The Gray Man (story)

"The Gray Man" is a short story by science fiction author Bob Shaw. The story is included in the fixup novel Ship of Strangers.

Concerning a crew of astrographical surveyors, the theme of "The Gray Man" is that of discerning a genuine thing from a fake, presented in the form of a logical puzzle.

The crew of the survey ship discover an alien species which is able to mimic anything at all to perfection, and control minds and bodies that fall within its range. In true genre tradition, the alien gets its come-uppance faced with human ability to reason logically.