Murder in the Cathedral is about the Archbishop of Canterbury's moral struggle against Henry II. Henry has stripped him of the chancellorship of England because the archbishop has excommunicated bishops Henry had appointed. Thomas Becket, the archbishop, arrives at Canterbury at odds with the King.
During the play, four tempters try to persuade the archbishop to violate his conscience. The first tempter tries to lure him to compromise with Henry because of the easy, successful life he could have were he reinstated as chancellor. The second reminds him of the worldly power Thomas would gain if he bent his morals to accommodate the king. The third tempter wants him to lead a band in overthrowing the king: then Thomas could place the church at the center of power. Thomas refuses all these temptations, which parallel Satan's temptations of Christ in the desert.
Then a fourth tempter arrives and tells Becket he should allow himself to be killed by Henry so that he can become a martyr and be remembered for all time ever after as a great man. Becket has a harder struggle but refuses this temptation. In the end, he takes the morally correct path: he refuses to compromise with Henry. He is killed by Henry's knights.
This play was commissioned in 1935 to critique both the brutal tactics of the Nazis in murdering anyone who got in their way, just as Henry kills Thomas when he becomes a problem, and to critique compromises with evil. At the end of the play, his priests, who can be seen as speaking for everyman-- "we acknowledge ourselves as a type of the common man," they say--ask forgiveness for a host of sins that were characteristic of their times, the Nazi era and today:
Who fear the injustice of men more than the justice of God .../who fear ... the push into the canal ... have mercy upon us.
The play is a moral message to the world, a cry for better and more courageous behavior on the part of people everywhere.