Piers Gaveston, in exile from England in his native France, receives a letter from his friend and probable lover, Edward II. Upon the death of his father, Edward II has been newly crowned King of England, and in his letter her reveals that he has revoked Gaveston's banishment and wants his favorite to come share in his own wealth and power. Gaveston eagerly complies, delighted at the prospect of seeing Edward but also hopeful that he can use the King's affection to his own advantage.
Tensions begin to surface, however, even before Gaveston makes his return to England known. As Gaveston watches from a place of hiding, Edward II argues with a group of nobles who regard Gaveston as a manipulative social climber and support his continued exile. Although the King's brother, the Earl of Kent, warns Mortimer Junior and the other nobles that they are dangerously close to committing treason, they stand firm in their opposition and leave for their homes threatening war. Gaveston then reveals himself to Edward, and the two share a joyful reunion during which Edward makes Gaveston Earl of Cornwall and gives him the authority to issue commands and draw money from the treasury in the King's name. The joyful reunion, though, is marred by the arrival of the Bishop of Coventry, who makes it clear that he also opposes Gaveston's return. With Edward's encouragement, Gaveston assaults and imprisons the Bishop and then confiscates his property.
The attack on Coventry further cements the nobility's low opinion of Gaveston, as does Edward's ongoing neglect of his wife Isabella, the sister of the King of France. Along with the Archbishop of Canterbury, the nobles Mortimer Junior, Mortimer Senior, the Earl of Warwick, the Earl of Lancaster, and the Earl of Pembroke together issue an order for Gaveston's exile. However, despite their insistence that Edward has obligations to both the Church and the English nobility, Edward at first refuses to sign the order. In response, the nobles forcibly arrest Gaveston and threaten rebellion, causing the King to reluctantly agree to his favorite's banishment. A sorrowful parting follows, during which Edward promises to send money to Gaveston in Ireland.
Shortly after Gaveston departs, however, Isabella begins trying to persuade the nobles to allow him to return: Edward, who suspects her of having an affair with Mortimer Junior, holds her responsible for Gaveston's exile and refuses to even speak to her while Gaveston is away. The nobles agree, reasoning that recalling Gaveston will at least provide them with an opportunity to murder him. Naturally, the nobles do not share this plan with Edward, and their apparent willingness to compromise brings them back into the King's good graces: Edward even entrusts Mortimer Senior with leading his army in Scotland. Meanwhile, Edward prepares for Gaveston's return by calling Gaveston's fiancée, Lady Margaret de Clare, back to court so that the two can be married (Lady Margaret is Edward's niece, so marriage to her will formally tie Gaveston to the royal family). Lady Margaret brings with her tutor Baldock and a family retainer named Spencer Junior—two men who hope to find employment with Gaveston and eventually become favorites of the King.
Virtually as soon as Gaveston has arrived back at court, however, another quarrel breaks out between him and the nobility. Matters only worsen when Mortimer Junior learns that his uncle, Mortimer Senior, has been captured and is being held for ransom by the Scots. Edward is unwilling to pay for Mortimer Senior's release himself, which prompts the nobles to list all the ways in which they feel the King has behaved irresponsibly—by spending money on art rather than the military, by jeopardizing diplomatic ties through his treatment of the Queen, by flouting the nobles' own opinions, and so on. Edward ignores all of this, decisively alienating not only the nobility but also his own brother, who goes to join Mortimer Junior, Lancaster, and the other earls in planning an assault on the court at Tynemouth to capture Gaveston.
The attack on the castle forces Edward and Gaveston to split up, fleeing in different directions. The nobles choose to pursue Gaveston, eventually overtaking him. Initially, they agree to allow Gaveston to see Edward one last time before he is executed. Warwick, however, is unhappy with the compromise and ambushes and kills Gaveston as he is being escorted to the King. When Edward learns of this, he swears revenge and prepares to go to war with the nobles, who are now demanding that Edward cease with his favoritism of Spencer Junior as well.
Edward's forces succeed in defeating the nobles in the initial battle, and Warwick, Lancaster, and Mortimer Junior are all arrested. The first two are eventually executed, but Mortimer Junior succeeds in escaping to France with Kent. There, they join forces with Isabella, whom Edward had initially sent with their son, Prince Edward, to negotiate with France regarding England's claims to the region of Normandy. Isabella's loyalty to her husband has finally worn thin, however, and she is now trying to find allies who will help her install her son as king instead (something the Prince himself does not want).
Edward believes that he can successfully curb the threat the Queen poses by buying off the French nobility, but Isabella and Mortimer—who now in fact are lovers—eventually manage to find a supporter in a nobleman named Sir John of Hainault. Together with Kent, they return to England and fight the King's forces in a battle at Bristol. This time, it is Edward who is defeated, although Baldock, Spencer Junior, and the king himself succeed in escaping to a monastery where an Abbot offers to hide them. A man who works as a mower, however, betrays their whereabouts and all three are arrested: Spencer Junior and Baldock are taken off to be executed while Edward is imprisoned in Kenilworth under the guard of the Earl of Leicester.
Together with the Bishop of Winchester, Leicester manages to persuade Edward to surrender his crown. Nevertheless, Mortimer Junior quickly dismisses Leicester for being too sympathetic to Edward's plight, replacing him with Berkeley, whom he then also dismisses. Mortimer finally settles on Gourney and Maltravers as guards, instructing them both to torment Edward as much as possible and to move him back and forth from Kenilworth to Berkeley in order to frustrate any escape attempts: Mortimer has learned that Kent now regrets the role he played in his brother's overthrow and is plotting to free him. Gourney and Maltravers comply with these instructions, mocking Edward and apprehending Kent when he tries to make contact with his brother.
Meanwhile, Mortimer Junior has made plans for Prince Edward's coronation. Since the new king is still a boy, Mortimer himself will wield de facto power as the Lord Protector, as well as the lover of the Queen Mother. Nevertheless, he feels his position will not truly be secure until Edward II is dead, and therefore arranges for an assassin—Lightborne—to murder him. Having sent Lightborne on his way, Mortimer quickly asserts his authority over Prince Edward (now Edward III) by ordering Kent's execution against the new king's wishes.
Lightborne arrives at Berkeley and explains his mission to Gourney and Maltravers, who—unbeknownst to him—have orders to murder Lightborne as soon as Edward is dead. Initially pretending to sympathize with Edward, Lightborne urges the deposed king to lie down on a feather bed, where he then kills him with a hot poker. Gourney then stabs Lightborne and flees, while Maltravers reports the crime to Mortimer Junior. As he does so, however, Isabella enters in a frenzy, reporting that Edward III has learned of his father's death and suspects both her and Mortimer of murder. Edward III then enters himself, accompanied by members of the nobility and bearing the written order Mortimer had issued for Edward II's death. Realizing the jig is up, Mortimer bids farewell to Isabella and stoically accepts his impending execution. Isabella herself continues to plead with her son but is ultimately unsuccessful: Edward III orders her imprisonment until he can learn whether and how she was involved in his father's murder. Finally, he orders a hearse to be prepared for Edward II, on which he places the severed head of Mortimer.