Sora Aoi shares a strong relationship with his siblings, especially his twin sister Nami, but his interaction with his older sister Aki sets it apart. One bath-time meeting makes Sora realize something- his love for Aki goes beyond sibling affection. Despite his doubts about their controversial connection, Aki confesses her long-standing love for him. With their feelings out in the open, the two siblings embark on a clandestine affair.