Raizou Hiraga, a 65-year-old former police officer, is forced out of retirement after learning that his grandson Shouta has fallen comatose due to an incurable disease. While searching for high-paying work, he comes across a job posting for the SS Agency, offering a salary that exactly matches the monthly one million yen needed for Shouta's treatment.

In the bureau located at Desperado, a second-hand bookstore, Raizou meets his new employer Kinume. Although she discloses little about his upcoming duties, his desperation for money brings him to accept the job. Convinced that he would serve as a hotel housekeeper, Raizou quickly realizes that his cleaning work is starkly different when he stumbles upon a dead body—assassinated by an 11-year-old girl, Miharu Suzukaze.

Nonetheless, Raizou's successful first mission leads him to become Miharu's hitman partner. Having protected political figures his entire life, Raizou is now brought into a world of murder as the pair slowly uncover the criminal scheme unfolding behind the scenes.