The narrator (Peter Bodganovich) explains the situation. The conditions in planet Earth have deteriorated, especially because the air is so polluted. There is need to look for somewhere else to live.
The main story is told as in the past, when astronaut Andre Freneau (Gennadi Vernov/Aldo Romani on footage) is already back on Earth after the spacetrip, but the film mentions it as though it would be the future, as if the year 1998 would be far away.
The first human-worked spaceship to Venus, which ended in disaster. Captain Alferd Kern (Georgi Tejkh/James David), Astronaut Howard Sherman (Yuri Sarantsev/Ralph Phillips) and Robert John are left stranded on the planet.
Six months later, another trip to Venus was finally ready to go. The second ship sent by the agency MARSHA- reaches the planet. It includes Commander William "Billy" Lockhart (Vladimir Yemelyanov/Roberto Martelli), Andre Ferneau and Hans Walter (Georgi Zhzhyonov/Murray Gerard).
Cue to the first accident. Robot John is included among its crew, so Robot John is the one who ones in one of the smaller transport to the planet. It looks like the ship has crashed: there was the sound of an explosion and John doesn't answer. However, Robot John is still alive, but without an arm. When the rest of the astronauts -Kern and Sherman- go down to the planet they see dazed, confused Robot John. They have to kill some sort of local dinosaurs, not huge in size but very dangerous and aggressive.
Back to the rescue team. The Commander, on landing, wonders whether the old cultures knew something when they named the planet after the goddess of love, Venus. They land without further ado. From the ship, they think they see or hear something, like sirens wailing, or singing. They walk but Andre gets attacked by a kind of huge octopus who wraps him and tries to take him away. The Commader goes to save him. Andre is grateful, but he says he didn't cry for help.
Kern and Sherman feel more and more tired, because infections spread through their ragged astronaut suits. However, they need to keep on walking, and it's Robot John who opens the way. They find a river with safe unpolluted water which cascades down. The astronauts get weaker and weaker, but Robot John finds a cave behind the cascade, where there is no water. Robot John awaits its orders, but the astronauts cannot give him any. They want John to stay with them, and then they pass out.
The group of three astronauts hear mermaid singing again. They follow the sound and they reach a rocky beach, on the vicinity of which there are some cute blonde girls dressed with elephant pants and shells, although the rescue team don't see them. Moana (Mamie Van Doren) and Verba (Mary Marr) feel there is time to go into the sea. The mermaids, including Twyla (Paige Lee) dive into the troubled oceans, where they fish with their bare hands fish and eat it raw without on the spot. A dinosaur bird attacks -or watches, at the beginning it was not clear- them (Margot Hartman, Irene Orton, Pam Helton, Frankie Smith, Robin Smith, Cathie Reimer, Adele Valentine, Judy Cowart) while on the water.
The group of three finally contacts by radio the Kern and Sherman, who are about to die. They tell Robot John to take a pill and give it to the passed-out astronauts. The Pterodactilus doesn't attack the flying ship of the rescue team, but it hits them by chance. The Pterydactyl attacks them later on. The astronauts kill the bird, but it causes damage to their ship, so they have to go down to the bottom of the sea. They can breath because of their astronaut suits. In the bottom of the ship, they find a statue with rubies on their eyes, so they realise there must be a civilisation on the planet.
Moana finds the dead dinosaur bird and talks telepathically to the other girls. They look at the bird. They take the dead bird shore to take it to the spirit. The mermaids see the astronauts at the bottom of the sea, and they think they are responsible for Twyla's death. They think the astronauts are demons. The astronauts go out fo the sea and start a fire. The two other astronauts are now all right because Robot John put the pills onto their mouths. . Kern and Sherman see a beautiful volcano exploding: its governed by the telepathic mermaids who want to revenge Twyla. Robot John takes the two astronauts on its back and he steps onto the river of lava to go down the slope. There comes a moment when Robot John can't walk anymore. Robot John malfunctions and attacks one of the astronauts, and is about to let him fall onto the river of lava.
The spaceship with the three other astronauts is approaching fast. Robot John looks to be done for good. The space ship rescues the two astronauts. Robot John falls onto the river of lava and dies. The Commander gets tear-eyed when he realizes what happened. The five astronauts argue about the adaptability of the human species, and also their wish to communicate with the inhabitants of the planet. The mermaids find Robot John's remains and they think it's a totem left behind by the astronauts. The leader of the mermaids still thinks that the deamons are dangerous and that they left the artifact behind to show how powerful there were, that is, as a threat.
The leading mermaid sees the astronauts returning to their spaceship. They pray to Twyla. The astronauts are already inside the spaceship, talking to MARSHA. The mermaids create a flood with land slidings. The astronauts have to take out the heavy boxes of alien materials and samples from the spacecraft and leave them behind in order to leave themselves. One of the astronauts has to hit a stuck door machine with a stone in order to open it. The stone gets torn to pieces and he realizes there is a small porcelain mask inside. He enters the spaceship overjoyed, thinking that the inhabitants of the planet are alike to humans. The mermaids feel disappointed when they see the spaceship leaving.
Moana realises that the demons are more powerful than their god, the dinosaura bird, so they hit it with rocks until it falls to pieces. The leading mermaid says that there is a stronger god, so they go to look for the remains of Robot John. They think its the strongest god.
A voice over says that the astronauts came back to Earth safely, and that there are not plans to return to Venus. Andre says that hell always try to visit the planet again, as he cant take it from his head, and he says that he may die trying.