On February 2nd 1933, in the town of Le Mans, in the West of France, two sisters, Christine Papin, 27, and her younger sister Léa Papin, 21, brutally killed their employer Mrs. Ancelin and her daughter Geneviève. Why did they commit this horrifying murder? As a result of an unhappy childhood? Because they could no longer stand being commanded and exploited by arrogant bourgeois employers? Because their incestuous relationship had been exposed? The film tries to answer these questions and make us understand why and how the two sisters turned from meek sheep to bloody monsters.