King John refuses the demands of Chatillon, an ambassador from King Philip of France, to yield the English crown to young Arthur, Duke of Bretagne. John then intervenes in a dispute between Robert and Philip Faulconbridge over their inheritance. It emerges that Philip is the bastard son of King Richard. Philip gives up his share of his inheritance in return for service to John, who knights him.
The English then prepare for France. The French king and his ally Austria are meeting to besiege Angiers when they receive news of John's arrival. The two sides parley, and then prepare to fight. Hubert of Angiers refuses to open his gates to either side until one is victor, but the matter is not resolved when both sides claim victory after a battle. The Bastard suggests that the two sides unite to attack Angiers, but Hubert forestalls this by proposing a marriage between John's niece, Blanche, and Lewis the Dauphin. The parties agree, and they prepare for the wedding.
The Bastard, angry at the way events have turned out, reflects on the madness of it all. Arthur's mother Constance angrily rejects the new alliance. Cardinal Pandulph arrives from Rome, and threatens King Philip with excommunication if he allies himself with John, who is considered a heretic because of his opposition to the Church in England. Philip, after some mind-searching, complies, and the battle recommences. The French are defeated; the Bastard kills Austria, and Arthur is taken prisoner, to Constance's great distress. John orders Hubert to kill Arthur, but Hubert is dissuaded by Arthur's heartfelt pleas.
Meanwhile, Pandulph persuades the Dauphin to continue the fight in England. John celebrates at home with a second coronation, and tries to keep the support of Pembroke, Salisbury, and other nobles. Their fears that John is plotting Arthur's death cause John to regret his request to Hubert, and he is delighted when he learns from Hubert that Arthur is alive. But Arthur falls to his death in trying to escape from prison, and the nobles defect when they find his body, believing that his death was due to John. John is forced to accept Pandulph's demands for reconciliation with Rome, in return for an assurance thet he will dissuade the Dauphin from attacking. But Lewis, whose confidence has been boosted by the support of the English nobles, refuses to do Pandulph's bidding.
The French make progress, though a fleet carrying reinforcements is wrecked. The English lords learn from Melun that the Dauphin plans to kill them after his victory, so they return to John. The Dauphin sues for peace.
The news arrives too late for John, who has been poisoned by a monk and dies soon after, leaving Prince Henry and the Bastard to reflect on the country's future.