Koutetsu no Majo Annerose is set in the dystopian city of Amidahara, where humans, demons, and criminals coexist in a lawless environment. The story follows Annerose, a powerful and feared witch who runs a private detective agency, and her undead servant Rikurou Tachibana, bound to her by a magical spell. When a girl named Miki seeks Annerose's help to find her missing brother, they become entangled in dangerous conflicts involving dark forces and a vengeful enemy from Annerose's past, threatening not only Amidahara but the entire world.