Kuroi otome: Q

Mei (Nana Asakawa) lives in an orphanage, but she is adopted by the wealthy and kind Uda family. The family already has an adopted daughter Rana (Kana Kita). While Rana seems weird, Mei and Rana find that they have a lot in common and become like real sisters. The girls then learn that the Uda family's business has collapsed and they can only keep one of the daughters. One of the girls will be returned to the orphanage in a week. Revenge is soon planned.