Rito Saionji is a detective in the Special Crimes Division. Five years ago, she lost her older sister Mao, whose death remains unsolved. Mao was also an outstanding detective in the same unit, involved in an undercover investigation of Mantis, an underground criminal organization, but died in an accident during the operation. The truth behind her death has never been revealed. To this day, Rito continues to pursue Mantis, the one responsible for her sister's death, vowing to uncover the truth.