The crime and the positions: CCTV footage shows that Hye-yoon and Si-ho beat a homeless man, leaving him critically injured. He later dies. At first the expected sides form. Jae-gyu, the ethical doctor, leans toward turning the children in, while Jae-wan, the lawyer, looks at it as a legal problem to manage.
The reversal: As the dinners continue, the positions flip. Jae-gyu sees his son's future collapsing and becomes desperate to protect him, and Yeon-kyung is just as determined. Jae-wan grows disgusted by his daughter's lack of remorse and decides the children must face justice. The cynic becomes the conscience, and the idealist gives up his principles.
The climax: In the last minutes, which NME called the film's most violent and terrifying moments, Jae-gyu runs his brother down with a car to stop him from reporting the crime. It deliberately echoes the road-rage case from the opening, which the "good" brother had once condemned. The ending is abrupt and, in IndieWire's words, "darkly poetic and anything but normal."
Hidden meanings in hindsight: Several early details turn out to be warnings: the grandmother's comment about her son's violent streak, Jae-gyu's self-righteous lectures, and Yeon-kyung's open contempt for those below her in the family. The respectable family was never normal. Its wealth and manners were hiding violence the whole time.