When Kim Ji Yong was a child, his mother was beaten to death in the street. The culprit only received three and a half years in prison. Kim Ji Yong—now an adult—sees that his mother's killer hasn't changed at all. Ji Yong takes matters into his own hands and beats him brutally. Subsequently, Kim Ji Yong begins living two entirely different lives. On weekdays, he's a model student at the police university. During the weekends, he punishes criminals who'd received light sentences and continue to commit criminal acts. Now called Vigilante, Ji Yong receives help from admirer Cho Kang Ok. Meanwhile, Detective Cho Heon chases after the man called Vigilante.