With school out for the summer, Margery and Clementine Butcher-Baker are taking advantage of the break to go on holiday. They plan to explore the village of St-Martins-on-the-Water and rest before the chaos of Mrs Smith's impending hen do. By the end of their first night, the local mayor lies dead on the floor of the hotel restaurant, having been poisoned by his meal. The villagers are convinced: The Poisoner is back.

As the residents turn to the duo with suspicion and begin to freeze them out, Margery and Clementine are left with more questions than answers. Everyone is convinced that the pair – and Clementine in particular – are bad news, but why? And as more people in the village start to become unwell, the question remains, is The Poisoner back, or is this the work of a new killer? And will they strike again?