What is the main action of the play The Crucible by Arthur Miller?

The main action in Arthur Miller's The Crucible is a trial. The play is a fictionalized take on the famed Salem witch trials that saw a group of young women tried for witchcraft during a period from 1692 to 1693 in Salem, Massachusetts. The actual trials were a brutal example of the puritanical beliefs and mob mentality that existed in the early New England colonies. Miller's play is loosely based on these actual historical events and he wrote the play as an allegory to the anti-communist fervor of 1950s America. Many people in this era of McCarthyism felt as if they were being persecuted and had the sense that the House Un-American Activities Committee (HUAC) was a kind of "witch hunt."
One of the main characters in The Crucible is Abigail Williams, who is portrayed as being the ringleader of a group of young women accused of Satanism and witchcraft. This starts a process which quickly gets out of hand. The play follows the development of the trial, which degrades into chaos as people turn on one another and accuse each other of witchcraft to cover up for a whole host of indiscretions.
In the end a number of the villagers are found guilty of witchcraft and have their property, and in some cases their lives, taken away. The play's main villain, if there is one, is Thomas Danforth, the chief judge in the trial. He refuses to stop the proceedings even as the whole process tears the community apart—he serves as a suitable stand-in for Senator Joseph McCarthy, architect of HUAC and the anti-communist craze of the 1950s.