Renzaburou Taki seems like an ordinary office worker, but in truth, he belongs to the Black Guards—a clandestine group that upholds the delicate balance between the human and demon worlds. As the deadline for renewing the peace treaty nears, Taki is assigned to protect the key human mystic Giuseppe Mayart, aided by a demon woman named Makie. Yet the job proves harder than it sounds; a radical demon faction has resorted to terrorism in an attempt to shatter the unity of the two realms, while Giuseppe is more interested in slipping away to chase women.