Tokichi Inaba is an ordinary, somewhat timid salaryman whose life revolves around his office job and his family, whom he loves dearly. His mundane existence is shattered one night when he accidentally witnesses a violent confrontation involving a legendary hitman known as Nicho. Mortally wounded, Nicho coerces the terrified Inaba into taking his place, ordering him to complete a hit to rescue a woman. To ensure compliance, Nicho threatens to murder Inaba and his entire family if he fails. Suddenly thrust into a high-stakes criminal conspiracy, Inaba reluctantly kills the target and saves the woman, a mysterious and beautiful troublemaker named Chinatsu.

Now bound to the underworld, Inaba finds himself forced to lead a dangerous double life. By day, he continues to work as a humble businessman, struggling to keep his new reality a secret from his wife, Misako. By night, he is the reluctant successor to the legendary hitman, a role enforced by a shadowy crime support organization known as "The Convenience Store," which handles logistics and cleanup for a fee. Alongside the alluring and enigmatic Chinatsu, who becomes his partner, Inaba is pushed into accepting more assassination contracts to pay off the debts incurred from his first night of violence. The narrative follows his desperate attempts to protect his family from this escalating spiral of danger, balancing his mundane responsibilities with the brutal demands of the criminal world, all while being targeted by rival organizations who see him as a threat.