Two years ago, Keiichi Akagi was a promising young police officer before he mysteriously vanished without a trace. The police eventually closed the case, and his family was left to grieve, with his younger sister Kei taking on the responsibility of caring for their home while their mother worked nights. Unbeknownst to them, Keiichi had not simply disappeared but had died and returned as a skeletal ghost, confused and without memory of how he met his end. He claims to have emerged from a dark forest inhabited by the dead who hold grudges against the living.

Kei discovers she possesses the rare ability to see not only her brother’s spirit but also other supernatural entities. These include malevolent ghosts born from the negative emotions of living people, which can possess individuals and compel them to commit evil acts. Defeating these spirits can restore the afflicted person to their former self. Unlike these hostile apparitions, Keiichi retains his desire to protect people, but as a ghost, he cannot directly interact with the living world. This limitation forces him to partner with his sister.

The story is set primarily at Kei’s prestigious high school, where a growing supernatural darkness seems to be influencing the student body. Together, the siblings form an unconventional detective duo. Kei handles the physical, real-world investigation while her brother uses his ghostly abilities to confront and battle the spiritual threats directly. Their joint efforts focus on solving the mysterious and often dangerous problems plaguing Kei’s classmates.

As they work to protect the living from these supernatural cases, an overarching mystery drives the narrative forward: the circumstances surrounding Keiichi’s death. The siblings remain haunted by the questions of how and why he died, as well as the true nature of the dark forest from which he returned. Their investigation into the ghostly incidents at school gradually intertwines with the personal quest to uncover the truth behind the brother’s demise.