The film explains why a famous Japanese writer, who was close to getting a Nobel prize in literature committed a ritual suicide by seppuku inside the Tokyo headquarters of the Japanese Ministry of Defence.
In the 70’s Yukio Mishima founded a military society which mostly included students and strived to regain the power of the Emperor and revive a traditional Japanese society. The society had bushido philosophy behind it, which is a set of moral rules, traditionally followed by samurai. According to wiki, Bushido stressed frugality, loyalty, mastery of martial arts, and honor to the death. Under the bushidō ideal, if a samurai failed to uphold his honor he could only regain it by performing seppuku.