Second-year middle school student Tobi Otogiri spends his days searching for his missing older brother. He carries a mysterious talking backpack named Baku, which he received on the day his brother vanished. Baku is an inhuman entity, or jin, that serves as Tobi's only constant companion. Isolated from his classmates, Tobi avoids speaking to anyone except Baku. One day, his classmate Ryuko Shiratama approaches him and reveals she can hear Baku as well. She also sees the same strange, invisible inhuman beings clinging to people around them that Tobi can see. As unsettling incidents begin at their school, Tobi and Ryuko realize these entities may be responsible. They are drawn into a hidden conflict between humans and inhuman forces, with Baku at the center.

The school janitor, Haizaki, appears to know something about these entities and becomes a key figure in the unfolding events. After an incident forces Tobi and Baku to consume two inhuman entities, they develop a dangerous appetite that begins to erode their thoughts and control. A transfer student named Monika arrives, claiming to know everything about Tobi and the inhuman world. The story explores the consequences of life consuming life and the growing hunger that threatens to consume Tobi and Baku from within, while the mystery of Tobi's missing brother and the true nature of the inhuman entities deepens.