While returning to Earth from a hazardous journey to Mars, the British rocket crew hear a mysterious distress call from the vicinity of Venus.
In answer to a distress call from a stranded astronaut; MR4 has diverted to Venus. By a trick, the rocket's univited guest Harcourt Brown has made pilot Henderson decide to land on the unknown world beneath the carbon cloud.
The crew of the British rocket MR4 have landed on Venus to rescue the stranded American astronaut. But he has vanished and his rocked has been ransacked.
The British rescue party on Venus have found the American's ransacked rocket. Evidence of life is soon confirmed.
Harcourt Brown has persuaded the American astronaut, Wilson to go with hims in search of a Venusian civilisation. The British party in pursuit suddenly find themselves trapped in a cave by a huge primitive man.
The British space explorers, searching inside a Venusian volcano for Captain Wilson, are hunted down by a primitive species of men. Making their escape, they are trapped by roof falls and molten lava as the volcano erupts.
Cut off from their rocket by an active volcano, the astronauts start the long trek across the Venusian desert - and are attacked by pterodactyls. But these pre-historic flying reptiles are only a foretase of what is to come.
The astronauts have trekked across the vast Venusian desert to return to their rocket. But as they near MR4, volcanic ash sets the jungle aflame - and MR4 is in the centre of the inferno.