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 ...