During the Edo period, the eighth Tokugawa shōgun, Yoshimune, officially had 17 families of oniwaban, his private ninja guards. They were known as "The Secret 18th Group." Months after Yoshimune's death, his fourth son, Hitotsubashi, asks Kane Ametaro, a hairdresser, to put the group back together. The other members of the secret 18th group include Miki, a doctor; Otojirō, a chef; and Inokichi, an acrobat. The group members have their doubts and internal conflicts about how to live their lives as "secret agents," but when various injustices occur in Edo... they have no other option but to unite and begin clearing the darkness together.