The frame narrator, Dr. Horace Bianchon, begins his story with a lengthy description of a dilapidated residence called La Grande Bretèche, which he discovered while practicing medicine in Vendôme. Although the residence is not ancient, Bianchon speaks of it as if it were an archaeological ruin—one whose mysterious power of attraction is so strong that he regularly scales its garden walls to contemplate the unknown catastrophe that caused it to collapse into such ruins. Bianchon continues to visit its garden in secret until, one day, he receives a call from Mr. Regnault, an attorney and executor of the estate, who forbids him to trespass on its grounds. Bianchon agrees to stop his clandestine visits but implores Regnault to tell him what he knows about the place’s catastrophic decline.
Regnault explains how Mrs. de Merret, the owner of the estate, called him late one night to make him the legal custodian of her estate. Curiously, the principal task that she assigned him was to ensure that the place would remain uninhabited and untouched for fifty years after her death. Regnault also passes along other facts about Mr. and Mrs. de Merret’s “catastrophic” separation; Mr. Merret’s subsequent fall into decadence in Paris; and Mrs. de Merret’s return to her native Chateau de Merret, where she withered away to a ghostlike existence. She eventually died clutching an ebony crucifix and uttering these cryptic last words: “Oh! my God!”
Mrs. Lepas, the nosy owner of the Vendôme inn in which Bianchon is staying, picks up the story where Regnault leaves off. She takes Bianchon back to the days before the separation of Mr. and Mrs. de Merret, to a time when Mrs. de Merret received romantic visits from a Spanish prisoner of war named Férédia. Férédia was a deeply religious man who read his Bible like a preacher and attended mass regularly. One day Férédia mysteriously disappeared, and his clothes were found near a river. Mr. Lepas concludes from this that Férédia either escaped or drowned; but, Mrs. Lepas, who knows that Mrs. de Merret died with Férédia’s ebony crucifix in her hand, suspects that Férédia’s disappearance was linked to the separation (and subsequent demise) of Mr. and Mrs. de Merret. Because Mrs. Lepas lacks direct evidence to support her theory, she sends Bianchon to see Rosalie, Mrs. de Merret’s former servant and the only surviving witness to these events.
The story that Bianchon persuades Rosalie to tell him is a chilling one. Mr. de Merret, who returned home later than usual one night, decided to check in on his wife. As he reached for her bedroom door, he heard another door close inside her room and immediately suspected that his wife was hiding a lover. Mrs. de Merret adamantly denied her husband’s accusation and warned him that if he were to look and find nobody in her antechamber, their marriage would be destroyed. Mr. de Merret did not look; instead, he had Rosalie send for her fiancé, a bricklayer, whom he had build a brick wall to seal off the antechamber so that he could observe his wife’s reaction. After she did not flinch, Mr. de Merret pretended to go to town to verify her story about purchasing her ebony crucifix at a pawnshop; he immediately doubled back to see what his wife would do next. When he returned, his wife was trying to knock a hole in the brick wall; the moment she saw her husband, she fell ill from agony. Her agony grew more severe, especially during Férédia’s final days. Each time that the Spaniard made a noise from behind the wall, Mr. de Merret would chide his wife: “But you swore on your cross that nobody was there.”