In the thermal town of La Bourboule, a man is found dead, killed by poison. His ex-girlfriend, Irene, chief-doctor of the thermal baths, owns an important stock of medicinal arsenic which could be the murder weapon. She is accused of the crime and she is arrested. Called to the rescue, her daughter Fanny, a young captain in the Crime section of Paris who she hasn't seen for years, will try to prove her mom's innocence and discover the truth. Between love interests, business interests and vengeance, Fanny's investigation is going to lead her much further than she could ever imagine, making her discover her own family's secrets.