Her third marriage (after 6 September 1410) was to Henry le Scrope, 3rd Baron Scrope of Masham. That year, Scrope was made a Knight of the Garter. He served Henry IV as treasurer, and was executed in 1415 following the failure of his plot with the Earl of Cambridge (Joan's former stepson, being the son of her first husband, and nephew by marriage, being the husband of Anne de Mortimer, her sister's daughter) to assassinate Henry V and place Edmund Mortimer, 5th Earl of March (Joan's nephew) on the throne. (The Earl of March had been the heir presumptive of Richard II. In 1399 Richard was forced to abdicate in favour of Henry IV, and for the next few decades Mortimer served as a focal point for conspiracies aimed at removing Henry IV and his heirs from the throne.) Lord Scrope and Cambridge were both beheaded on 5 August 1415 at Southampton Green, Hampshire, England. Cambridge's then four-year-old son, Richard Plantagenet, ultimately championed his father's cause, which evolved into the Wars of the Roses and the Yorkist claimants achieving the throne.