While the residents of the Québec village of Three Pines take advantage of the deep snow to ski and toboggan, to drink hot chocolate in the bistro and share meals together, the Chief Inspector finds his holiday with his family interrupted by a simple request. He's asked to provide security for what promises to be a non-event. A visiting Professor of Statistics will be giving a lecture at the nearby university. While he is perplexed as to why the head of homicide for the Sûreté du Québec would be assigned this task, it sounds easy enough. That is until Gamache starts looking into Professor Abigail Robinson and discovers an agenda so repulsive he begs the university to cancel the lecture. They refuse, citing academic freedom, and accuse Gamache of censorship and intellectual cowardice. Before long, Professor Robinson's views start seeping into conversations. Spreading and infecting. So that truth and fact, reality and delusion are so confused it's near impossible to tell them apart. Discussions become debates, debates become arguments, which turn into fights. As sides are declared, a madness takes hold. Abigail Robinson promises that, if they follow her, ça va bien aller. All will be well. But not, Gamache and his team know, for everyone. When a murder is committed it falls to Armand Gamache, his second-in-command Jean-Guy Beauvoir, and their team to investigate the crime as well as this extraordinary popular delusion.

When Chief Inspector Armand Gamache is asked to provide crowd control at a statistics lecture given at the Université de l'Estrie in Quebec, he is dubious. Why ask the head of homicide to provide security for what sounds like a minor, even mundane lecture? But dangerous ideas about who deserves to live in order for society to thrive are rapidly gaining popularity, fuelled by the research of the eminent Professor Abigail Robinson. Yet for every person seduced by her theories, there is another who is horrified by them. When a murder is committed days after the lecture, it's clear that within crowds can lie madness. To uncover the truth, Gamache must put his own feelings about the divisive Professor to one side. But with her ideas gaining ground, the line separating good and evil, right and wrong, is quickly blurring - especially when the case leads unexpectedly close to home.