The director of the celebrated black comedy Tatie Danielle, Étienne Chatiliez returns to the realm of dark humor with Tanguy. When their eponymous son is born, Paul and Edith Guetz (André Dussolier and Sabine Azema) are so besotted with the new arrival that they make him the fateful promise he can live with them forever. Twenty-eight years later, with Tanguy still under their roof and showing no intention of relocating, they begin to regret their promise. Although she is proud of her son, who is both excessively smart and handsome, Edith is soon driven to distraction, and makes plans to bundle Tanguy off to Asia. When this doesn't pan out, Edith convinces Paul that they must resort to more serious measures.