overheard them. Outside, Higgins and the crew encounter the vicious creature and discover that it is unharmed by bullets, but is frightened by the explosion of a gas grenade. To avoid the electrified wall, the crew digs a tunnel underneath. Luther and Larson, meanwhile, are being held hostage in their rooms and are unable to communicate with each other. In a sacrificial chamber, Duessa and her sisters have bound Hestia to an altar, where they perform dance rituals. Duessa claims first marrying rights as eldest daughter, and plans to kill Hestia as a sacrifice to the goddess Aphrodite, as punishment for Hestia’s transgression. After Higgins, Anderson and Stanhope are captured in the garden, they are held captive in the chamber. Soon after, Luther discovers how to escape from his room, and releases Larson. They elude a group of women who have come to take them to the sacrificial chamber, but when they overhear the women talk about their male hostages, Luther realizes he must rescue the crew. Luther and Larson re-arm themselves after finding their weapons, then find their way to the chamber. When Prasus awakens, he goes into the garden and is murdered by the creature, who has entered via the crew’s tunnel. The creature then enters the chamber moments after Luther and Larson arrive, and climbs the altar. At Higgin’s shouted advice, Luther lobs a gas grenade, which causes the creature to fall into the sacrificial fire to its death. After Luther frees Hestia, and Larson releases the crew, the sisters abandon their plans for a sacrifice, and join with the men. Upon discovering Prasus’ body outside, Hestia is declared the new ruler. However, she temporarily cedes power to her sister Neesa so she may join the voyage back to Earth. With promises to return with more men for the New Atlantis women, Luther, his new love Hestia, and the crew board the rocketship and leave for Earth.