Returning to Earth from a Mars mission Henderson and crew receive an SOS message from the direction of Venus. An American astronaut's spaceship has been damaged by a meteorite. Henderson and a Russian supply rocket begin a rescue attempt.
The MR4 arrives at Venus. Scheming Harcourt Brown is convinced that he has seen indications of advanced life on the surface of Venus and lies to Henderson telling him that Capt. Wilson has already been forced to crash land on the planet.
Geoff and Margaret locate Wilson's crashed spaceship but are alarmed to discover it empty and worse still something with very big teeth has ransacked the interior. Wilson and Brown set out to find Brown's Venusian city.
The hairy thing with the big teeth traps Professor Meadows and Margaret in the MR4 rocket but luckily for Geoff it vanishes before he comes to the rescue. The three of them set off into the jungle to find Henderson, Brown and Capt. Wilson.
Hamlet is almost eaten by a plant. The explorers become lost and separated in a cave system under a mountain which is home to a clan of primitive cave people. To add to their woes they realize that the area is prone to volcanic activity.
Margaret's new little cave girl pal Kiki helps the crew escape from the lava caves. Harcourt Brown's Venusian city turns out to be a disappointment and Professor Meadows becomes ill after she receives an insect bite.
The crew plan to cook a giant flying reptile's egg for lunch but cancel that when the annoyed parents turn up. The Russian supply rocket is soon due to arrive in orbit so the long walk back to the MR4 begins.
A jungle fire puts the MR4 rocket at risk. Brown plans to sabotage the MR4 to safeguard Venus from future human exploitation. Kiki helps Capt. Wilson back to the rocket after he gets thumped by a Venusian Neanderthal.