Realising that laws and justice won't catch criminals, police chief Nam Gu-hyeon reaches out to Oh Gu-tak, an ex-officer known for his mercilessness.

On the way to the crime scene, Jeong Tae-su, the former hit man, suddenly stops the van and asks to visit a nearby neighbourhood.

The police fail to respond in time to a kidnapping incident. Enraged by their incompetence, Chief Nam again turns to Gu-tak and the `Mad Dogs'.

The city is gripped by an increase in human trafficking cases. While free from prison, each of the three convicts revisits people from their past.

There are multiple casualties at a city park when a man randomly opens fire. Gu-tak and his special investigation team happen to pass the site.

Following Woong-cheol's GPS signal, Gu-tak and the team end up in an isolated area in the woods and come face-to-face with a gang of black-suited men.