From the publisher:

Mackenzie blasted off into the void, carrying sealed orders. With his exploration vehicle in orbit he watched the sidereal-chron ticking off the seconds. Mac opened his orders and read the microfilm like a man who reads his own death-warrant.

The X 12 was an experimental vessel and Mac was growing accustomed to strange, dangerous assignments. This was the most sinister work he had yet undertaken. His orders directed him to an enigmatic planet on the galactic rim, a world from which no astrogator had yet returned, a world protected by a screen of barrier-rays which defied all attempts at analysis. Mac thought he had succeeded and then the true nature of the ray field disclosed itself and he wished that death had been swift and merciful...

Somehow he staggered back to civilization. The terrible changes which had destroyed Mac's blemished body were not confined to him alone. The unknown menace began to spread....

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