Sora Aoi has always been close to his siblings. While Nami is his twin sister, he shares a different kind of bond with his older sister, Aki. An encounter with Aki in the bath leads Sora to a startling realization: he is in love with her. Although Sora hesitates over these forbidden feelings, Aki confesses that she has always loved him. With their feelings revealed, the two begin a secret relationship.