An MSDF submarine collides with a U.S. nuclear submarine, crushing all 76 people on board, including its CO, Shiro Kaieda. However, the crew survives. The accident is a cover story to get the MSDF submarine’s crew on board the Seabat, a nuclear submarine secretly built by the Japanese and U.S. governments. However, Kaieda loads the Seabat with nuclear missiles and suddenly mutinies and flees.