Loudun, October 1947. Leon Besnard and Marie celebrate their eighteenth wedding anniversary with friends and Ady, a former German prisoner they have "adopted". A few days later, Leon dies. Louise, a friend of the couple's and probably Leon's mistress claims that, on his deathbed, the deceased told her that Marie was poisoning him. The whole town soon condemns Marie and she is arrested and sent to jail. Did she really kill Léon as well as twelve other members of her family as she finds herself charged with?

Une notable fut, après-guerre, accusée d'avoir assassiné son mari et onze autres personnes de son entourage, suite à des rumeurs qui firent d'elle "l'empoisonneuse du siècle". Marie Besnard n'avoua jamais ces assassinats à l'arsenic. Elle finit par être acquittée après plus de dix années de procès et d'expertises et emporta son secret dans la tombe...