In the Kyōhō era under the Tokugawa shogunate, a conspiracy to usurp the household unfolded at Shinshū Kurohime Castle. The lord, Yūki Takanobu (played by Kurama Hayatarō), was targeted by his concubine O-Ran (Ayukawa Tōshiko) and her son Harunosuke, conspiring with the evil steward, Ōnuki Jūtayū (Kagawa Ryōsuke). Takanobu was assassinated by Jūtayū's swordsman Totsuka Denpachirō (Morozumi Keijirō) during a hunting expedition. The righteous elder retainer, Mizuki Shōzaemon, faced a crisis after supporting the legitimate heir, Takechiyo.