"Sin of a Family" involves the kidnapping and murder of a young child in a small South Korean city. Detective Jo (Hyeon-jun Shin) becomes involved in the case. At first he acts without a sense of duty, but changes as he faces the truth of a horrible case.

After a body was found at the lake, the police identifies it as Jeong Min-hwan, a young autistic boy, who went missing two months earlier.

While struggling in his troubled relationship with his son Gyeong-su, Detective Jo Chang-shik is assigned to the case. As part of the investigation, he visits the victim's family.

When he eventually unearths their family secrets, he suspects the father may be responsible for Min-hwan's death.