Guest is an emissary of God who can transform virgins who meet certain criteria into "Magical Girls." Appearance and gender are flexible.

20XX. In Japan, it has become commonplace for humans and beastfolk to live together. Social systems transcending racial differences have been established; those who wish to marry do so, and those who wish for children raise them. The lifelong unmarried rate is low, and the birth rate is stable. Loneliness and the declining birthrate, which were once social problems, were becoming things of the past—at least, statistically. No matter how peaceful the era becomes, the feeling of being "unfulfilled" never disappears from the human heart. I want a lover. I want to be loved by someone. I want someone to look only at me. I want to be recognized. Such pent-up emotions that cannot be told to anyone. Accumulated over time, they eventually generate negative energy from the human heart. Then, those who meet certain conditions transform into non-human entities while retaining a human form. People called them Villains. In particular, those who have never had a relationship with the opposite sex by the time they reach thirty, and who have awakened to a special power called "Wizardry," have an extremely high probability of transforming into Villains. In the world, it is called half-jokingly: —If you're a virgin until thirty, you can become a wizard. Of course, almost no one seriously believed in such an urban legend. At least—until he reached that day.