The narrative opens with an intense philosophical debate between the protagonist Durtal and his friend Des Hermies regarding the superficiality of modern naturalism in literature. Durtal is struggling to reconcile his artistic need for realistic precision with a deeper, spiritual hunger that standard materialism fails to satisfy. This existential weariness drives him to historical research, specifically into the life of the infamous fifteenth-century French nobleman Gilles de Rais, also known as Bluebeard.
As Durtal immerses himself in the historical documents of Gilles de Rais, the narrative explores the dual nature of a man who was once a valiant military captain and a devoted companion to Jeanne d'Arc. The author details how immense wealth, devastating war, and unbridled curiosity push the Marshal from high-minded mysticism into alchemy, and ultimately into the darkest realms of satanism. Surrounded by unscrupulous scholars, transmuters of metals, and sacrilegious priests like Francesco Prelati, Gilles descends into unspeakable acts of cruelty, including the abduction and murder of hundreds of children.
The text contrasts the spiritual purity of Flemish Primitives and the haunting imagery of Grünewald's Crucifixion with the grotesque rituals of black masses and occult evocations. Durtal grapples with the unbridgeable gulf between reason and religious belief, finding temporary solace in the sublime music of church bells as experienced through his friend Carhaix. Throughout these intellectual and spiritual wanderings, Durtal becomes entangled in a mysterious, passionate correspondence with a married woman, Mme. Chantelouve, which mirrors the themes of spiritual torment and carnal obsession.
The narrative builds toward a harrowing exploration of diabolism, incubacy, and human perversion, examining how the insatiable pursuit of the forbidden leads inevitably to spiritual ruin. Durtal's historical investigations into Gilles de Rais mirror his own internal battles with faith, disgust for contemporary bourgeois society, and the eternal struggle between the divine and the infernal. The book concludes on a contemplative note, leaving the reader with the lingering horror of man's capacity for evil and the eternal mystery of the supernatural.