In 1215 King John had agreed to the terms of Magna Carta, but he then reneged on his word, plunging the kingdom into war. The rebellious barons offered the throne to the French prince Louis and set off the chain of events that almost changed the course of English history.

Louis first arrived in May 1216, was proclaimed king in the heart of London, and by the autumn had around half of England under his control. However, the choice of a French prince had enormous repercussions: now not merely an internal rebellion, but a war in which the defenders were battling to prevent a foreign takeover. John's death in October 1216 left the throne in the hands of his 9-year-old son, Henry, and his regent, William Marshal, which changed the face of the war again, for now the king trying to fight off an invader was not a hated tyrant but an innocent child.

1217 charts the nascent sense of national identity that began to swell. Three key battles would determine England's destiny. The fortress of Dover was besieged, the city of Lincoln was attacked, and a great invasion force set sail, and unusually for the time, was intercepted at sea. Catherine Hanley expertly navigates medieval siege warfare, royal politics, and fighting at sea to bring this remarkable period of English history to life.