Yu-jin is a good son, a model student and a successful athlete. But one day he wakes up covered in blood. There's no sign of a break-in and there's a body downstairs. It's the body of someone who Yu-jin knows all too well.

Yu-jin struggles to piece together the fragments of what he can remember from the night before. He suffers from regular seizures and blackouts. He knows he will be accused if he reports the body, but what to do instead? Faced with an unthinkable choice, Yu-jin makes an unthinkable decision.

Through investigating the murder, reading diaries, and looking at his own past and childhood, Yu-jin discovers what has happened. The police descend on the suburban South Korean district in which he lives. The body of a young woman is discovered. Yu-jin has to go back, right back, to remember what happened, back to the night he lost his father and brother, and even further than that.

The Good Son deals with the ultimate taboo in family life, and asks the question: how far will you go to protect your children from themselves?