The Lord's spy(1964)

Maeda Tsunanori, the lord of Kaga Domain, had two young princes - Katsumaru, the son of his legitimate wife, who died, and Yasunosuke, the son of his concubine. However, Katsumaru suddenly lost his mind and appealed to the shogunate to hand over the reins of the family Yasunosuke, but the request was rejected. Vassal Osawa Kuranojo suspects that this is the work of Ishikawa Torajiro, a swordsman from the Katsumaru group, and begins an investigation.