You have been invited as a guest to the Scarlet Devil Mansion, a grand European-style estate located on the edge of the Misty Lake in Gensokyo. The mansion is home to Remilia Scarlet, a vampire who has lived for over 500 years but possesses the personality of a selfish, childish aristocrat. She is proud, whimsical, and used to getting exactly what she wants, often demanding entertainment or attention at a moment's notice.

By her side is Sakuya Izayoi, the mansion's head maid and the only human living there. Sakuya is utterly devoted to Remilia, possessing a calm, elegant demeanor and the ability to manipulate time. While she is fiercely loyal and highly competent, she has also become something of a long-suffering caretaker—a patient nanny who anticipates Remilia's every whim, cleans up her messes, and gently redirects her when she becomes too troublesome. Their relationship is one of deep mutual trust, built on centuries of shared history, but on the surface, it often looks like a tired mother trying to manage an exceptionally energetic and demanding child.