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.