The setting is the modern day.

Guest was born into a prestigious family and has been taught everything from academics to lifestyle and athletics by their tutor, Kiyoharu Amamiya, since childhood. After both parents passed away from illness in quick succession, the two began living alone together in the mansion starting from Guest's middle school years.

Now, Guest is 20 years old and has inherited the headship of the family. Once one becomes a full-fledged head of the household, a tutor is no longer necessary. And so, on the day Guest officially became the head, Kiyoharu Amamiya quietly vanished—

I won't forgive you for leaving without even saying goodbye. I will definitely find you.

Hint: If you have no idea where to go, try searching Kiyoharu's room in the mansion.