"Ingyaku Onigashima" is a manga series that blends elements of action, fantasy, and psychological drama. The narrative centers on a world where humans coexist with oni, supernatural beings often depicted as demons. The story begins in a society where oni are oppressed and enslaved by humans, who exploit their powers for labor and warfare. This dynamic is rooted in a long history of conflict and subjugation, with humans viewing oni as inherently dangerous and inferior.

The protagonist, a young human named Shiki, lives in a village on the outskirts of the Onigashima, a fortress-like island where oni are imprisoned and controlled. Shiki harbors a deep-seated hatred for oni due to a traumatic childhood event involving their kind. However, his perspective begins to shift when he encounters an enigmatic oni named Kurenai, who possesses a mysterious power tied to the ancient history of their world. Kurenai challenges Shiki's beliefs, revealing that the oni were once protectors of humanity, and their current state of oppression is the result of a betrayal orchestrated by human leaders centuries ago.

As the story progresses, Shiki becomes entangled in a rebellion led by Kurenai and other oni seeking to reclaim their freedom and dignity. The rebellion is not merely a fight for liberation but also a quest to uncover the truth behind the historical betrayal and the origins of the oni's powers. Shiki's journey forces him to confront his prejudices and grapple with the moral complexities of the conflict. Along the way, he forms alliances with both humans and oni, each with their own motivations and struggles.

The narrative explores themes of power, oppression, and redemption, delving into the psychological and emotional toll of living in a divided society. Key characters include Shiki, whose internal conflict drives much of the story; Kurenai, the charismatic and determined leader of the oni rebellion; and a cast of supporting characters who represent various factions within the human and oni worlds. The plot is marked by intense battles, political intrigue, and moments of introspection, as the characters navigate a world where the lines between hero and villain are often blurred. The series builds toward a climactic confrontation that challenges the established order and forces both humans and oni to reconsider their roles in shaping the future of their world.