Dr. Tom Anderson (Van Cleef), an embittered scientist, has made contact with a Venusian creature, while using his radio transmitter. The alien's secret motivation is to take complete control of the Earth by enslaving humanity using mind control devices; the alien claims it only wants to bring peace to our troubled world by eliminating all emotions. Anderson agrees to help the creature and even intends to allow it to assimilate his wife (Garland) and friend Dr. Nelson (Graves).
The Venusian then disrupts all electric power on Earth, including motor vehicles, leaving Dr. Nelson to resort to riding a bicycle.
After killing a flying bat-like creature which carries the mind control device, Dr. Nelson returns home to find his wife newly assimilated. She then attempts to force his own assimilation using another bat-creature in her possession, and he ends up being forced to kill her in self-defense. By then, the only people who are still free from the Venusian's influence are Nelson, Anderson, Anderson's wife and a group of army soldiers on station in the nearby woods.
Nelson finally persuades the paranoid Anderson that he has made a horrible mistake in blindly trusting the Venusian's motives, allying himself with a creature bent on world domination. When they discover Tom's wife has taken a rifle to the alien's cave in order to kill it, they hurriedly follow her, but the creature kills Claire Anderson before the two doctors can rescue her. Finally, seeing the loss of everything he holds dear, Dr. Anderson viciously attacks the Venusian by holding a blowtorch to the creature's face; Anderson dies at the alien's hand as it expires.
It Conquered the World was written by Lou Rusoff, but before being completed, Rusoff's brother died and he had to leave for Canada. Corman then called in Charles Griffith to do a final rewrite, two days before filming began; Griffith, however, didn't want his name to be credited on screen. Griffin does has a small part as a scientist.
The design of the creature was Corman's idea, and he thought that coming from a big planet, It would have evolved to deal with heavy gravity and would therefore be low to the ground. Corman later admitted this was a mistake, saying the creature would have been more frightening had It been larger or taller. When Beverly Garland first saw the creature, she commented "That conquered the world?" and kicked It over. Based on this action, the design of the creature was reworked.
The creature's working pincers were broken on the first day of shooting, but its arms could still be raised. The melting eye effect was completed using chocolate syrup.
Griffith on the creature prop:
I called it Denny Dimwit and somebody else called it an ice-cream cone. I was around when Paul Blaisdell was building it, and he thought the camera would make it look bigger. I have some photographs of it in construction, probably the only ones in existence. I asked for my name not to be on that picture, so I was unbilled. Surprisingly, it got good reviews.
It Conquered the World was released theatrically by AIP in July 1956 on a double bill with The She Creature.
The movie originally received an "X" certificate in the UK, meaning the picture could only been seen by adults. The issue was that the scene of the creature being destroyed by a blowtorch was seen as animal cruelty. However, producer Samuel Z. Arkoff was able to convince the film board that the violence was against a otherworldly person, and not an animal, earning the film a passing certificate.
During the 1960s, the film was syndicated to television by American International Television. VHS versions appeared in the 1990s on the US home video market (RCA Columbia Home Video) but these are no longer in distribution, nor is the film available on DVD or Blu-ray in the US or in the [UK.
Film historian and critic Leonard Maltin called It Conquered the World, "... well acted and interesting but awkwardly plotted". Variety found the movie a cut above normal, despite its low budget, and praised the "remarkable adult questions" asked by the screenplay.
Time Out magazine, however, gave the film a negative review, criticizing its poor special effects. Critic Tony Rayns opined, "You have to see a movie like this to realise that film-makers who feel they have nothing to lose are rarer than you'd think".
The Chicago Reader gave the film a generally positive review, saying, "Amazingly, this 1953 [sic] picture isn't half bad".Allmovie gave the film 3 out of 5 stars, calling it an "above-average quickie".In 1966, It Conquered the World was remade in 16mm color by self-proclaimed "schlockmeister" Larry Buchanan after he secured rights from AIP; he retitled his rewritten remake Zontar, the Thing from Venus and then sold it to television syndication.
Frank Zappa's 1974 live album Roxy & Elsewhere referred to the film in the introduction for the song "Cheepnis".
The end of the film was shown at the beginning of Elvira, Mistress of the Dark (1988).
In 1991, It Conquered the World was the subject of the comedy television series Mystery Science Theater 3000; joke topics included poor monster props, occasionally wooden acting and an overblown closing monologue.
Audio samples from the film were included in the song "Facing That" from M83's self-titled debut album, released in 2001.