Dared by his friends, Nagisa climbs Mount Osore to enter the feared Hone-jo, a centuries-old ruined castle rumored to make people disappear.
Inside the castle, Nagisa comes across the corpse-eating monster who haunts the halls and rooms of Hone-jo. He's saved from certain death by Seiho, a man bound to the castle for life by a deal he made with the creature.
Though warned not to return to the Castle of Bones, Nagisa comes back as he can’t get Seiho out of his head and his heart. But their love has no future because Seiho can’t leave the castle. Determined to save him from this tragic fate, Nagisa goes to Osore-zan Temple to seek help from the itako--the shamans of the temple.
When they refuse to offer aid, Nagisa vows to find a way to destroy the creature and give Seiho back his liberty.