Lee Jung Do delivers food for his father's chicken restaurant. He seems like an ordinary guy who likes to play video games with his pals at internet cafes and talk with his friends over drinks. Yet, he is a prodigy in the martial arts. He has achieved the highest rank in taekwondo, kendo, and judo. When he sees someone in trouble, he doesn't think twice about helping them. One day, Lee Jung Do happens to see a martial arts officer being attacked by a probationer wearing an ankle monitor, and he saves the martial arts officer. After saving the officer, Lee Jung Do begins to fill in for the injured officer for 5 weeks. He is partnered with probation officer Kim Sun Min. As a martial arts officer, Lee Jung Do's job is to help during physically dangerous situations. Kim Sun Min's manages violent offenders who are on probation. While Lee Jung Do works with Kim Sun Min, Kim Sun Min notices his talent for fighting and recommends that he work full-time as a martial arts officer.
