Raizou Hiraga, a 65-year-old retired police officer, is compelled to leave retirement after discovering that his grandson, Shouta, has fallen into a coma caused by an incurable illness. While looking for high-paying employment, he finds a job advertisement from the SS Agency, which offers a salary precisely equal to the one million yen per month required for Shouta's medical care. At the agency's office, situated within a second-hand bookstore called Desperado, Raizou meets his new employer, Kinume. Although she reveals very little about his future responsibilities, his financial desperation leads him to accept the position. Believing he will work as a hotel housekeeper, Raizou quickly discovers that his cleaning duties are vastly different when he encounters a corpse—murdered by an 11-year-old girl named Miharu Suzukaze. Despite this, Raizou's successful first assignment results in him becoming Miharu's partner as a hitman. Having spent his life protecting political figures, Raizou is now drawn into a world of murder as the duo gradually uncovers the criminal conspiracy operating behind the scenes.