The human world and the demon (youkai) world have overlapped, allowing humans to walk the streets of life as usual, while demons walk the very same streets, hidden from the naked eye. A seemingly young woman named Yurine and her servant, the Karas (derived from the Japanese word for "Crow"), have long maintained order and balance between the overlapped worlds, ultimately keeping demons from interrupting human lives. However, humans have come to forget and jest at the existence of demons, no longer understanding the privilege of living without fear. Disgusted by this arrogance, an old Karas turns his back on the laws he once upheld. In his human form, named Eko, he creates an army of Mikura, or mechanized demons, to ready an attack on the human race. A young man named Otoha inherits the powers of the Karas and takes his place at Yurine's side, who claims that his soul called out for her while he lived the life of a human. They live in the world of the demons. It is now up to Otoha to prove himself as a Karas and restore the balance that Eko threatens to upset. Meanwhile, a superstitious police officer named Sagisaka and his rational new recruit, Kure, follow the trail of the murders dealt by Eko's Mikura, as well as the trail of a rogue Mikura named Nue. The prophecy unfolds from here into a grave revelation for all in the city.