One day in December 1979, in Hersange, an imaginary town in the East of France, an entire class is missing and returns the next day as if nothing had happened. Was it a concerted action or a coincidence? A tormented principal, a policeman with a head for maths, and an unabashed teacher investigate in a creeping, equivocal atmosphere littered with false clues. How can an apparently minor event become a real enigma?