Awakening her dormant abilities as a devil one day, Yuuko Yoshida aka Shadow Mistress Yuuko, is entrusted with the mission to defeat the Light clan's shrine maiden, a magical girl, by her ancestor Lilith. Yuuko meets magical girl Momo Chiyoda through her classmate Anri Sada, and challenges her to a duel, but loses quickly due to her lack of strength.
Since then, Yuuko has struggled with her role as a devil and her duel with Momo, and borrowed help from her regularly. However, by a strange coincidence, Yuuko also weakens Momo by taking away her power to cooperate in protecting the peace of Tama city.
The story revolves around Kaiman, who does not remember who he was before he was transfigured by a Magic user. This transformation left him with a reptile's head, and a desire to find out the truth about who he really is. Accompanied by Nikaido, his female companion, he tracks down Magic Users in "The Hole" and unceremoniously chomps down on their head, hoping to find out who it was that put him in this state. One by one, they witness this "second man" inside the head of Kaiman, and after pulling them back out of his mouth he asks them all a question... "What did the guy inside my head say?"