Baron Bisclavret of Brittany disappears every week for three days. His wife begs him to tell her why, and he finally relents, explaining that he turns into a wolf. He tells her he hides his clothes so that he can find them after three days and turn back into a human.
His wife is a little freaked out by this revelation, and doesn't want to be with him any more. She tells a knight who has loved her top follow him and steal his clothes so that he cannot return to human form. The baron fails to return to his people, a search for him is to no avail, and the "widow" marries the knight.
A year later, while hunting, the king comes upon a wolf that rushes at the king and kisses his foot and leg. Amazed at the behavior, the king decides to bring the wolf back to the castle. The wolf's gentleness is remarkable, until...
...at a large celebration, the knight who married Bisclavret's wife arrives and is attacked by the wolf. The king threatens Bisclavret, who backs down. The court assumes that the knight has somehow wronged the wolf.
Later, the king is visiting Bisclavret's former barony, and takes the wolf along. Bisclavret's "widow" comes to the king bearing gifts, but when the wolf sees her, he rushes at her and tears off her nose. A wise man links this unusual attack with the first attack: that the two are married, and the woman was married to the missing baron. The king has the woman tortured, whereupon she confesses what she did to her husband. The produce the baron's clothing, and he becomes the baron Bisclavret once more. The king restores his lands and exiles the baroness and her knight. Her descendants are born nose-less afterward.
Marie de France claims she heard this performed snd translated it from Breton, along with a collection of other stories. We have several stories from her, and I'll tell you more about her tomorrow.