Well, the big plot twist is that the town was the site of a horrible slaughtering of indigenous people, and that this small town's proud history has largely been either fabricated or misrepresented to make their ancestors seem like heroes. And that even the name of their town was derived from a racial slur.
And the one man in the town who wanted to make this history widely known, to atone for their ancestors' crimes, was fired and silenced. All of this is revealed to Armie Hammer's character, who has been kept in the dark throughout the play, and he speaks out against their willful ignorance.
Then the other big stylistic twist is that the play suddenly becomes more surreal, and all of the city counsel member begin a kind of tribal ritual which involves smearing themselves with blood. This is the twist that I think had some people scratching their heads, and feeling a bit frustrated by the sudden change in style. Though I personally thought it worked.