"Bisclavret" ("The Werewolf") is one of the twelve Lais of Marie de France written in the 12th century. Originally written in French, it tells the story of a werewolf who is trapped in lupine form by the treachery of his wife. The tale was popular and was reworked as The Lay of Melion, and is probably referenced in Sir Thomas Malory's Le Morte d'Arthur with the story of Sir Marrok, who has a similar story. 
Marie de France claimed that she translated this lay, as well as the other eleven she wrote, from the Breton language, in which she claimed to have heard them performed. There have been many translations of her work into the English language, the translation noted below was done by Eugene Mason.
Bisclavret, a baron in Brittany who is well loved by the king, vanishes every week for three full days. No one in his household, not even his wife, knows where he goes. His wife finally begs him to tell her his secret and he explains that he is a werewolf. He also says that while in werewolf form he needs to hide his clothing in a safe place so he can return to human form. The baron's wife is so shocked by this news that she tries to think of ways she can escape her husband. She no longer wants to "lie beside him anymore."  She conspires with a knight who has loved her for a long time. The following week, the baron's wife sends the knight to steal her husband's clothing. When her husband fails to return, she marries the knight. The baron's people search for him but finally relent, feeling that their absentee ruler has left for good.
A year later, the king goes hunting and his dogs corner Bisclavret, now fixed in wolf form. As soon as he sees the king, Bisclavret runs to him to beg for mercy by taking the king's stirrup and kissing his foot and leg. This behavior so astounds the king that he has his companions drive back the dogs and everyone marvels at the wolf's nobility and gentleness. The king takes Bisclavret, still in wolf form, back to the castle to live with him.
The knight who had married Bisclavret's wife is invited to the castle for a celebration along with all the other barons. As soon as Bisclavret sees the man he attacks him. The king calls to Bisclavret and threatens him with his staff. Because the wolf had never acted so violently before, all in the court think that the knight must somehow have wronged him. Soon after, the king visits the area where the baron used to live and brings the werewolf along with him. Bisclavret's wife learns of the king's arrival and takes many gifts for him. When Bisclavret sees his former wife, none can restrain him. He attacks her, tearing off her nose.
A wise man points out that the wolf had never acted so before and that this woman was the wife of the knight whom Bisclavret had recently attacked. The wise man also tells the king that this woman is the former wife of the missing baron. The king has the wife questioned under torture. She confesses all and yields up the stolen clothing. The king's men put the clothing before the wolf, but he ignores it. The wise man advises them to take the wolf and the clothing into a bedchamber and let Bisclavret change in privacy. Bisclavret does so and when the king sees again his beloved baron, he runs to him and embraces him, giving him many kisses. The king restores Bisclavret's lands to him and exiles the baroness and her knight. Many of the wife's female posterity were afterwards born without noses and all of her children were "quite recognizable/ in face and appearance."
The Word “Bisclavret”
In the first part of the poem, Marie de France seems to use the Norman French word for werewolf, garwaf, interchangeably with the Breton term, bisclavret. However, she draws a distinction between ordinary werewolves and Bisclavret. One scholar specifies three evidences for this. “First, [Marie de France’s] statement implies that he is unlike the violent werewolves that she has just described; second, her use of the definite article combined with the fact that Bisclavret is capitalized also implies that he is unique, that he is perhaps the only Bisclavret. Finally it is also noteworthy that Marie uses the term “garwalf” when describing the traditional werewolf. She thus once again distinguishes it from Bisclavret.”
- Scudder, Vida (1917). Le Morte D'Arthur of Sir Thomas Malory & its Sources. E.P. Dutton & Co. p. 33. ISBN 9781437270334. Retrieved 1 May 2013.
...and Sir Marrok the good knight, that was betrayed by his wife, for she made him seven years a werewolf. Sir Marrok is a suggestive figure whose story, told by Marie de France, happens to be extant...
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- Schwerteck, Hans. “Eine Neue Etymologie von “Bisclavret.” Romanische Forschungen 104.1–2 (1992): 160–63.