In a Japanese city haunted by nature spirits and demons, a secret war is raging between the renegade demon Eko and a spectral warrior known as the Karas. Eko has mechanized himself and his followers to defend against the unearthly fury of the Karas; in addition to an armored humanoid form, the Karas can transform into any of various heavily armed and armored vehicles. Eko and his followers prefer to live in the material realm in human form, but to continue doing so they must feed on the fluids of human bodies. Their periodic feasts result in massacres that leave only a single traumatized survivor; these "inexplicable" tragedies are the obsession of a marginalized police inspector whose daughter was the survivor of one such massacre. Opposing Eko are the renegade demon Nue, who also lives in the material world in human form but refuses to drink blood or take a mechanized form, and Otoha, a former human who is summoned by the mysterious Yurine to become the new Karas.