Astrophysicist Ryoichi Shiraishi, his fiancee Hiroko, his sister Etsuko, and his friend Joji Atsumi attend a festival at a local village near Mount Fuji. Shiraishi then tells Atsumi that he has broken off his engagement with Hiroko but gives no reason other than an undisclosed obligation to remain in the village. Then, a mysterious forest fire flares up, burning more rapidly than normal and emanating from the ground, and Shiraishi rushes out to investigate and disappears during the confusion.
The next day, Atsumi is at the local observatory, where he meets with his mentor, head astronomer, Tanjiro Adachi. He hands the doctor a report written by Shiraishi which concerns a newly discovered asteroid that Shiraishi theorized was once a planet between Mars and Jupiter. He has named it Mysteroid. However, Adachi does not believe in his radical theory and also points out that the report is not complete.
Meanwhile, the village in which the festival was held is completely wiped out by a massive earthquake. While investigating the area, Atsumi and a group of police officers stumble upon a giant robot, Moguera, which bursts from the side of a hill. It emits rays which decimate the investigation team; only Atsumi and the lead policeman survive. The robot then advances to a town near Koyama Bridge that night, and is met by heavy resistance from Japan's self-defense force. However, the conventional artillery has no effect on the war machine, and the automaton continues its rampage until it tries to cross the Koyama Bridge, which is detonated sending the machine crashing down to the ground below, destroying it.
At the Diet Building, Atsumi briefs officials on what has been learned about the robot. The remains of the giant machine reveal that it was manufactured out of an unknown chemical compound. Shortly afterwards, astronomers witness activity in outer space around the moon. They alert the world to this discovery, and not long after the aliens emerge, their gigantic dome breaking through the Earth's crust near Mount Fuji. As a combined military and scientific entourage observe the dome, a voice calls out to request five scientists, including Dr. Adachi, who is among the observers, come to the dome for a conference. The men agree to this meeting and are formally ushered into the dome, where the Mysterians, a scientifically advanced humanoid alien race, list their demands from the people of Earth: a two-mile radius strip of land and the right to marry women on Earth. The reason for this is that 100,000 years ago their planet - Mysteriod, the once fifth planet from the sun - was destroyed by a nuclear war. Fortunately, some Mysterians were able to escape to Mars before their planet was rendered uninhabitable. However, due to the nuclear war, Stronium-90 has left 80 percent of the aliens' population deformed and crippled. The proposed interbreeding with women on Earth would produce healthier offspring and keep their race alive. The latter part of their demands is downplayed as they admit to already taking three women captive and reveal two others that they are interested in, one of which is Etsuko.
Japan quickly dismisses this request and begins the mobilization of its armed forces around Mount Fuji. It is also discovered that the missing Shiraishi has sided with the advance race due to their technological achievements. Japan wastes no time, though, and quickly launches a full-scale attack against the Mysterians' dome. However, the modern weaponry is no match for their technology, and Japan's forces are easily fought back. Distraught by this setback, Japan sends their plea to other nations that they might join together to remove the threat of the Mysterians from Earth. The nations around the world answer the plea, and in no time issue another raid against the Mysterians' dome, this time utilizing the newly developed Alpha and Beta class airships. Sadly, this attack meets failure as well.
The Mysterians then increase their demand, asking for a 75-mile radius of land, as the Earth continues to develop a new method of attack. Earth's efforts in this matter pay off as a Markalite FAHP (Flying Atomic Heat Projector), a gigantic lens that can reflect the Mysterians’ weaponry, is designed. Meanwhile, the Mysterians kidnap Etsuko and Hiroko, causing Atsumi to search for, and locate, a cave entrance to a tunnel under Mysterians' dome.
In the meantime, the Markalite FAHP's are deployed by large Markalite GYRO rockets and the final battle against the Mysterians' base of operations commences. Atsumi enters the dome and finds the women, alive and unharmed, in an unguarded room. Taking them back to the tunnel, Atsumi finds Shirashi, who admits the Mysterians duped him and have no good intentions, then returns to the dome and sacrifices himself in a final attack on the base from the inside while the Markalite FAHP's assault the base from above ground. The dome collapses and then explodes as Adachi and the women reach safety in the hills above the Mysterians' occupied land. A few enemy spaceships are observed fleeing into space, out of range of Earth weaponry, and Dr. Adachi comments on the need for continued vigilance.Kenji Sahara as Jōji Atsumi
Yumi Shirakawa as Etsuko Shiraishi
Momoko Kōchi as Hiroko Iwamoto
Akihiko Hirata as Ryōichi Shiraishi
Takashi Shimura as Dr. Kenjirō Adachi
Susumu Fujita as General Morita
Hisaya Itō as Captain Seki
Yoshio Kosugi as Commander Sugimoto
Fuyuki Murakami as Dr. Nobu Kawanami
Tetsu Nakamura as Dr. Kōda
Yoshio Tsuchiya as Mysterian Leader
The Mysterians was released in 28 December 1957 in Japan. The film was re-issued theatrically in Japan on 18 March 1978.
In the USA, The Mysterians was originally purchased by RKO Radio Pictures, which provided the dubbing, but was sold to Loew's Inc. for release due to RKO's failing fortunes. The film was double-billed with Watusi and released in 15 May 1959 via MGM. According to MGM records the film made the studio a profit of only $58,000 in the USA.
From contemporary reviews, The New York Times called the film "an ear-splitting Japanese-made fantasy, photographed in runny color and dubbed English," and concluded: "This Metro release is crammed with routine footage of death rays and scrambling civilians, not one of whom can act." Variety called it "well-produced", noting "special effects involving sliding land, quaking earth and melting mortars are realistically accomplished proving the facility with the Japanese filmmakers deal in miniatures." but found the film "As corny as it is furious" noting that "While Junior may be moved by the arrival of outer-space gremlins, big brother and all like him will laugh their heads off." The review commented on the English dub, stating that it was "understandable enough, but one might easily believe something was lost in translation." The Monthly Film Bulletin noted that the version the reviewer viewed was a "banal American-dubbed version" and that its "main weaknesses are a slight and confused plot, under-developed characterisation and artless acting;" The review praised the films "imaginative art direction and spectacular staging" which the review stated as "possibly the most dazzling display of pyrotechnics in the genre to date."
In a retrospective on Soviet science fiction film, British director Alex Cox compared The Mysterians to First Spaceship on Venus but remarked the latter as "more complex and morally ambiguous." AllMovie praises the film for its special effects. In a retrospective review, Sight & Sound found that "Its space-age visuals and colourful design anticipate the spectacular fantasies Honda would go on to make for Toho in the [1960s], including Mothra, Godzilla vs. The Thing, Ghidrah The Three-Headed Monster and Invasion of the Astro-Monsters."