It is the early twenty-first century. Man is seeking signs of life elsewhere in the univrse, but all exploratory ships have been lost without a trace--except for Deep Space Probe 15. Thirty years after leaving Earth, and given up for lost, DSP 15 suddenly appears on radar screens at the space station at Mildenhall, England.
Standard procedure for the DSP's is to have the crews frozen to prevent aging, and as the ship settles to a landing, Dr. Richard Warboys eagerly waits with other scientists for word of what DSP 15 has found. But there is no crew, only a message scratched into a metal surface, signed by the captain.
"If this ship returns to Earth, then mankind is in deadly peril--God help you--"
And so Earth becomes accidentally involved in a cosmic battle against a virtually omnipotent alien power, in a story suspensful and exciting from cover to cover.