King Henry is doing ritual penance for his suspected role in the assassination of Thomas à Becket, archbishop of Canterbury, formerly his friend and chancellor of England. While the king wonders aloud where their friendship went wrong, Thomas’s ghostly presence appears before him, telling him to pray instead of talk. The scene then shifts to the early days of the two men’s boon companionship. Henry makes an impulsive appointment of Becket to the position of chancellor, a move intended to give the king more control of a rebellious clergy. Equally mistrusted by the bishops and by the king’s own henchmen, Becket nevertheless performs his duties with grace and skill, earning the grudging respect of both sides.

The king, however, fails to understand his friend’s true motivations. While riding through the woods shortly after Becket’s appointment as chancellor, the two are caught in a downpour. The king’s questions to Becket show the king to be quite ignorant regarding his own subjects and the laws that govern them. The king then becomes enamored of the young peasant girl in the shack where they take cover. Becket pretends to want the girl for himself in order to parry the king’s indiscretion. Becket soon thereafter loses his mistress to the king’s misguided playfulness: The king decides that Becket should return the favor that the king granted Becket in the shack. Specifically, the king should sleep with Becket’s mistress. Becket, circumspect as ever, agrees to share Gwendolen’s favors in exchange for those of the peasant girl. Gwendolen stabs herself rather than sleep with the king. Henry, oblivious as ever, concludes that he has survived an assassination attempt on Gwendolen’s part.

As chancellor, Becket finds himself obliged to educate and civilize the king and his henchmen-barons, instructing them in manners and in enlightened self-interest, especially when dealing with conquered enemies such as France. Becket also perceives a growing domestic threat from the British clergy. Upon learning of the death of the archbishop of Canterbury, Henry decides to move against that threat by naming his friend Becket to the post, much against Becket’s wishes and better judgment. Becket, a former candidate for the priesthood, resolves to do, as usual, the best possible job of whatever is handed to him, giving away all of his possessions to the poor. The king, meanwhile, faces trouble in his household as well as in his reign. He is henpecked by his wife and mother, both of whom envy his friendship with Becket, and he is unable to control his adolescent sons.

Becket’s resignation as chancellor, on the grounds that he cannot serve God and king at once, drives the king to the desperate act of plotting against Becket with Gilbert Folliot, bishop of London and chief among Becket’s enemies within the church. Soon Becket, condemned on patently false charges of embezzlement and witchcraft, seeks refuge, going first to France and then as far as Rome for an audience with the pope. Becket then returns to France. The French, however, find him too worrisome a fugitive for permanent asylum. The king of France, seeking compromise, arranges for Becket to meet with King Henry on neutral ground in an effort to reconcile their differences. Both men still care about each other, but neither will abandon his principles and the meeting fails. Returning from France, knowing his life to be in danger, Becket waits at Canterbury for the inevitable, a murderous assault by the henchmen of the king. The play’s final scene, replicating the first, portrays the king’s ritual flagellation for his part in the assassination plot. Such penance is in fact a deft political move, defusing a revolt mounted against the king by his own sons. The king is showing, in his canny statecraft, how much he has learned from Becket. A final irony is that Becket, from the grave, manages at last to transform Henry II from a petty tribal chieftain into a true monarch.