Robot Monster (also released as Monster from Mars) is a 1953 American independent 3D science-fiction horror film produced and directed by Phil Tucker, and starring George Nader, Claudia Barrett, and George Barrows. The film follows an alien robot who, while on a mission to destroy Earth and humanity, betrays the orders of his master, the Great Guidance, by protecting a woman he was tasked to murder.

The film's production lasted only 4 days and took only $16,000 to produce. Production subsequently wrapped on March 23, 1953. Due to the constraints of the film's minuscule budget, Tucker opted to hire Barrows who had made his own Gorilla Suit, along with other adjustments. It was shot in only a handful of locations including Bronson Canyon. The finished film was also printed in a dual strip, polarized 3D, which many critics found to be very high quality, and also added an additional $4,000 to the budget. While containing numerous stock footage found in other productions, Robot Monster notably also beared plot similarities to Invaders From Mars (1953), a film that was distributed by 20th Century Fox a month earlier.

The film was distributed in the United States by Astor Pictures on June 24, 1953. The film received mixed reviews from contemporary critics, and was a box-office success, grossing $1 million. However in later decades, Robot Monster is often remembered as one of the worst films of all time, with its plot and production values receiving criticism.

In December of that year following the film's release, Tucker attempted suicide due to his lack of compensation for his work on the film. Tucker survived following detectives responding to his suicide note which he had posted in a newspaper prior. The reason why Tucker was not compensated remains unclear.

Ro-Man Extension XJ-2, referred to as just Ro-Man (a creature with an ape-like body and an antenna-equipped helmet covering its head and emitting electric flashes), has destroyed all human life on Earth with a Calcinator death ray to prevent Humanity from challenging his race, except for eight humans who remain alive in a protected facility. The survivors are a professor, whose name is never mentioned; his wife; their two daughters, Alice and Carla; their young son, Johnny; the professor's assistant, Roy; and space pilots Jason and McCloud (neither of whom is seen or heard). Both pilots depart in a rocket ship for an orbiting space platform. All eight have developed an immunity to Ro-Man's death ray, having received an experimental antibiotic serum developed by the professor intended to cure all diseases.

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 Jason and McCloud's spaceship that had been sent into space to escape, along with the space platform. The Professor uses the machine intended to contact Jason and McCloud to instead contact Ro-Man, pleading for him to let them go as they pose no threat to him. Ro-Man instead demands to talk to Alice alone, who agrees despite her family's protests. When Johnny leaves to find Ro-Man, Alice ventures out with Roy to find him. Johnny confronts Ro-Man and reveals his immunity to the death ray, causing Ro-Man to begin planning a method to counteract this immunity.

Alice and Roy find Johnny, and after inexplicably kissing each other while still in danger, return to the facility. Alice and Roy announce to the professor their desire to marry. The Professor then declares it the biggest event of the year. Ro-Man contacts Great Guidance (referred to as "The Great One"), leader of the Ro-Man Empire, to announce his plan, and is told that he must complete this goal before the Earth finishes revolving. The marriage is conducted, with the Professor asking the Lord for a victory in Mankind's struggle. Ro-Man discovers Carla outside the facility and strangles her.

Ro-Man's mission is waylaid, however, when he develops an illogical attraction to Alice and cannot bring himself to eliminate her, asking the Great One if he may keep one human, but the Great One demands that he continue the mission. Ro-Man rediscovers Alice and Roy, and throws Roy off a cliff. He explains to Alice that the source of his energy lies in his cave, where he takes her and attempts to woo her before being interrupted by the Professor's broadcast. They ask for a quick death at the ravine, and Ro-Man agrees. Immediately after this, the Great One contacts him, angry at him for thinking up the plan to keep the girl and accusing him of bearing free will in violation of Ro-Man law. He gives Ro-Man one last chance to destroy the girl, which Ro-Man refuses to. Johnny distracts him, allowing the family to save Alice. Both Johnny and Ro-Man are suddenly killed by the Great One with a Calcinator blast. The Great One continues the genocide with Cosmic Q-Rays, which cause prehistoric reptiles to appear; and psychotronic vibrations, which "smash the planet Earth out of the universe".