For research purposes, Venus was visited by a spaceship of earthlings. On the way back, Dr. John Crawford was going about his routine—reviewing the examination results of each crew member—when he discovered something unexpected. One of the blood samples gave abnormal indicators with which a person simply cannot be alive! The paranoid doctor puts forward a crazy idea at first glance: what if some Venusian creature, capable of mutating, killed one of the earthlings and entered the ship, taking the form of a man?.. Could this turn out to be true?