Aimi Koenji is a high school student afflicted by asthma and cat allergies, complications that intensify when feline presence enters her life. She nurses a longstanding, unreciprocated romantic attachment to childhood friend Hiroki Koizumi, a one-sided devotion causing her profound emotional anguish as Hiroki grows increasingly withdrawn and despondent after his girlfriend vanishes.

Her protective instincts manifest when she shields Hiroki from classmates' physical bullying and teachers' severe disciplinary measures. Despite Hiroki's emotional detachment and outright rejection during her home visit, she persistently voices care for his welfare. This vulnerability draws supernatural notice; she becomes prey to possessed teacher Miss Ogawa, who imposes an unjust cleaning punishment. Hyoi Kagezaki's intervention disrupts this encounter, escalating supernatural confrontations.

Aimi's emotional distress and despair over unrequited love ignite the story's climax. Her potent negative emotions accidentally rouse and magnify latent evil forces within peers and educators, mutating them into tentacled monstrosities. Ultimately, Aimi experiences a grotesque metamorphosis, emerging as a colossal amalgamated entity incarnating the collective darkness she unleashed.

Resolution arrives when Hiroki confronts Aimi's monstrous manifestation, acknowledges her feelings, and returns her affection. His sincere love confession acts as a purifying agent, nullifying the evil corruption. Their souls—united and cleansed—ascend together, signifying transcendence beyond physical strife.