Another world exists beyond the one that humans can see–a world populated by demons. For centuries, demons have crossed into our world, some looking for a new life, adventure, or just a job. But many are intent on causing chaos and destruction, feasting on human flesh and fear. A tenuous Peace Treaty is the only barrier between the worlds, the only thing protecting humankind from utter devastation.

Every hundred years the Treaty must be renewed—and the time for that secret ceremony is at hand. Violent demonic splinter factions that are hell-bent on destroying the ancient pact are preparing to assassinate a key figure who is vital to the signing ceremony.

Enter the Black Guard, a secret police force that protects the boundary between the dark world and our own. It's the job of two Black Guard agents—Makie, a gorgeous demon, and Taki, a hard-boiled human—to form an unlikely partnership, stop the assassination, and avert an all-out war. But to save both their worlds, they must risk not only their lives, but their very souls!