Kanna Kusakabe is a second-year high school student in 1948 Tokyo. Her life takes a turn when the new literature teacher, Akihiko Chuzenji, arrives. After a classmate's mysterious death is linked to a ghost rumor, Kanna investigates the school at night and discovers the supposed ghost is actually Chuzenji, who uses a hidden library of forbidden books. To protect his secret, he spreads a story that Kanna exorcised the ghost herself.

This incident accidentally earns Kanna a reputation as an occult expert among her peers. Though dismayed, her innate desire to help others compels her to investigate the various strange and paranormal-looking occurrences that begin plaguing the school. Her bubbly personality often leads her into trouble, and she regularly seeks aid from the surly Chuzenji-sensei in the library preparation room.

Kanna is a teenage girl with neck-length brown hair and brown eyes, typically dressed in her school uniform and a neckerchief. Through their repeated collaborations, she and Chuzenji form a duo that navigates these eerie events. She acts as the public investigator while he works behind the scenes to provide rational explanations and solve the mysteries. He often scolds her, but a mentorship defines their dynamic.