The capucine monk Medardus drinks from a devil's elixer which makes him sin: he desperately falls in love with a young girl, Aurelie, who confesses to him to be in love with him but then disappears. For Medardus, this is the beginning of a mad quest for her love; he wants to possess Aurelie whatever it takes. He leaves his monastery 'in order to heal from sin', but he is actually going to find Aurelie. Under a false name, he gets to live at both the baron's and the ruler's residence, where he meets her again. But she is afraid of him, and above all, he is a monk - so they cannot be together. Under another false name and a new apperance, he meets her once more in the palace of the ruler. When he is about to marry her on the ruler's wish, he rages in madness and kills her - at least he thinks so.
On his travels, Medardus is accompanied by a mysterious, mad double, and it wasn't until late in the book that I knew for sure this wasn't just a vision or a creature of the mind. He travels to Italy and repents for his crimes, and then returns to his home monastery. There, he meets Aurelie and his mad double once again: Aurelie becomes a nun, but his double murders her when she has just spoken her vow. Medardus, happy that Aurelie has passed free from sin, dies a year after.