Eliot wrote his play for an audience expected to know the historical story of Thomas Becket and King Henry II. For that reason, a brief review of that story, contained in the "About Thomas Becket and King Henry II" section of the Note, will greatly aid comprehension of this summary.
Murder in the Cathedral opens in the Archbishop's Hall on December 2nd, 1170. A Chorus, comprising women of Canterbury, has gathered at the cathedral with some premonition of a terrible event to come. In a long speech, they reflect on how their lives are defined by suffering and reflect on their archbishop, Thomas Becket. He has been in exile from England for seven years, after a terrible clash with King Henry II. The women worry that his return could make their lives more difficult by angering the king.
Three priests enter the hall and also lament Thomas's absence and debate the ramifications of his potential return. A Herald arrives, bringing news that Thomas has indeed returned to England and will soon arrive in Canterbury. The Herald quashes their hopes that Thomas's return indicates reconciliation with Henry and confesses his own concern that violence is soon to follow the archbishop's return.
Once the heralds leave, the priests reflect on Thomas's time as Chancellor of England, when he served as secular administrator under Henry. The Chorus, listening to the priests discuss the matter, confess their disappointment at his return, which they believe will bring them more suffering. They admit their lives are hard but predictable, and they would rather "perish in quiet" than live through the turmoil of new political and spiritual upheaval (180).
The Second Priest insults them and insists they fake happiness to welcome Thomas. However, Thomas enters during this exchange and stresses that the priest is mistaken to chide them, since they have some sense of the difficulty that awaits them. He stresses that all should submit to patience, since none can truly know God's plans or intentions.
A series of tempters enters, one by one, each attempting to compromise Thomas's integrity. The First Tempter reminds Thomas of the libertine ways of his youth and tempts him to relinquish his responsibilities in favor of a more carefree life. The Second Tempter suggests Thomas reclaim the title of Chancellor, since he could do more good for the poor through a powerful political post than he could as a religious figure. The Third Tempter posits a progressive form of government, in which a ruler and barons work together as a "coalition." In effect, he offers Thomas a chance to rule and break new ground in government. Thomas easily rejects all three tempters; after all, they are forms of temptation that he has already rejected in his life.
A Fourth Tempter enters and suggests the idea of martyrdom, which he notes would give Thomas the greatest dominion over his enemies. He would be remembered throughout the ages if he allowed himself to die for the church, while his enemies would be judged and then forgotten by time. Thomas is shaken by this temptation, since it is something he has often entertained in his private moments. He recognizes that to die for pride, which is "the wrong reason," would compromise the integrity of a martyrdom, so he must overcome that impulse if his death is to have meaning.
While he considers the dilemma, all of the characters thus far mentioned (except the Herald) give a long address considering the uncertainty of life. When they finish, Thomas announces that his "way [is] clear" – he will not seek martyrdom from fame, but instead will submit to God's will. He has accepted his fate. Part I ends here.
Between Part I and Part II, Thomas Becket preaches a sermon in an Interlude, in which he restates the lesson he learned at the end of Part I. The Interlude is set in the cathedral on Christmas morning, 1170. In the sermon, Thomas considers the mystery of Christianity, which both mourns and celebrates the fact of Christ's death – Christians mourn the world that made it necessary, while celebrating the sacrifice that enables others to transcend that world. He suggests that the appreciation of martyrs is a smaller version of that mystery, and defines "the true martyr [as] he who has become the instrument of God, who has lost his will in the will of God, not lost it but found it, for he has found freedom in his submission to God" (199). He closes his sermon by admitting he might not preach to this congregation again.
The first scene of Part II is set in the Archbishop's Hall on December 29th, 1170. The terrified Chorus begins with an ominous address, after which four boorish knights enter. They insist they are there on Henry's business from France and demand an audience with Thomas despite attempts by the priests to distract them.
Thomas arrives and is immediately insulted and chided by the knights for what they perceive as disloyalty toward Henry and misuse of the archbishop's position to incite opposition to England. Thomas denies their interpretation of events but also reveals a serenity and readiness to die when necessary. The knights attempt to attack him but are interrupted by the priests. A more specific political argument follows, during which Thomas continues to deny their claims and insults them as overly concerned with petty, political matters. Angry, the knights threaten the priests with death if they let Becket escape, and then the knights leave.
The Chorus gives a brutal, evocative speech, and Thomas comforts them. He acknowledges that by bearing necessary witness to the ritual of his death, their lives will grow more difficult. But he maintains that they can find comfort in recollection on having been here this fateful day.
As the knights approach again, the priests beg Thomas to flee, but he refuses. The knights force him from the hall and into the cathedral, against his protestations. As the scene changes, the women of the Chorus steel themselves for the death soon to follow.
The priests bar the doors, which the knights then begin to besiege. The priests' arguments do not convince Thomas, who accuses them of thinking too much of cause-and-effect, rather than accepting God's plan. Finally, the priests open the door and the knights drunkenly enter. They demand Thomas lift all the excommunications he has put upon English rulers. He refuses, and they murder him. While Thomas is being murdered, the Chorus gives a long, desperate address lamenting the life they will now have to lead in the shadow of Thomas's martyrdom.
After the murder is done, the four knights address the audience directly. They wish to explain themselves and defend their actions. The First Knight admits he has no facility for argument, and so acts as an MC to introduce the other knights. The Second Knight says he understands how the audience and history will hate them, but begs the audience to realize the knights were "disinterested" in the murder; they were merely following orders that were necessary for the good of England (216). The Third Knight presents a long, complex argument suggesting that Becket was guilty of betraying the English people and hence was killed justly. The Fourth Knight suggests that Becket willed his own death by pursing martyrdom for the sake of pride, and hence is guilty of suicide, making the knights not guilty of murder.
Once the knights leave, the priests lament Thomas's death and worry about what the world will become. The Chorus gives the final speech, revealing that they have accepted their duty as Christians. They acknowledge that living up to the sacrifice Thomas made is difficult, but that they will be spiritually richer for undertaking this challenge, and they beg mercy and forgiveness from Thomas and God.