Astronauts Glenn and Fuji are sent to investigate Planet X, a newly discovered planet located just beyond Jupiter. Back on Earth, Fuji's younger sister Haruno meets with her boyfriend inventor Tetsuo Torii. He has developed a sound device that he sells off to a toy company represented by a woman named Namikawa. On planet X, Glen and Fuji encounter mysterious aliens known as the Xiliens. They seek the aid of Earth's people to help save their world from "Monster Zero". In exchange for borrowing Godzilla and Rodan, the Xiliens offer a cure for cancer. As Glenn investigates, he develops a romance with Namikawa and uncovers the Xilien's true intentions.