After Margaret de Clisson and her sons failed in their plot to overthrow John V, Duke of Brittany and assert their claim to Brittany between 1420 and 1422, the Penthièvre family was forced to appear before both the French Parliament and the Breton Estates at Vannes. Two of William's brothers were found guilty of felonies and treachery on 16 February 1425 and their properties in Brittany were confiscated. William himself had already been given over to the duke of Brittany as a hostage on 29 July 1420.