Being a single parent to her son Doyeon, Yeon-hee, a responsible police officer, had to maintain a fine balance between her duties and her family. As Doyeon grew up, he imbibed his mother's values of empathy and compassion and became the person she had always hoped he would be. However, their peaceful lives were disrupted by a series of brutal murders that posed a difficult question for Yeon-hee: should she focus on protecting her son or assist her colleagues in tracking down the killers?