Natsuki, the son of a criminal investigator, deeply cared for Mika, the daughter of a hitman from the Mochizuki Group killed by his father, treating her like his own sister. However, Mika is brainwashed, and Natsuki becomes entangled in the conspiracies of Yukiko and Saezuka, using her as a human shield. The story portrays the suffering engraved in her heart and the harsh reality she faces.