Based on the erotic game by Black Lilith. 
 
The stage is set in an abandoned city called Amidahara, inhabited by monsters, criminals and armed refugees. There lives Annerose, a female witch blade, who runs a private detective agency. 
 
One day, a girl named Miki comes by and asks Annerose for help in finding her brother. Starting from her request, Annerose gets involved in fierce power struggles between the dark forces in the city... 
 
(Source: DLsite English)