A young girl named Coco lives alone in a large mansion on the outskirts of a strange town. Her only companion is her beloved black cat, Kuro. While Coco sees her life as idyllic, filled with fun-filled daily adventures with her pet, a far more sinister reality lurks just beneath the surface of her peaceful existence.

Unbeknownst to Coco, monstrous entities roam the area around her home, a constant threat that the townspeople are all too aware of. Most residents receive regular injections from childhood that allow them to perceive these creatures, but Coco has not, leaving her blissfully ignorant of the danger. The townsfolk themselves have varied attitudes toward the girl; some are friendly and supportive, while others remain distant or openly hostile, their resentment often directed at the unusual cat she keeps. This mistrust stems from a mysterious, tragic incident connected to Coco's family manor that occurred years ago, an event that also robbed Coco of her early childhood memories.

The narrative follows the delicate balance of Coco's happy ignorance and the grim reality surrounding her. Her friends, Maria and Milk, try to maintain their relationships with her, though they are visibly uncomfortable around Kuro, whose true nature is anything but feline. When Coco is not looking, Kuro reveals his otherworldly essence, his eyes shifting or sharp tentacles emerging from his mouth. The town doctor theorizes that Kuro is the result of a parasitic monster worm infecting a dying cat, creating a previously unseen symbiotic relationship. Other key figures include Brenda, Coco's tutor, and her young daughter Sesame, who are among those who care for the girl.

As the story progresses, the underlying tensions begin to surface. Kuro's appetite diminishes, and Coco's health starts to decline, causing her to become increasingly reclusive and withdrawn from the town. Concerned for their friend, Maria and Milk enter the manor to help Coco. This intervention triggers the return of Coco's suppressed memories, forcing her to confront the tragic past she has been protected from. In the climactic sequence, Coco must face the truth about Kuro, herself, and the monsters that have always surrounded them, leading to a surreal and ambiguous resolution where Kuro makes a final sacrifice to cure Coco's self-deception, though he mysteriously reappears in the epilogue as an ordinary cat.