Chiaki and Mao become stepsisters due to their parents' remarriage. Chiaki is overjoyed to have an adorable younger sister, but little does she know, Mao carries a darkness within her. Manipulated by Mao's devious schemes, Chiaki becomes isolated from her friends, loses the person she loves, and unwittingly finds herself in loneliness... What could be the reason behind Mao's plot?