The play opens with The Barker introducing the audience to the main characters in the play in a prologue, and explaining that Arturo Ui is more dangerous than Richard III. The action begins in the early 1930s with a group of men, who are part of a Cauliflower Trust in Chicago attempting to figure out how to get their crops sold to local grocers. If they don't, the cauliflower will rot and they will lose everything. We see Arturo Ui, a known gangster hanging around, and no one wants anything to do with him. The Trust go to a shipping tycoon named Sheets and strong-arm him in order to give up his shares of his company, which the Trust then give to Dogsborough, a man who has an impeccable reputation. The reason they do this is that with Dogsborough in control of Sheet's shipping business he can convince the city to give him a loan that will repair the docks. But, the Trust desires to use the loan to help their business with no intention of using it for repairing the docks.
The city then decides to investigate the loan and determine if the money is being spent appropriately. This is when Arturo Ui comes in. A man named Bowl comes to Ui telling him how the Cauliflower Trust has taken the loan money which Dogsborough was able to approve from the city, and he knows that the investigation will cause the Trust and Dogsborough to be exposed of corruption.
Ui takes this as his opportunity to find his way into the seat of power. He controls Bowl, who is the link to the corruption, thus he controls the Trust and Dogsborough who don't want to get caught. With this Ui and his men begin to extort the local grocers into paying protection money to them and purchasing the cauliflower from the Trust. The grocers only do this after one of Ui's men burns down a block of stores while the grocers are refusing. Next, a trial over the fires occurs and Ui and his men drug the man accused of the crime, who didn't actually do it, and they have the judge overseeing the case on their side. Finally, Ui's men gun down Bowl who is coming in to testify.
With this, Ui has kept Dogsborough and the Cauliflower Trust out of the spotlight, but they must now give power to Ui which they do, and Ui becomes the leader of the city through manipulation and extortion. He desires to take it even further, to go to more cities and gain control over them as well. As he does he learn that Cicero, the next city on his agenda won't accept his offer unless Ui's right hand man and best friend, Ernesto Roma is out of the picture. Ui cannot believe it, he cannot betray Roma. But in the next scene he has Roma killed. He then takes Cicero and demands from Ignatius Dullfeet, the newspaper editor, that he stop printing articles that paint him as a monster. Dullfeet does as Ui asks, but Ui kills him anyway.
Ui then has a dream that Roma has come to him as a ghost with a bullet in his head. He is haunted and even commands his bodyguards to shoot at the ceiling where he believes Roma is. Finally, the grocers of Chicago and Cicero have assembled to speak out against Ui, they want to vote him out. But, Ui gives a speech to them stating that whoever is not for him is against him, and they shall suffer the consequences. The grocers know that he is a dangerous man and they all vote for him to remain in power.
The play ends with Ui taking off his mustache to speak directly to the audience, asking them to learn to look instead of gawking. The play is a satire that is based upon Hitler and his Nazi party's rise to power in Germany and Europe in the 1930s. All of the events that occur in the play parallel actual events that happened which allowed Hitler to come to power.
