Joanna of Flanders, Countess de Montfort and Duchess of Brittany, abruptly vanished from public life after 1343 amid the Breton Wars of Succession during the Hundred Years War. As wife of the late Duke John de Montfort, Joanna’s rightful place was in Brittany as regent of the duchy for their five-year-old son and heir John of Brittany. Despite her fame for the defense of Hennebont in 1342 during her husband’s imprisonment, she along with her children had accompanied Edward III of England to Britain in February 1343 and seemingly never departed. She resided in England in Tickell Castle, Yorkshire, in comfortable obscurity until her death around 1374. What happened to her and why? The answers to those questions belie the core complexities of medieval social structures, the care of the vulnerable, and the custody of women.