Juuna Tono is a boy without a past. Orphaned by a tragic accident that claimed the lives of his parents and his memory, Juuna and his younger twin sister Senna have only survived by their commitment to support each other. But when a strange girl greets him as "Big Brother" (onii-chan) Juuna turns to her and says, "Haru...hi?" It is someone from his forgotten past, who Senna claims is his childhood friend - or is she?