Evil alien robot Ro-Man Extension XJ-2 , referred to as just Ro-Man, has seemingly destroyed all human life on Earth with a Calcinator death ray, except for eight humans that remain alive. The survivors are an older scientist named George, his wife Martha, his two daughters Alice and Carla, his young son Johnny, his assistant Roy, and two space pilots that shortly take off in a spaceship for an orbiting space platform. All eight have now developed an immunity to Ro-Man's death ray, having received an experimental antibiotic serum developed by the scientist.
Ro-Man must complete the destruction of all humans, even if it means his physically killing them one by one, before his mission to subjugate the Earth is complete. After fruitless negotiations, Ro-Man destroys the spaceship headed for the orbiting platform, killing the two pilots aboard. He later strangles Carla and tosses Roy to his death over a cliff.
Ro-Man's mission is waylaid, though, when he develops an illogical attraction to the elder daughter Alice. He refuses to eliminate her, forcing the alien leader, the Great Guidance, to kill the disobedient Ro-Man. The Great Guidance then attempts to finish the genocide by releasing prehistoric dinosaurs and a massive earthquake on the remaining survivors.
But Johnny is alive, having just awoken from a concussion-induced fever dream. Up to now, all that has happened has just been his nightmare. His parents, who had been looking for him, rejoice and take him home.
Suddenly, The Great Guidance, his arms raised in a threatening manner, walks out of the cave.
- George Nader
- Claudia Barrett
- Selena Royle
- John Mylong
- Gregory Moffett
- Pamela Paulson
- George Barrows
- John Brown
- The film's composer, Elmer Bernstein, later became known for composing memorable scores for such films as The Ten Commandments, The Magnificent Seven, The Great Escape, Animal House and Ghostbusters. It is likely that he took the job of scoring a low-budget film such as Robot Monster due to being "grey-listed" after being called before the House Un-American Activities Committee, where he was accused of writing reviews for a Communist newspaper.
- Robot Monster was reportedly shot in only four days. It utilized no sets, and was entirely filmed outside.
- Robot Monster was made on a budget of $16,000. A promotional piece in Variety listed a technically-true figure of "under $50,000." The producers Tucker and Zimbalist feared that potential viewers would dismiss the film without consideration if the actual budget was made public.
- Robot Monster was originally released in 3-D and was paired with the Three Dimension Pictures short Stardust in Your Eyes, starring nightclub comedian Trustin Howard as Slick Slaven.
- Robot Monster is generally acknowledged to be the first science-fiction film with stereophonic sound.
- Robot Monster grossed $1,000,000 during its initial theatrical release, more than 62 times its original investment, making it more successful than its reputation might tend to indicate. It also received positive reviews from some critics at the time.
- Director Phil Tucker originally intended Ro-Man to have a more conventional robot design. However, all of the robot suit performers he contacted were out of his price range. This prompted Tucker to conceive of having George Barrows (a friend) pair his gorilla suit with a diver's helmet.
- The scenes on the view screen presented by Ro-Man come from a variety of sources: among them, the shots of New York in apocalyptic ruins are matte paintings by Irving Block from Captive Women (RKO, 1952); the shots of the headquarters of the Great Guidance (a rocket ship in launching position) was originally created for Rocketship X-M (1950, produced by Robert L. Lippert), also painted by Block.
- The footage of the dinosaurs battling after the lightning flash comes from other films. The large lizards are from One Million B.C. (1940) (Hal Roach, 1940), supervised by Roy Seawright; the one brief shot of two stop-motion triceratops fighting is from Lippert Pictures' Lost Continent (1951), animator unknown.
- The film was not entirely filmed at Bronson Canyon. The scenes at the ruins of the home were shot in a residential hill area elsewhere.
- Close examination of the Ro-Man's helmet reveal it to be very similar to the helmets worn by the moon-men on the lunar surface in Republic Pictures' serial Radar Men from the Moon (1952).
- Ro-Man's helmet is slightly more elaborate than that of the Great Guidance. It is actually the Great Guidance, not Ro-Man who walks out of the cave at the end of the film.
- Received four nominations in The Golden Turkey Awards series: Most Ridiculous Monster, Worst Credit Line of All Time (for the Billion Bubble Machine), Most Idiotic Ad Line in Hollywood History, and Most Laughable Concept for an Outer Space Invader. It "won" the first one, and lost the others respectively to The Taming of the Shrew (for, "With additional dialogue by Sam Taylor"), Kwaheri, and The Creeping Terror.