Richard I, also known as Geoffrey Plantagenet, also known as Richard Cordelion is killed by a man named Austria. As left in Richard's will, his youngest brother John becomes Richard's successor to the crown of England. However, Constance, widow of Richard's younger (and John's older) brother Geoffrey, feels that her adolescent son, Arthur, should have become the new king of England. Constance appeals to the King of France, Philip, to help her oust John from the throne and place Arthur on it. A third claim to the throne appears (though none of the characters ever acknowledge him) in the personage of Philip, a bastard son of Richard I, actually older than Arthur, and much more similar in manners and looks to Richard I than Arthur is. John knights the Bastard (as he is called throughout by Shakespeare) and allows him to accompany him to the city of Algiers (in France) where they, along with Queen Elinor (the mother of both King John and Richard I), confront King Philip (of France). King Philip is actually helped by the man Austria, supposedly since Austria is sorrowful for having killed Richard I. King Philip's son, Prince Lewis (the Dauphin), also helps his father threaten King John.
The two kings and their armies fight one another to prove which is the true king of England to Hubert, a leader in Algiers. Hubert cannot be convinced, and instead offers a compromise whereby Prince Lewis marries Blanch, daughter of Richard I and niece to King John. The kings agree and the marriage is settled, with the dowry including some outlying British lands and peace between King John and King Philip. To appease Arthur, and more-so his mother Constance, King John makes Arthur the Duke of Britain and Earl of Richmond. Constance does not appreciate the titles, since she only wants her son Arthur to be king. The Bastard does not approve of the marriage and entitlements, and fears bad things will become of it.
On the wedding day, Cardinal Pandulph (a legate under the Pope) arrives and orders King John to allow the Papal chosen Archbishop of Canterbury to take office, an act that King John had not been allowing. King John continues to disobey the Pope's wishes, and consequently, Pandulph excommunicates King John. Out of fear of repercussions, King Philip abandons his peace with King John and war breaks out again. During battle, the Bastard kills Austria (in revenge for Austria killing Richard I, the Bastards's father), King John captures Algiers, and John captures Arthur. John orders Hubert to return to England with Arthur and to kill him, hoping Arthur's death will secure John's title to the throne (reminiscent of Richard III). Pandulph suggests to Prince Lewis that he try to become King of England, playing on the English subjects' inevitable outrage over Arthur's sure-to-come future murder by King John.
Hubert tries to burn out Arthur's eyes (an unexplained shift from murder), but cannot, though he tells King John that Arthur is dead. The English Lords denounce King John for killing Arthur and secede to help Prince Lewis. In sorrow over the kidnapping and death of her son, Constance dies. Queen Elinor also dies, though reasons are not given. Hubert then tells John that Arthur is in truth alive, cheering him up, though unbeknownst to anyone, Arthur has leapt to his own death from a castle wall. King John repents to the Pandulph and is reinstated into the church. War on English soil ensues with the Bastard actually leading the army and acting as the King, since King John falls ill and seems incapable of making decisions. The Bastard's army wins the day's battles. A dying Frenchman, Melun, warns the English Lords that Lewis plans on beheading them as soon as the battle with the English is over, so the Lords switch back to King John's side. Resting at a monastery, a monk poisons King John, though the monk himself dies after tasting the food for King John. King John's son Prince Henry shows up (a fourth claim to the English throne) in time to witness his father's death. Pandulph convinces the French to make peace and return to France, and Prince Henry is named the new king. Time period is approx. 1210-1216 A.D. Also, King John signed the Magna Carta, though Shakespeare makes no mention of this.