A drowned nun and a flood of anonymous letters draw Dame Beatrice Lestrange Bradley into the cloistered world of a convent school. The nuns of the Order of Companions of the Poor summon the eminent psychiatrist and sleuth to investigate a series of unsettling anonymous letters. Upon her arrival, the prime suspect in the letter-writing campaign is found drowned in the convent school pond, clutching her Family Bible. Dame Beatrice must navigate the sniping gossip of paying guests and the rumors of ghosts among the school children to uncover the truth.

Gladys Mitchell's 'Convent on Styx' is a classic example of Golden Age detective fiction, featuring her signature blend of intricate plotting and eccentric characters. The novel is praised for its atmospheric setting within a convent and the intellectual prowess of Dame Beatrice Lestrange Bradley, a formidable female detective. Readers often appreciate Mitchell's ability to weave complex mysteries with psychological insight, making the unraveling of the truth a satisfying intellectual exercise. The narrative's engagement with themes of faith, suspicion, and the hidden lives within institutions contributes to its enduring appeal.