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 Arcturus, will be sent from Earth, but will not arrive for two months.
The cosmonauts aboard Sirius and Vega decide that some sort of landing and exploration is better than waiting. Ivan and Allan go down from Vega in the glider, leaving Masha in orbit. All contact is lost after they land in a swamp. The Sirius lands nearby and the three-man crew set out in their hovercar to find them.
During their travels they hear an eerie woman's song in the distance and encounter prehistoric beasts both benign and threatening. Ivan and Allan, meanwhile, must fight off some man-sized T-rex beasts as they head to meet the men of Sirius. The two fall ill with a fever. Their robot, John, stands watch.
The Sirius crew must submerge their hovercar to escape a pterodactyl. In doing so, they discover what might have been an ancient city. Alyosha finds a strange triangular rock and a statue of a pterodactyl with rubies for eyes.
Once on dry land, the Sirius crew contact the robot John and tell him to administer an anti-fever drug. Ivan and Allan recover just as a volcano sends down rivers of lava. They order John to carry them across, but he malfunctions half way there. The hovercar shows up just in time to rescue them while John is lost to the lava.
All five return to Sirius, but worry that Masha had landed the Vega somewhere, stranding them all. An earthquake and flood from rain threaten to strand the Sirius, so they must take off immediately. Alyosha discovers that his odd triangular rock is really a sculpture of a woman's face, proving that there might still be intelligent life on Venus. They blast off and find that Masha remained in orbit and together they head home.
How many return to Sirius?
All five