A kid inherits his father's submarine and discovers that knowing how a machine works doesn't prepare you for what people will do to take it from you. The underwater setting strips away escape routes, you can't run from what's hunting you down there, only go deeper. Helmet Party isn't a shadowy empire waiting in act three; they're closing in from episode one, and every depth the Super 99 descends becomes another cage. This is a show built on the principle that a genius weapon in the wrong hands doesn't create a hero story, it creates a hunted kid who has to think faster than he ever imagined. The real conspiracy isn't what Susumu's chasing. It's what chases him back.

When the gifted scientist Dr. Oki, creator of a new kind of submarine, goes missing, his son Susumu senses that an unknown group is behind something sinister. Susumu understands every detail of his father's vessel, the Super 99, from its equipment and weapons to its functions and limits. He believes that tracking down his father means exposing a secret organization called the Helmet Party and halting its plot. Joined by his friends and the Marine Corps, he heads beneath the waves on a quest whose dangers he does not yet grasp. The result is a science fiction action adventure told over thirteen episodes.