Returning from their unexpected trip to Mars the crew of MR4 intercept a distress signal from Captain Wilson, a U.S astronaut, and must change course for Venus to attempt a rescue in space. However, through the space periscope, Brown sees what appears to be a settlement or a city of some sort. Desperate to investigate, Brown edits Wilson's message and tricks the crew into landing Venus so as they can rescue him.

Professor Norman Wedgwood and his team are planning another rocket mission to the Moon. Once again, Geoff, Valerie and Jimmy are on hand to witness the launch of a new rocket MR1, along with journalist friend Conway Henderson. However, when the automatic supply ship MR2 fails to launch Henderson and the children make a desperate rescue attempt with Jimmy's pet Hamlet.

A secondary mission in a new rocket, MR4, to the Moon takes off from Buchan Island. This time Henderson takes the lead role as pilot accompanied by Professor Wedgwood's oldest son Geoff as radio operator, Professor Mary Meadows, Henderson's niece Margret along with Hamlet. However one of the crew turns out to be science writer named Harcourt Brown who has plans to divert the ship to Mars determined there is life on the planet. Brown succeeds in getting MR4 to Mars, but with the length of the journey, the crew decide that the only way to get home is to find water on Mars.