February 1912: The curtain of the Opéra-Comique rises to reveal medieval Brittany. Washerwomen gossip about lepers who walk among the healthy, in defiance of the law. The women hint to Maria that her son, Ervoanik, is in love with the beautiful Aliette, who, it is rumoured, is one of these freely roaming lepers. Ervoanik does love Aliette, and when he informs his parents of his decision to marry her, his father explodes in rage. Doesn't he know that she is the daughter of a leper? Ervoanik refuses to believe it. The hideous appearance of Aliette's mother, Old Tili, betrays her own leprosy as she attempts to lure young children with her singing to eat her infected breads. When Aliette and Ervoanik stop at Old Tili's cottage to rest before making a holy pilgrimage, Tili assumes that Aliette intends to infect him, just as she has infected so many others. When she realizes that Aliette truly loves Ervoanik and plans to live with him chastely, Tili lies, telling her that he already has a wife and children. Aliette confronts Ervoanik, and he claims it is true, as a joke. She immediately touches her lips to a cup of wine and offers it to him. He drinks. In the third act, Ervoanik, now a leper, awaits the procession of the dead, which leads him to his house of exile. Aliette joins him.