'Zombies' is Krillie's name for them - although Kemlo and Kerowski explain that the men engaged in the Sonic Wave Experiment are risking their lives in the attempt to revolutionise man's powers of communication through space.

Yet even the leader of the Space Scouts and his friends are shaken at the odd behaviour of some of the S.W.V.s under text conditions thousands of miles out in space. Perhaps after all there are stronger forces at work than the earth scientists had anticipated. Kemlo is determined to find out.

But they have another problem as well. What is the mysterious weakness which haunts Alvin Searle, the stern and dominating figure in charge of the experiment - and can the space-born boys help him overcome it?