A messenger from France arrives in the English court, demanding that King John abdicate his throne in favor of his nephew Arthur. The messenger speaks for King Philip of France, who supports Arthur's claim as the rightful heir to the throne; when John refuses to step down, France threatens war.
The Bastard and his younger brother enter to dispute their inherited lands. John rules that the Bastard has the right to the lands, because a wife's offspring become a father's heir, no matter who the actual father. John's mother, Eleanor, takes a liking to the Bastard, because it is rumored that his father was her son and John's brother, Richard the Lionhearted. She proposes that he leave his lands to his younger brother and join her armies under the name of the Bastard of Richard the Lionhearted. He agrees, and John knights him.
In France, Philip and his forces prepare to attack the English-held town of Angers unless its citizens swear allegiance to Arthur. John and his armies arrive; Arthur's mother, Constance, and Eleanor insult each other, as various members of each side argue. Each king asks Angers's citizens whom they support as the King of England, but the citizens say they support the rightful king. Philip and John's armies go to war, but are so equally matched that neither side wins. The citizens of Angers still won't decide between them. The Bastard suggests that the English and French armies unite to conquer the disobedient town of Angers, then fight each other later. They agree and prepare to attack. At this point, the citizens of Angers suggest an alternative. Marry Philip's son Louis to John's niece Blanche, they suggest, then peace can be settled. Louis and John are pleased with the idea, because it strengthens John's ties to the throne, and Louis gains English-held French territory. The Bastard marvels at the changeable minds of the nobles.
Constance is upset by the turn of events and blames Philip for having abandoned his support for Arthur when presented with a more fruitful bond to the English throne. Louis and Blanche are married when Pandolf, an ambassador from the pope, arrives. He charges John with having disobeyed the pope in the matter of a posting of an archbishop, but John is not about to obey the orders of the distant pope. Pandolf excommunicates John and charges Louis with the duty of overthrowing John. Philip, whose family has just been linked to John's by marriage, hesitates while his nobles try to influence him. Pandolf reminds him that his ties to the church predate his connection with John and threatens excommunication. Finally Philip gives in and breaks with John.
After inconclusive battles in which the English capture Arthur, John prepares to return to England, leaving his mother in charge of English-held French territories and sending the Bastard ahead to collect the finances of the English monasteries. John instructs Hubert to look after Arthur and then asks him surreptitiously to kill him. Meanwhile Pandolf tries to encourage the French to fight, suggesting to Louis that he now can lay the same claim to the throne of England as Arthur, because he has married into a branch of the English royal family. Louis agrees to attack England.
Hubert tries to kill Arthur but he is so enchanted with Arthur's innocence that he is unable. He tells Arthur that no one can know he is alive. Meanwhile John's lords have asked for Arthur to be released, so John agrees to order Arthur's freedom. Hubert enters and reports that Arthur is dead; the lords believe Arthur was assassinated and depart to join Louis's army. The Bastard returns from the monasteries, reporting that the people are not happy about John robbing the monasteries, and they predict John's downfall. John yells at Hubert, accusing him of having tricked him into ordering the death of Arthur, which he claims he never wanted. Finally Hubert reveals that Arthur is alive. A relieved John sends him after the departed lords to report the news.
Arthur tries to flee England but foolishly leaps off a castle wall and falls to his death. The lords come upon his body and are horrified at the brutality they believe was used to kill the boy. Hubert enters and reports that Arthur is alive; the lords point out Arthur's body and accuse Hubert of having killed him. Hubert says Arthur was alive when he left him. The lords depart to meet Louis.
John strikes a deal with Pandolf; he agrees to honor the pope if Pandolf can turn away the French army. The Bastard arrives to report the departure of the lords; John tells him about his deal with Pandolf. The Bastard wants to fight the French and leads John's army at his behest.
John's departed lords swear allegiance to Louis. Pandolf arrives with news that John has reconciled with Rome and tries to dissuade Louis from attacking, but Louis says he won't be ordered around by anyone. The Bastard arrives to speak to Louis and threatens him with terrible destruction at the hands of the English armies unless Louis retreats. Each side prepares for battle.
A French lord is wounded, and he tells the English lords that Louis planned to kill them if he won. He urges them to rejoin John, and they do so. Louis's reinforcements are lost at sea, greatly dimming his prospects of victory. Meanwhile the Bastard meets Hubert, who reports that John has been poisoned by a monk at a monastery, where he had been awaiting reports from the Bastard. The English lords and John's son Prince Henry gather around the ill King. The Bastard reports that he has lost many of his men, who drowned in a tide.
John dies from the monk's poison. The Bastard prepares to attack Louis, but the nobles report that Pandolf had just brought them a peace treaty from Louis. The Bastard and the lords swear allegiance to Henry, and the Bastard speaks of England and how it will never be taken by foreign conquerors unless it is first damaged by internal strife.