Submarine Super 99

Submarine Super 99 (潜水艦スーパー99, Sensuikan Sūpā Nain Nain) is a Japanese manga series written and illustrated by Leiji Matsumoto from 1970 to 1972. It was adapted into an anime that aired on AT-X from May 8 to July 31, 2003.

Plot
In 20XX, Earth's ocean is taken over by a mysterious "ocean empire". The empire's military commander in chief, Hell Deathbird, demands that all shipping request his permission for sailing. Earth's only hope is the submarine called Super 99, a mysterious submarine designed by Dr. Juzou Oki.

The story begins as Japanese forces decided to investigate the mysterious threat beneath the ocean. Goro Oki gives his younger brother, Susumu, a rifle, a weapon modified from Type 99 rifle of the World War II era; Susumu is also given a key bearing a "99" marck by his grandfather Juzou. Goro and Juzou Oki set off in an exploration submarine to investigate the attacks in the Japanese Trench; soon later, their vessel is attacked by a mysterious submarine hiding in the Trench. The surface survey ship loses contact with the survey submarine, and two spies storm her command bridge, taking the crew hostage. The spies are overpowered as the mysterious submarine attempts to torpedo the survey ship, and Susumu manages to destroy the incoming torpedo with his rifle.