An alien civilisation prepares to conquer Earth - and only four men can save her...
The great fleet was drawing near the Earth - the twenty-one leading spheres flying in crescent formation and, behind them, the others stretching back into Space for hundreds of miles...
Jet Morgan and the crew of the Discovery return to Mars on the most dangerous and vital mission ever undertaken by man - to obtain the Martian plan for the conquest of Earth. Too late, they find themselves part of the invasion fleet.