Based on the adult game by Lune Team Bitters.

After Iori fell in love with Ken since childhood, She intends to be his wife Ken who is more than anything. And it turns out Ken also has the same feelings towards Iori. But on a night when dating, suddenly appeared a big monster and killed Ken. Moments later comes a mysterious man who offers Iori to become Mahou Shoujo who can save his life Ken. However, Mahou Shoujo's contract was necessary by sex with the mysterious man. Can Iori save Ken?

(Source: Translated and Edited from Nekopoi)