The movie starts out in 1988 with pilot Koji Miyoshi's return to Japan in response to electromagnetic interference that is plaguing the world following a comet scare. Miyoshi is quickly reunited with old friends and acquaintances upon his arrival, although the reunion is cut short as UFOs began to appear all across America as the UN's Space Station Terra is also destroyed in orbit. This series of events prompts Miyoshi to meet with Masato Takigawa and request that the United Nations Space Forces' Gohten Project - construction of the UNSF Gohten, is completed. The request falls on deaf ears until the aliens from the distant planet of Yomi in Messier 13 who have been behind the recent disturbances bungle a plan to kidnap Takigawa, at which point work on the Gohten commences amongst global chaos from attacking Hell Fighter UFO's. Following a fierce battle at the Gohten’s base, the craft is finally launched as it makes its way to Venus, the suspected location of the invaders’ base of operations, to put an end to the alien threat.
Orville (Lou Costello) is the oldest orphan at the Hideaway Orphans Home. He accidentally winds up inside a truck heading to a top-secret laboratory, where he is placed under the guidance of lab worker Lester (Bud Abbott) to help load supplies onto a rocket ship. While on board with Lester, Orville hits the ignition button and the rocket blasts off, flying across the country to New Orleans, where Mardi Gras is in progress. They exit the ship and witness "hideous creatures", which are actually costumed celebrants, and conclude that they have landed on Mars.
Off the coast of a small Sicilian fishing village, two fishermen watch in amazement as a spaceship pierces the skies and crashes into the sea. The men, Mondello (Don Orlando) and Verrico (George Khoury), row out to the site and pull two space travelers from the capsized craft before it shudders and sinks into the sea. Meanwhile, in Washington, D.C., General A. D. McIntosh (Thomas Browne Henry) discovers that the government's missing spacecraft, piloted by Col. Bob Calder (William Hopper), has been located off the coast of Italy. As McIntosh flies to Italy, Pepe (Bart Braverman), a little boy who lives in the fishing village of Gerra, finds a metal capsule that has washed up on the beach. Upon opening the capsule, he finds a jelly-like glob inside and sells it to Dr. Leonardo (Frank Puglia), a visiting zoologist who is studying sea creatures. Meanwhile, Leonardo's medical-student granddaughter Marisa (Joan Taylor) is summoned to town to take care of the injured Calder and his companion, Dr. Sharman. When Calder regains consciousness, he finds Sharman in the last throes of the fatal disease that decimated his crew.
Dr. Tom Anderson (Lee Van Cleef), an embittered scientist, has made contact with Zontar, a Venusian creature, while using his radio transmitter. The alien's secret motivation is to take complete control of the Earth by enslaving humanity using mind control devices; the alien claims it only wants to bring peace to our troubled world by eliminating all emotions. Anderson agrees to help the creature and even intends to allow it to assimilate his wife (Beverly Garland) and friend Dr. Nelson (Peter Graves).
After a botched attempt at suicide, Nora King (Kathleen Crowley) revives and discovers that her building has no electricity or water. She soon realizes that Chicago is deserted when she wanders out into the empty, quiet streets. Stumbling over the body of a dead woman, she encounters Frank Brooks (Richard Denning), another person who has just recently revived after being beaten in a robbery.
Capt. Patterson (Eric Fleming) and his space crew (Dave Willock, Patrick Waltz and Paul Birch) crash land on Venus and are captured. They learn the planet is under the dictatorship of cruel Queen Yllana (Laurie Mitchell), a masked woman who has banished men from the planet. In the palace the astronauts are aided by a beautiful courtier named Talleah (Zsa Zsa Gabor) and her friends (Lisa Davis, Barbara Darrow and Marilyn Buferd). The women long for the love of men again and plot to overthrow the evil queen. When Patterson has the opportunity to remove the Queen's mask, he discovers she has been horribly disfigured by radiation burns caused by men and their wars. In a fury, the Queen decides to destroy Earth and its warlike peoples but she dies in the attempt. The Venusians are free again to enjoy the love of men.
The film begins in 1985, when engineers involved in an industrial project to irrigate the Gobi Desert accidentally unearth a mysterious and apparently artificial "spool". When found to be made of a material unknown on Earth, the spool is circumstantially linked to the Tunguska explosion of 1908. The "spool" is seized on as evidence that the explosion, originally blamed on a meteor, was actually caused by a spaceship. Professor Harringway deduces the alien craft must have come from Venus. The spool itself is determined to be a flight recorder, and partially decoded by an international team of scientists led by Professor Sikarna and Dr. Tchen Yu. When radio greetings sent to Venus go unanswered, Harringway announces that a journey to Venus is Earth's only alternative. The recently completed Soviet spaceship Cosmostrator I (Kosmokrator in the original), intended to voyage to Mars, is redirected to Venus, a 30–31 day trip. During the voyage, Sikarna works furiously to translate the alien message on the disc using the ship's computer.
Three Soviet spaceships, Sirius, Vega, and Capella, are on their way to the planet Venus. The Capella is struck by a meteorite and destroyed. The remaining two ships, Sirius and Vega, continue on, despite the fact that the planned mission required three ships. A replacement spaceship, the Arktur, will be sent from Earth, but will not arrive for two months.
At a dinner party with their wives, NASA scientist Dr. Keith Ritchie (Tony Huston) reveals to his colleague Dr. Curt Taylor (John Agar) that he has secretly been in communication with a three-eyed, bat-winged alien from Venus named Zontar who he claims is coming to Earth to solve all of the world's problems. However, as soon as Zontar arrives on Earth via a fallen laser satellite it quickly becomes obvious that the skeletal black creature has a hidden agenda as it begins causing local power outages that stop telephones, automobiles and even running water from working and it starts taking control of people's minds using flying lobster-like "injecto-pods" that sprout from its wings. Only after his wife is killed does Ritchie finally realize that Zontar has come not as a savior but as a conqueror, and he goes to confront the hideous alien in the sulfur spring-heated cave that it has made its secret base.
Astronauts landing on Venus encounter dangerous creatures and almost meet sexy Venusian women. The astronauts kill a creature that is worshiped by the Venusian women who then attempt to use their powers of nature to kill the astronauts but fail. At last, the astronauts leave the planet and their robot that was burnt by the volcano fire is placed as a god by the Venusian women who have already destroyed the statue of their previous god (a bird).
A spy (Essie Lin Chia) discovers that the Chinese government has created a doomsday device (the "key" to which, "only Chairman Mao has") capable of destroying the Earth and it will be activated in 72 hours. Soon after, Astra – a two year return mission to Venus by the United States Space Program – has its time of launch speeded up and half of the male flight crew are replaced by women shortly before take-off, including one Russian. Shortly before blastoff military alerts are put into effect.
The film is set in a fictitious past, in which space travel is pioneered by roughnecks. The logic of space travel is not addressed, and most technology was designed for the film to appear as if "it could be repaired with a hammer". Samuel Curtis, an interplanetary trader, is approached by an acquaintance of his, the Blueberry Pirate, with a mission. He is to provide the women of Venus with a new king for mating. In exchange for this new stud, they will relinquish their currently deceased king, whose family is offering an immense reward for his body. Curtis sets off in search of the perfect replacement, the Boy Who Actually Saw a Woman's Breast, and soon discovers that his old nemesis Professor Hess is hot on his trail.