Lee Jung-do is a black belt in taekwondo, kendo, and judo who spends his days playing video games with his nerdy friends when not working as a delivery rider for his father's fried chicken restaurant. After one such delivery, Jung-do helps probation officer Cho Min-jo in subduing a violent parolee who was caught violating his probation. After receiving a commendation for his help, probation department manager Kim Sun-min persuades Jung-do to join up as a "martial arts officer", who use their skills to handle aggressive probation violators, with Sun-min serving as his partner and mentor. After taking on the job, the two become fast friends and Jung-do soon considers Sun-min to be like an older brother to him.