Hello! In a nutshell, Murder In The Cathedral is about a power struggle between the English king, Henry II, and the Archbishop of Canterbury, Thomas Becket, in the 12th Century. Here is a link for a short historical background of the story. If you can carve out some time in your busy schedule, try reading this short article; I promise you that it will go a long way towards helping you make sense of the story.
The play starts out with the local populace waiting for the return of their archbishop. They wonder if things will now go from bad to worse between their archbishop and the bad-tempered king. Furthermore, their lives are miserable and they're not too happy that it could get worse.
You must realize that apart from the king, the position of Archbishop of Canterbury is the highest church position a man in Becket's position could have. King Henry had once made Becket his Lord Chancellor. Lord Chancellors in Henry's time were predominantly important churchmen. Why? The clergy knew how to read and write. They advised the king on spiritual and government matters, they were involved in all levels of judicial government, and they were Keepers of the Great Seal.
However, Henry was not satisfied with merely having Becket as Lord Chancellor. You see, the king was greatly troubled that his power in the kingdom did not seem to extend to the Church. He felt that punishments for lay clergy were too lenient; he rather preferred the clergy to be tried in his own secular courts rather than ecclesiastical (religious) courts. That's why he wanted Becket to be Archbishop of Canterbury. After all, his reasoning was that they were already great friends: why not make good use of that friendship? Henry actually thought that giving Becket such a coveted and powerful church position would extend his power into the Church. Becket warned Henry about making him Archbishop, but Henry wouldn't listen.
Unfortunately for Henry, Becket was one of those men who took his religious responsibilities seriously. When he became archbishop, he also gave up the party life: no more rich banquets to attend and certainly no more drinking oneself to oblivion. Henry did not like this one bit. The king was angry that Becket would not deliver church authority into his hands. Hence, you may now understand this great historical quote:
"What sluggards, what cowards have I brought up in my court, who care nothing for their allegiance to their lord. Who will rid me of this meddlesome priest."
Becket fled to France rather than agree to any of Henry's demands. The play ends with Becket being cut down with swords at the hands of four knights supposedly loyal to King Henry.
Themes: 1)The major one being the separation of church and state. Should the king also have power over religious matters?
2) Absolute power corrupts absolutely. This is true whether it references religious leadership or secular leadership.
I hope I have made Murder In The Cathedral a little more accessible to you. Thanks for the question!