Tatewaki Shoutarou is an average high school student in Asahikawa City, Hokkaido Prefecture. However, hh has an unusual acquaintance by the name of Kujou Sakurako, who loves bones more than life itself. One day, on a bone gathering expedition along the shores of Mashike, they find a human skull...

Shoutarou finds a little girl at a convenience store in the dead of night. Other than her nickname, Ii-chan, no one can figure out her real name and address. Finding a bloody handprint on her backpack, Shoutarou asks Sakurako for help. But their inquiries only lead them to a shocking realization...

While on a bone-hunting expedition in the mountains, Sakurako and Shoutarou again find the skeletal remains of a human being. Later, Shoutarou finds out that the body belonged to Kougami's grandmother, whose death the police have written off as a suicide. In disbelief, Kougami asks Sakurako for help...