King Richard I, the revered 'Lionheart', is dead. His brother John has become King of England, but the French argue that the throne should belong to the boy Arthur, son of John's deceased older brother Geoffrey. Matching the dispute over the throne is a dispute over inheritance in the noble Falconbridge family. It is discovered that Philip Falconbridge (the 'Bastard') is the illegitimate son of Richard I; he is accordingly knighted 'Sir Richard and Plantagenet'. French and English forces fight for the town of Angiers in France; a citizen proposes that the opponents should be united by a marriage between Lewis the Dauphin, heir to France, and John's niece, Lady Blanche. Arthur's mother Constance is furious that the French have given up on her son's claim. John is excommunicated for failing to agree to the Pope's choice for the post of Archbishop of Canterbury. The papal legate Cardinal Pandulph stirs the French to resume war against the English. Arthur is captured and John commissions his servant Hubert to execute the boy; young Arthur dies from a fall while trying to escape. John changes his mind and agrees to the Pope's wishes. A French invasion force is shipwrecked. John falls sick and dies. His son Henry becomes king, though the Bastard remains the most forceful character.
Robert the third earl of Oxford, helped force King John to sign the Magna Carta at Runnymede. He was elected one of the Charter's twenty-five guardians. He was excommunicated by the pope for insolence. He committed treason when he joined a rebellion to hand the throne over to the French Dauphin. In response, King John laid siege to Castle Hedingham. The French Dauphin returned to France and John retained his throne. In the play, Robert the third earl of Oxford is not mentioned.
The Shakespeare King John was based on John Bale's version, which was Protestant propaganda. Bale's version was available only in manuscript, and never published after the 1560s. De Vere likely saw a performance of Bale's version in 1561 at Ipswich, or at least would have access to the manuscript, since de Vere's father was one of the longtime patrons of John Bale.
In 1563, shortly after the death of de Vere's father, his elder half sister Katherine and her husband threatened a lawsuit against de Vere and his sister Mary, claiming he was a bastard and illegitimate claimant to his father's estates. The lawsuit was unsuccessful at the time (but later resurrected). Queen Elizabeth reportedly once called de Vere a bastard - an event which angered the proud de Vere. De Vere himself gave birth to a bastard son - Edward Veer - through Anne Vavasour.
The key character in King John is Philip the Bastard. The historical Philip the Bastard was inconsequential. At the play's beginning, Philip the Bastard is introduced into court with a disinheritance scheme not unlike de Vere's case. Throughout the play, Philip the Bastard utters memorable speeches and immortal lines.
Mary Queen of Scots abandoned the Scottish throne in 1568, driven by a murder scandal in which Burghley's agents may have played a role. She was a legitimate heir to the English throne should Elizabeth die. As such, she was considered a threat to those around Elizabeth, who wanted her dead. In 1586, evidence was brought forward that she was plotting to overthrow Elizabeth. De Vere was one of 45 jurors who condemned Mary, Queen of Scots to death in October 1587. Elizabeth procrastinated in consenting to the execution until February 1588. The execution itself was botched; the executioner required to chops. Elizabeth disowned responsibility, laid responsibility with her secretary, and imprisoned him. In de Vere's King John, King John - like Elizabeth - sidesteps responsibility for sanctioning the death of Arthur - the Catholic heir to the throne. As a propaganda piece, King John blunted the criticism Elizabeth faced for the beheading of Mary Queen of Scots.
The historical Shakespearean plays were the product of de Vere's arrangement with Queen Elizabeth in which she paid de Vere an annuity of £1,000 and in return de Vere writes propaganda plays for the masses to promote the virtues of and loyalty to the regime.