Dr. Karloff prisons an alien from the Leguminous Planet and calls his friend Dr. Marins to help him in his experiences, because the Leguminous Monster is an anarchist intellectual full of hate against the human beings. While the scientists discuss about the monster, Caquinha is convinced for him to let him go. Done that, the Monster kills the scientists and kills everyone he finds in his way.
After refusing to pay their property taxes, a house of cannibals prey on the government workers who come to collect.
The day after the carnage initiated by the Martian scientist, a fleet of spacecraft from Mars is called upon to invade Earth, but Dr. Rottenberg's plans are hampered by two NASA officials, Detective Johnson and Dr. Nagib, called in to investigate. the case.
