Intent on being an independent young woman, high schooler Himeyuka lives on her own in an unremarkable apartment complex in a corner of the city. But one day, she discovers her ordinary building has turned into something extraordinary! Her beloved “castle” is covered in childish scribbles— - both inside and out! And waiting for her at the end of this rainbow-colored mess…is the perpetrator of the crime—: a little boy named Rozione, who seems to know quite a bit about Himeyuka, though she has no idea who he is. Is he really just someone’'s lost child, or is there a greater mystery behind his appearance?