"Since a long time, Rumia lives with a couple which wasn't blessed with an own child. They love her deeply, even while knowing of her dangerous nature.
In this story, a foreseen tragedy is bound to happen."

From Gensokyo.org: One night out of nowhere, Rumia appears in a couple’s home and eventually becomes part of their life. However rosy things may seem, is it what you would call a family?