In 1928, Emily Locke's final year at the isolated Briarley School for Girls is derailed when Violet, the school's brightest star (and a cunning beauty for whom Emily would do anything), falls to her death on her eighteenth birthday. Emily and her buttoned-up rival Evelyn are, for once, in agreement: Violet's death was no accident. There's an obvious culprit, the French schoolmistress with whom Violet was getting a little too close - they just need to prove it.

Desperate for answers, Emily and her classmates turn to spiritualism, hoping for a glimpse of wisdom from the great beyond. To their shock, Violet's spirit appears, choosing pious Evelyn as her unlikely medium. And Violet has a warning for them: the danger has just begun.

Something deadly is infecting Briarley. It starts with rotten food and curdled milk, but quickly grows more threatening. As the body count rises and students race to save themselves, Emily must confront the fatal forces poisoning the school. Emily's fight for survival forces her to reevaluate everything she knows: about Violet, Evelyn, Briarley, and, ultimately, herself.