Like his other historical plays, King John is rooted in factual events and explores, with some liberty for creative expression, the reign of King John of England who ruled from 1199-1216. John was the son of Eleanor of Aquitaine and Henry II, and fathered Henry III of England.
The play begins as a French ambassador enters King John's court. His request is dramatic, the ambassador demands on behalf of King Philip of France that John renounce his throne. King Philip believes his nephew Arthur has claim to the throne that King John occupies.
While John presides over a dispute over inheritance between Robert Faulconbridge and Philip the Bastard, his older brother, it becomes evident that Philip the Bastard is the first illegitimate son of King Richard I and therefore John's half-brother. Their mother Eleanor also sees their brotherly resemblance. As Queen she suggests that Philip let go of his Falconbridge claim and accept a knighthood from John, who gives him the new knighted name of Richard.
Meanwhile in France, King Philip takes the English town of Angiers, proclaiming to the citizens that they will be attacked unless the pledge their allegiance to Arthur instead of King Philip. France and King Philip are supported by Austria. As English forces arrive, Queen Eleanor and Arthur's mother Constance bicker. King Philip and King John make their case to the citizens of Angiers', but the people vow to support whomever the rightful King is whenever that is decided upon.
The bastard now known as Richard makes a proposal that calls for England and France to penalize the citizens of Angiers' for their non partisanship. The citizens are not eager to be punished and propose an alternative plan. They want Louis the Dauphin, King Philip's son, to marry King John's niece Blanche. This clever scheme will enable John more claim to the throne while allowing Louis to obtain more French territory. Arthur's mother Constance is furious at Philip but the pair are married, leaving Arthur's claim seemingly disregarded.
King John finds further trouble when the Cardinal Pandolf arrives and accuses him of appointing an archbishop without the consent of the church. John does not back down and is excommunicated by the church, and the Cardinal stands behind Louis.
A bloody war ensues, leaving the future in turmoil and Arthur imprisoned in an English cell. With French plans to invade England, Louis told he now has a strong claim to take the English throne. King John orders Arthur's execution, but the executioner Hubert can't stomach the murder. John is advised to release Arthur, but believes it is too late as he receives inaccurate news from Hubert that Arthur is already dead.
Once King John's noble advisors hear that Arthur is dead, they join sides with the French Louis. The French monasteries are unhappy with the English King's attempt to steal French gold. John learns that Arthur is in fact still alive.
Just as news of his life delighted John, Arthur jumps to his death from a castle wall. Rumors swirl around who is to blame for his death, most blaming John. The war rages on with a massive losses of life on both sides of the battlefield, and most English nobles return to John's side after a warning from a French soothsayer.
John is poisoned by an angry monk and dies surrounded by his nobles. In his death he brings peace as the English unite around his son as the rightful heir. The bastard Richard reflects to the audience that bickering within England could open the door to foreign attack.