Hiromi Oka, a first-year student at West High School, joins the tennis club admiring Reika Ryuzaki, aka Mrs. Butterfly, and senior Todo. She is a complete beginner, but as soon as she joins the club, the devilish coach, Munekata, sees her talent.
An alien woman named Maya crash-lands on Earth. Her homeworld was destroyed by the Blasters, a cruel alien race who steals the natural resources from other planets. She falls in love with a scientist and gives birth to a human boy named Kentaro. When the Blasters invade the Earth, Kentaro must defeat them by fighting with Astroganger, a robot made from living metal.
