A Martian spacecraft crash-lands near the observatory of Cyrus Layton, designer of Earth's first spaceship. The survivor, forerunner of an invasion, can assume the form of any earthman. Calling himself the "purple monster," the humanoid invader sets about gaining control of Layton's rocket project. Opposing him is Craig Foster, former Secret Service man, who episode by episode tries to thwart the monster's attempts to acquire rocket components. Will Craig ever suspect that his closest associate is really the monster?