The Mysterians

Astrophysicist Shiraishi Ryoichi, his fiancée Hiroko, his sister Etsuko, and his friend Atsumi Joji are witnessing 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 to astronomer Tanjiro Adachi. He hands the doctor a report written by Shiraishi. The report concerns a newly discovered asteroid that Shiraishi theorized was once a planet between Mars and Jupiter. He has named it Mysteroid
Atsumi, and a group of police officers stumble upon a giant robot, Moguera.
Mysterians, a scientifically advanced 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. Interbreeding with women on Earth would produce healthier offspring and keep their race alive, begins the mobilization of its armed forces to attempt to stop them with every weapon available.