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The bodies of young boys were found in six cases. Shiro Sakaki, a professor considered Japan's leading expert on butterflies, turns himself in. He has recorded everything in a report published on the Internet, from his motive to the process of his creation. The interrogation proceeds as if tracing his distorted record, leading to a sensational suspense story where beauty and madness intertwine.

Shiro starts to express his artistic viewpoint during interrogation. The art camp is canceled when Rumi falls ill. Shiro goes to the mountain to take the boys home. In the warehouse, there are six cases on display. Shiro feels emotional upheaval due to his expectations toward Itaru. As the detective investigates further, a big mystery arises, and the interrogation gets to the crux of his story.

After creating five specimens, Shiro's final target is his beloved son, Itaru. Despite the memories with Itaru, Shiro only sees his son as a butterfly. Shiro makes up his mind to create his last specimen. At last, the report named "Human Specimen by Shiro Sakaki" seems to have ended. However, the letters in the report start to get distorted, and the story shifts to a new perspective.

Itaru stays home while Shiro leaves for an observation survey of butterflies. Itaru stares at his father's specimens and invites his friends over. He starts recording his observations in his own report. When Shiro returns, they have dinner and share memories. Shiro opens his son's computer with a strange feeling about the specimens in his room.Three years later, Shiro receives a letter from Anna.

Anna seemed proud, but she longed for a successor while looking at her mother, in the spotlight for her art book. After the art camp is canceled, Anna invites the boys to the mountain house. She insists she made the Human Specimen. Shiro understands Itaru's hopes to become his father's specimen. The truth of Itaru's death and the mystery of Anna's confession will be unraveled.