Diderot and a few friends conceive a hoax to lure the Marquis de Croismare, an acquaintance who had retired to the provinces, back to Paris.. They forge letters purportedly from a young nun, "Suzanne Simonin," pleading for help to escape her convent, letters so plaintive and convincing that the chivalrous Marquis is nearly moved to intervene. As Diderot composes these fictional pleas, he finds himself increasingly engrossed in Suzanne's fate. The prank takes on a life of its own: Diderot begins crafting a full story around this invented nun, and soon La Religieuse emerges, first in epistolary fragments and eventually as a complete novel (though it will not be published until long after, in 1796).
The resulting story is one of grim authenticity and pathos: Suzanne, forced into the convent against her will, endures psychological torment, sexual harassment by a lascivious Mother Superior, and the dehumanizing strictures of cloistered life. Diderot spares no detail in depicting the convent as a social tomb, a place where young women (often mainly imprisoned by families) suffer under a rule that stifles nature and reason.