"Inmu Gakuen" is a supernatural horror anime that revolves around a mysterious and eerie high school setting. The narrative centers on a group of students who become entangled in a series of disturbing and otherworldly events after encountering a cursed artifact within their school. The artifact, an ancient mirror, serves as a gateway to a malevolent realm, unleashing supernatural entities that prey on the students' deepest fears and insecurities.

The protagonist, a reserved and observant student named Akira, discovers the mirror during a routine cleaning session in an abandoned classroom. Upon touching it, Akira begins experiencing vivid and terrifying hallucinations, which soon spread to other students who come into contact with the mirror. These hallucinations manifest as grotesque and surreal visions, often tied to the characters' personal traumas or hidden desires. As the curse intensifies, the affected students are driven to paranoia, madness, and, in some cases, violent behavior.

Akira, along with a small group of classmates, including the pragmatic and resourceful Yui and the skeptical but loyal Ryo, attempts to uncover the origins of the mirror and break its curse. Their investigation leads them to uncover the dark history of the school, which was built on the site of a tragic incident involving a former student who was ostracized and ultimately met a grim fate. The group learns that the mirror is tied to this student's vengeful spirit, which seeks to exact retribution by trapping others in a cycle of fear and despair.

As the curse spreads, the students must confront their own inner demons while navigating the increasingly hostile environment of the school. The narrative explores themes of guilt, trauma, and the consequences of collective neglect, as the characters are forced to reckon with their past actions and the role they may have played in perpetuating the cycle of suffering. The story builds toward a climactic confrontation with the vengeful spirit, during which the students must make difficult choices to either break the curse or succumb to its influence.

The anime blends psychological horror with supernatural elements, creating a tense and unsettling atmosphere. The characters' development is closely tied to their interactions with the curse, as each must grapple with their own vulnerabilities and moral dilemmas. The plot is driven by a series of escalating events, with the stakes rising as the curse tightens its grip on the school and its inhabitants. The resolution of the story hinges on the characters' ability to confront the truth and find a way to break free from the cycle of fear and retribution.