The background of the story is set in the middle and late stages of the Pacific War. The Japanese Navy was defeated by the U.S. Navy in the Battle of Midway; four aircraft carriers of the First Mobile Fleet were sunk; the Commander-in-Chief of the Combined Fleet, Admiral Isoroku Yamamoto, died in the battle, and various islands in the Pacific were destroyed. The Japanese garrison troops were wiped out one after another, and the Japanese navy was in a seriously disadvantageous situation.