The story follows Kathy and Frank, a couple who have raised their son, Julian, in a carefully constructed bubble of normalcy. During a family trip to Belgium, they visit Barbara, an old friend of the family. The vacation takes a dark, psychological turn when an intense attraction sparks between Julian and the older woman. As the parents watch the situation escalate, their frantic attempts to intervene reveal a twisted secret: Barbara isn't just a friend, and the "taboo" connection between her and Julian is rooted in a biological truth the parents have hidden for years.