“Mr. Pratt is well known for .… his excellent knowledge of warfare, and what a future war might be like. In this story he combines that knowledge with a vivid and fertile scientific imagination to construct an interplanetary story …. many scientists believe that life may originally have come to earth in the form of spores, from other solar systems and other universes. We therefore might really have had our home dim ages ago, on worlds distantly removed from our earth. The ability to travel the interstellar spaces, however, might also be possessed by other creatures—creatures driven by fear, necessity and by the will to conquer. And if they come, in mighty waves, with scientific powers far beyond us, to dominate the earth, a terrible time will face the puny human race. And in this story they do come, and provoke some of the strangest and most exciting adventures that have yet been recorded. (Summary excerpted from the Preface.)