Invasion of Astro-Monster, known in The United States as Godzilla vs. Monster Zero, is a sequel to Ghidorah. The film was released in 1965 in Japan and in the U.S. five years later. It is notable because it is the last Godzilla film to feature the creative team of director Ishiro Honda, screenwriter Shinichi Sekizawa, and special effects director Eiji Tsuburaya.

The film opens with a rocket ship heading toward Planet X, “a mysterious planet…discovered beyond Jupiter.” The crewmembers are Japanese astronaut Fuji and American astronaut Glen. When they get to the planet, they discover aliens who live underground because a creature known as Monster X has made the surface uninhabitable with its wanton destruction.

The alien leader is known as The Controller. He requests the Earthlings’ help in battling Monster X, who turns out to be King Ghidorah, last seen flying away from a battle with Mothra, Rodan, and Godzilla. The aliens request that they be allowed to bring Monster 01, Godzilla, and Monster 02, Rodan to their planet. In exchange with being allowed to retrieve the monsters from Earth, The Controller offers a miracle drug to cure all disease. As the astronauts leave, The Controller issues a maniacal laugh.

Once the astronauts return, the Supreme Council agrees and seeks to determine if the monsters are where The Controller said. As they search a lake Godzilla is supposed to be in, The Controller and his spaceships are discovered already on Earth, casting suspicion on their actions. After Rodan is collected, Glen and Fuji return with the aliens to Planet X. The monsters battle and Ghidorah is driven away.

But something isn’t right as Glen discovers the women on Planet X all look like his girlfriend back on Earth. The astronauts return home with the medical secrets, but unbeknownst to them they’ve been had. When the scientists play the audiotape that is supposed to contain the miracle cure formula, the people of Earth are order to surrender and become a colony of Planet X. The aliens threaten the people by explaining that they control the monsters through magnetic waves and set a deadline of 24 hours for humanity’s capitulation. Of course, by boasting of their plans, the aliens provide a solution.