King John is visited by an emissary from France, demanding that he hand his throne over to his nephew Arthur, whom the French King Philip believes is the rightful heir to the throne. If John refuses to abdicate, war is threatened.

John oversees a land dispute between Robert Faulconbridge and his older brother Philip (known as 'the Bastard'), during which it becomes apparent that Philip is the illegitimate son of King Richard I. Queen Elinor, mother to both Richard and John, recognises the family resemblance and suggests that he renounce his claim to the Faulconbridge land in exchange for a knighthood. John knights the Bastard under the name Richard.

In France, King Philip and his forces besiege the English-ruled town of Angers, threatening attack unless its citizens support Arthur. Philip is in turn supported by Austria, who is believed to have killed King Richard. The English contingent arrives, and Elinor and Arthur's mother Constance trade insults. Kings Philip and John stake their claims in front of Angers' citizens, but to no avail - their representative says that they will support the rightful king, without committing themselves as to who that might be.

The Bastard proposes that both England and France unite to quell the rebellious citizens of Angers, at which point they propose an alternative: Philip's son, Louis the Dauphin, should marry John's niece Blanche, a scheme that gives John a stronger claim to the throne, while Louis gains territory for France. Though a furious Constance accuses Philip of abandoning Arthur in favour of this new scheme, Louis and Blanche are married.

Cardinal Pandulph arrives from Rome bearing a formal accusation that John has disobeyed the pope and appointed an archbishop contrary to his desires. John refuses to recant, whereupon he is excommunicated. Pandulph pledges his support for Louis, though Philip is hesitant, as he has just established family ties with John. Pandulph brings him round by pointing out that his links to the church are older and firmer.

War breaks out, Austria is beheaded by the Bastard (in revenge for his father's death), and both Angers and Arthur are captured by the English. Elinor is left in charge of English possessions in France, while the Bastard is sent to collect funds from English monasteries. John orders Hubert de Burgh to kill Arthur. Pandulph points out to Louis that he now has as strong a claim to the English throne as Arthur (and indeed John), and Louis agrees to invade England.

Hubert finds himself unable to kill Arthur. John's nobles urge Arthur's release. John agrees, but is wrong-footed by Hubert's announcement that Arthur is dead. The nobles believe he was murdered, and defect to Louis' side. The Bastard reports that the monasteries are unhappy about John's attempt to seize their gold. Hubert has a furious argument with John, during which he reveals that Arthur is still alive. John, delighted, sends him to report the news to the nobles.

Arthur is killed after falling from a castle wall. The nobles believe he was murdered by John, and refuse to believe Hubert's entreaties. John attempts to make a deal with Pandulph, swearing allegiance to the Pope in exchange for Pandulph negotiating with the French on his behalf. John orders the Bastard, one of his few remaining loyal subjects, to lead the English army against France.

While John's former noblemen swear allegiance to Louis, Pandulph explains John's scheme, but Louis refuses to be taken in by it. The Bastard arrives with the English army and threatens Louis, but to no avail. War breaks out with substantial losses on each side, including Louis' reinforcements, who are drowned during the sea crossing. Many English nobles return to John's side after a dying French nobleman, Melun, warns them that Louis plans to kill them after his victory.

John is poisoned by a disgruntled monk. His nobles gather around him as he dies. The Bastard plans the final assault on Louis' forces, until he is told that Pandulph has arrived with a peace treaty. The English nobles swear allegiance to John's son Prince Henry, and the Bastard reflects that this episode has taught that internal bickering could be as perilous to England's fortunes as foreign invasion.