The setting is modern day Tokyo, where humans, spirits, ghostly apparitions and demons – both seen and unseen – co-exist in two overlapping dimensions. For time immemorial the balance and order between the two dimensions has been maintained by a guardian known as the Karas, a raven-like creature, and its masters. But now that balance is under threat from a former Karas named Eko. Disgusted by human arrogance, Eko has turned his back on the ancient laws he helped uphold for so long and is determined to seize power and bring order to the city's streets through force by leading a group of mechanized demons in an attack against the humans. Standing against Eko is Yurine, a spiritual entity who represents the will of the people, accompanied by a newly risen Karas of her own.

Meanwhile, a detective named Kure, who works at the police division in charge of demon-related incidents, opens a case involving a series of bizarre murders that appear to be the work of a water demon. It is the first evidence of events that will eventually lead to an ageless battle, one that will stretch across both dimensions, killing humans and spirits alike. It will prove to be a conflict in which humanity's only hope will lie with the Karas.