As the sky is illuminated by meteors, Shion Pavlichenko becomes a Contractor for the first time. Despite her brother's transformation, her twin sister Suou continues to lead a typical middle school life. However, that all changes when a masked man clad in black invades their home, shattering Suou's sense of normalcy. Unbeknownst to her, Suou possesses latent Contractor abilities which change her life forever. She must navigate the treacherous world of violence and espionage as she struggles to find her own purpose while reconciling with her family and friends. In Tokyo, the collapse of Hell's Gate has sparked an investigation marked by ominous prophecies that allude to a silver-haired doll.