Ichirou Aototou practices Western medicine, an unconventional approach in the Edo period. This medical role serves as a cover for his primary assignment: covertly safeguarding Usugumo, the daughter of Shogun Tokugawa Yoshimune and a Christian courtesan. Political conflicts surrounding Usugumo's lineage make her a target for court assassins seeking to eliminate the Shogun's secret.

He collaborates with Usugumo and the thief Kikunosuke, forming the thief group Kikugumi. Within this group, he engages in activities combining vigilantism with protective operations, confronting threats including the aggressive heir of the Kaga clan and assassins targeting Usugumo. He integrates his medical knowledge into both his undercover work and combative support for the group.

The character operates within Edo's Yoshiwara red-light district, where Usugumo works as a high-ranking courtesan. His relationships define his role—maintaining a professional yet protective bond with Usugumo and a cooperative partnership with Kikunosuke during their missions.