The monk Medardus is entrusted with a relic for safekeeping, the devil's elixir, which awakens sensual desires in humans. He drinks from it and is seized by passion for a woman he does not know, whose confessor he is and who resembles an image of Saint Rosalia in the church. When he is sent on a mission to Rome by his abbot, he breaks his vow and sets out to search for his beloved. This leads to a journey filled with misadventures, during which Medardus becomes a criminal, commits several murders, changes his identity multiple times, and repeatedly encounters his doppelgänger, a mad monk. He finds Aurelie, the woman he was searching for, must flee again, and ultimately awakens from deep unconsciousness in a Roman monastery. Here, he reads the writings of an old painter, who turns out to be the creator of the image of Saint Rosalia. Medardus recognizes the painter as his great-grandfather and learns that the people he murdered during his search were his half-siblings. The subsequent writing of his memoirs becomes a confession and penance for Medardus. In his horror novel, Hoffmann illustrates the path of a man losing his identity almost to the brink of madness, a journey that is only reversed through the realization of his own origins and thus through self-discovery.