Night after night, the same dream of a guillotine. A headhunting serial killer. And the "knights" in black who follow soon after. Irrationality that defies the realm of common sense begins to assault and devour the city.

A virtual world. Gunshots and bombs. A ruined city. Yet the only thing Kou remembers is the name of a case: "Gray Christmas". As the whole history about it comes back to him, his peaceful days come to an end.

Former magic student Kurou knows little of grimoires, but what he does know terrifies him--enough that he left his university life behind, and now practices a failing detective business. But just when he thought his Academy past was buried, it catches up with him and pulls him back into the dark shadows of the occult.

A parallel timeline where nuclear weaponry was never developed, Japan continued to fight through World War II, ending aggressions only when Hokkaido and Kyushu were occupied by Russia and America, respectively. Takeda Akane is a swordsman driven by a desire to face his one true rival. What will stand in his path to his goal?