In late Meiji-era Tokyo, where Western influences intermingle with lingering traditions and the hidden world of yōkai, the folklore scholar and exorcist Kantarou Ichinomiya makes his living by solving supernatural troubles. Kantarou was ostracized as a child for his ability to see spirits, but he grew up befriending them. Years earlier, he freed a legendary oni-eating tengu from a long seal, naming him Haruka and binding him as his servant through a name contract. The human master’s command is absolute, yet Kantarou calmly insists that “yōkai are friends,” and over time Haruka begins to trust him. They live together with Youko, a fox yōkai who serves Kantarou and often worries about their perpetual lack of money.

However, their peaceful daily life begins to fracture when Haruka’s instincts as a “oni-eater” awaken. The military commander Minamoto no Yorimitsu, a powerful state-sanctioned exorcist and head of the Genji clan, has long sought Haruka—specifically, the immortal-giving “gyoku” (sphere) inside Haruka’s body. Yorimitsu manipulates events to separate the pair, and during that time a deeper secret emerges: Kantarou has already merged body and soul with his ancient nemesis, the Sixth Heaven Demon King (Dairokuten Maou), who once devoured his own mother. No longer fully human, Kantarou sees only one way to end the threat—he wants Haruka to eat him, consuming the demon king along with his own flesh. Haruka, horrified by the urge he feels to devour Kantarou and by the betrayal of the trust they built, must confront whether eating his master is the only salvation or whether a different path exists.

The central arc of Tactics Shinsetsu follows this tragic dilemma. Kantarou conceals his intention while Yorimitsu and his allies scheme to obtain Haruka’s sphere, and other yōkai—such as Sugino, a tengu who considers Haruka an old friend, and his wife Mu-chan—get drawn into the conflict. The narrative resolves in a final confrontation where Haruka must choose: kill Kantarou to stop the demon king or find a way to save him without breaking their bond. The series brings the long-running story of the original Tactics to a close, weaving together themes of loyalty, sacrifice, and the blurred line between predator and protector.