King John opens with King Philip of France supporting Arthur, the young son of John’s deceased brother Geoffrey, as the rightful heir to the English throne. John’s mother, Queen Eleanor, and the French court engage in heated disputes over legitimacy. Meanwhile, the Bastard (Philip Faulconbridge), illegitimate son of Richard the Lionheart, joins John’s cause after being legitimized. War breaks out between England and France, with the strategic town of Angiers caught between the two armies.

The conflict temporarily resolves when John’s niece Blanche marries the French Dauphin Lewis, uniting the two kingdoms. However, Cardinal Pandulph arrives from Rome and excommunicates John for refusing to accept Stephen Langton as Archbishop of Canterbury. The Cardinal persuades France to break the peace and resume war against the excommunicated English king. In the ensuing battle, Arthur is captured by John’s forces.

John orders Hubert to kill the young Arthur, but Hubert cannot bring himself to murder the child and instead hides him. When Arthur later dies attempting to escape from his prison tower, the English nobles believe John has murdered him and defect to join the invading French forces led by the Dauphin. As John faces rebellion from within and invasion from without, he submits to the Pope’s authority to regain legitimacy. However, he falls ill and is poisoned by a monk at Swinstead Abbey.

The Bastard rallies the remaining English forces and discovers that the French nobles plan to kill their English allies after victory. This intelligence helps reconcile the English lords to their king. John dies, and his young son Henry is crowned King Henry III. The Dauphin withdraws his forces, and Cardinal Pandulph negotiates peace. The play concludes with the Bastard delivering a patriotic speech about England’s strength when united against foreign threats.