Twenty years ago, monsters with black wings suddenly appeared in the world. People called these creatures "Devils". In response, a new profession was born: Devil Sweepers; mercenaries hired to hunt and exterminate these monsters.

In the present day, a cynical young man named Lake Llandovery makes his living as an Devil Sweeper in a rural town. Haunted by a dark past and harbouring deeply pessimistic views, Lake directs his frustrations into hunting Devils. During one of his missions, his life is saved by a mysterious woman with white wings named Vene Ansbach, who subsequently suffers from amnesia. Feeling obligated to repay his life debt, Lake embarks on a journey alongside Vene to help her recover her lost memories.