A woman with no known past and no official identity, Rei is the top operative of the Zero Department, an unofficial and utterly deniable shadow division of the Tokyo police that functions as a team of government-sanctioned assassins. Her life is one of solitary routine, moving from one contract killing to the next with cold, mechanical efficiency. The story begins with her on a mission to eliminate a German underworld figure, a hit she executes with lethal precision by drowning the much larger man in a swimming pool before gunning down his bodyguards and escaping. However, this assignment goes wrong when her photograph is obtained by a rival criminal organization, which marks her for death. Now hunted herself, Rei finds her isolated existence disrupted on two fronts. She reconnects with a man from her past, a chef named Shiina, and begins a tentative romantic relationship that threatens to melt her icy exterior. Simultaneously, her ruthless superior, Muto, becomes displeased with her apparent loss of focus and growing emotional attachments, which he views as a liability. To eliminate the complication, Muto authorizes the Zero Department to turn on its own, deploying a new, ruthless assassin to eliminate Rei. Forced to flee from the very organization she served, Rei must use all of her lethal skills not to execute a target, but to survive a relentless manhunt while trying to protect the innocent people who have come to care for her, leading to a grim confrontation that determines whether a killer can ever escape her past.