The plot concerns an American astronaut, Frank Douglas, who mysteriously disappears from his spacecraft as it parachutes to Earth. The policeman in one scene inspect the landing site of Douglas's capsule and notices a burned patch, only to dismiss it as a prank. The vanished astronaut is apparently replaced by or turned into a large, radioactive, humanoid monster. This is revealed when it comes into the scene and kills off Dr. Logan. A team of scientists and military men also attempts to capture the monster ? and at one point succeed and imprison it in the lab, only to have it escape. Neither the capture nor the escape is ever shown, and both are simply mentioned by the narrator.
At the end of the film, the scientists corner the monster in a sewer under Chicago, but the monster suddenly disappears. The scientists receive a telegram stating that Douglas is in fact alive and well, having been rescued in the North Atlantic, perhaps implying the monster was an alien impersonating Douglas. The narrator provides the film's closing dialogue:
As if a switch had been turned, as if an eye had been blinked, as if some phantom force in the universe had made a move eons beyond our comprehension, suddenly, there was no trail! There was no giant, no monster, no thing called "Douglas" to be followed. There was nothing in the tunnel but the puzzled men of courage, who suddenly found themselves alone with shadows and darkness! With the telegram, one cloud lifts, and another descends. Astronaut Frank Douglas, rescued, alive, well, and of normal size, some 8,000 miles away in a lifeboat, with no memory of where he has been, or how he was separated from his capsule! Then who, or what, has landed here? Is it here yet? Or has the cosmic switch been pulled? Case in point: The line between science fiction and science fact is microscopically thin! You have witnessed the line being shaved even thinner! But is the menace with us? Or is the monster gone?
