In AD 1120, the realm of England was shaken by the early death of the heir apparent to King Henry I, William Adelin. Henry had no other sons - only a daughter, Matilda. He worked hard to force his barons to accept her as his heir, but on Henry's death in 1135, his nephew Stephen of Blois siezed the throne and plunged England into the cataclysmic Anarchy.
But what if, instead of dying so young, William Adelin had just been impossibly, dangerously inept? What if Henry, instead of attempting to push his daughter as a one-off, had changed the laws of succession to make the eldest child of the monarch, rather than the eldest son, the heir to the throne? What if Matilda had managed to hold onto her throne, and pass it on - first to her son, and then to his firstborn daughter?