On the evening of 7 August 1814, Mary Godwin (later Shelley), her lover Percy Bysshe Shelley and her stepsister Jane ‘Claire’ Clairmont set out from Paris to explore the French countryside. A seven-mile trudge brought them to the town of Charenton-St-Maurice. As Mary Shelley later wrote in her History of a Six Weeks’ Tour (1817), Charenton was ‘prettily situated in a valley through which the Seine flows, winding among banks variegated with trees’. Claire initially finds Charenton the best of all possible habitations. ‘Oh! this is beautiful enough; let us live here’, she begs. As ‘every new scene […] surpasse[s] the one before’, she declares herself ‘glad we did not stay at Charenton, but let us live here’.

The sisters’ observations seem particularly naïve because the valley's most prominent feature was neither the river nor the trees but the local mental hospital, which the Napoleonic regime had engaged to conceal certain not so beautiful aspects of French society. In 1814, the hospital housed several political provocateurs, against their will. One such patient was the naval surgeon, philosopher, and pamphleteer Victor Mariette (alias Victor Wreight). Another, the disgraced revolutionary Jacob Dupont, had attended the National Convention, where he preached atheism. But the most notorious of Charenton's political detainees was Donatien-Alphonse-François, Comte de Sade. At his death in December 1814, Sade's undesired stay at Charenton had lasted thirteen years.

Was Mary Shelley's Charenton day trip her closest encounter with Sade, or did she also read his fiction? Critics have long doubted that she did. Mario Praz's seminal work of Romanticism, La carne, la morte e il diavolo nella letteratura romantica (1930), translated in 1933 as The Romantic Agony, claims that Mary Shelley named the Frankensteins’ persecuted servant girl Justine, ‘like Sade's unhappy virtuous heroine’ only ‘by an odd coincidence’. Praz finds the Justine imagery in Mary Shelley's Valperga equally accidental. ‘All Mrs. Shelley did’ for Sade's literary legacy ‘was provide a passive reflection of some of the wild fantasies’ invented by Percy Bysshe Shelley and Lord Byron, whom he assumes had in fact read Justine. In other words, Mary Shelley appropriated those men's knowledge of Sade's text, having none of her own.