Following the Cold War, the Japan Maritime Self-Defense Force jointly and secretly developed a nuclear submarine, the Seabat, with the United States Navy.

On the Seabat's maiden voyage, Kaieda abruptly takes control of the submarine, re-names it the Yamato and declares it to be an independent state.

Kaieda and his crew intend sailing to Tokyo to form a formal alliance with Japan, and the United States Navy musters its entire Pacific fleet (including its most modern warships) to stop him.

In the second OVA episode, "Voyage 2", the Yamato enters Tokyo Bay and a special meeting is arranged between Kaieda and the Japanese government.

At this meeting, which is attended by the world press and televised live, a formal Treaty of Mutual Assistance is eventually concluded between Japan and the Yamato.

The film is directed by Kohei Yoshino, with scripts written by Hikaru Takai, and Shinzō Matsuhashi and Takao Osawa serving as producers.

The film was released by Toho and premiered in Japanese theaters on September 29, 2023,[13] and was streamed on Amazon Prime Video as 8-part web series on February 9, 2024.