When both the police and the prosecution give up on a case, the victim's family has no choice but to fight for the truth themselves. In a world where justice does not always prevail, these individuals strive to right the wrong. The Guardian is about such group of people whose worlds turned upside down by the sudden loss of their beloved ones. The main character, Jo Soo Ji, loses her one and only daughter after she falls from a building. To make it worse, it was not an accident but a homicide. She finds out that the killer is a young boy whose father is a very influential being. After the loss of her daughter, Soo Ji decides to only live for her late daughter and to keep the promise they made. Through their full control over the CCTVs, hacking phones and computers, and strong motivation to punish those that the law could not, the Guardians become the protectors for the weak.