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An alien is sent on a mission to Earth from the planet Davanna, where the population is dying as the result of constant atomic warfare. The alien assumes the appearance of a human, calls himself Paul Johnson and never allows himself to be seen without dark glasses. As Johnson's assignment is to send samples of human blood back to Davanna for study and potential use in saving the planet's citizens, he is systematically killing people at random and draining their blood with a machine he carries in an aluminum briefcase. In need of blood himself, Johnson visits the offices of Dr. F. W. Rochelle. After Rochelle studies Johnson's alien blood under a microscope and discovers that it is self-destructing, Johnson hypnotizes Rochelle into helping him and mentally blocks the doctor from revealing the details of his case to anyone. Johnson then hires Rochelle's head nurse, Nadine Storey, to move into his house to supervise his transfusions. There Nadine meets Johnson's servant, Jeremy Perrin, whom Johnson had threatened to kill when he spied him storing flasks of blood. Later, Johnson telepathically communicates with his commander on Davanna, who informs him that they will soon need a live specimen for vivisectional research. When Johnson tells the official that his survival depends upon his tolerance for Earth inhabitants' blood, the commander advises Johnson that, if he lives, Davanna will invade and take over Earth, but if Johnson thinks he is going to die, he is to destroy Earth first. Later, Johnson receives a visit from an overeager, vacuum cleaner salesman, whom he kills by his customary method of projecting a burning beam from his blank, pupil-less eyes. After extracting the salesman's blood, Johnson disposes of his body in the cellar furnace. Later, when Jeremy is driving him to an appointment, Johnson sees three drunken bums at the roadside and orders Jeremy to invite them to dinner that evening. Meanwhile, alone in the house and puzzled by her patient, Nadine goes investigating and, in the cellar, finds flasks of blood. Nadine is interrupted by the arrival of Rochelle and her boyfriend, Harry Sherbourne, who is a motorcycle patrol officer. When Johnson and Jeremy return shortly thereafter, Harry recognizes Jeremy as a minor crook. Rochelle, still working under Johnson's hypnotic command, assures Johnson that he is hoping to discover the reason for Johnson's disintegrating blood. While Nadine and Harry are on a date, Jeremy brings the drunks to the house and Johnson kills them, then sends more blood samples by his spatial transportation device to Davanna. Meanwhile, the local police are mystified by the number of corpses they have discovered with burned eyes and brains and very little blood in their bodies. While prowling the streets at night, Johnson finds a Chinese-American man, hypnotizes him, takes him back to the house and sends him to Davanna via his space-dimension-warp device. Nadine becomes concerned when Jeremy tells her that the drunks he brought to the house appear never to have left. At a newsstand, a Davannan woman wearing dark glasses contacts Johnson and informs him, telepathically, that she has escaped from the war-torn planet and that the transmission of the live specimen was unsuccessful. As the woman is in urgent need of blood, Johnson breaks into Rochelle's office after hours and transfuses her with blood stored in Rochelle's refrigerator, but is unaware that the flask contains rabies-infected blood. Johnson then directs the woman to a hotel, but she becomes ill, returns to Rochelle's building and dies on the steps outside it. While at dinner with Harry and Rochelle, Nadine becomes alarmed when the doctor refuses to discuss Johnson's case, leading her to believe that Johnson has some kind of hold over him. Later, after Rochelle and Harry learn the details of the dead woman's condition and note the similarity of her appearance to Johnson, Harry phones Nadine to warn her that Johnson may be an alien and that she should leave the house immediately. However, she and Jeremy decide to stay and search for evidence of Johnson's true identity. Soon after, Johnson returns and listens in as Rochelle phones to inform Nadine how the woman died. After finding a human skull in the furnace, Jeremy tries to shoot Johnson, but is killed by his burning gaze. When Johnson attempts to kill Nadine, her loud screams interrupt him, as he cannot tolerate loud noise, and she is able to escape from the house. Johnson then creates a small, teapot cosy-like creature, which he dispatches to Rochelle's office, where it settles over the doctor's head and kills him. Nadine reaches a phone and summons Harry, but Johnson eventually finds her and hypnotizes her into returning to the house. Planning on sending her to Davanna to become the replacement live specimen, he instructs her on how to activate the beam device. After Harry arrives at the house, Johnson drives off in his car and Harry chases him on his motorcycle. While Johnson is attempting to fix Harry with his gaze, the car crashes over a cliff and Johnson is killed, breaking his hold on Nadine, in the house, just as she is about to experience interplanetary travel. Later, as Nadine and Harry leave Johnson's grave, a male Davannan, dressed identically to Johnson and carrying an identical briefcase, approaches from the distance.