A monster appears underground in Atsumi City, S Prefecture, a city famous for its hot springs. The supply of hot springs has stopped, putting the town's tourism industry in danger. Suffering from unpopularity, Mayor Kan'ichi Mangaki personally pilots a drill machine developed by his late father, Tetsuzaemon Mangaki, and heads underground to defeat the monster. Awaiting him there are giant monsters and invaders. "Fight, Kan'ichi!" When you shout, the drill machine transforms into a steel warrior, the Bass Diver! Can the young second-generation politician defeat the monster, save the town of Atsumi, and restore his approval rating?