The first scene of Act I begins with a boy named Howard who is scouring the ground for worms and taunting his companion, Melinda, with it. He says that her family was once a pack of worms too just like everyone else. Melinda runs off. The Hillsboro minister’s daughter, Rachel, comes and asks Mr. Meeker, the bailiff, to see Bert Cates. She urges him to not teach evolution in the local school again as it is a crime. Cates does not see it as a crime and says that he was merely teaching from the biology textbook. Rachel and Cates keep arguing about it and they hug after a while. When Meeker comes again, they pull away. Meanwhile, everyone in the town is anticipating Matthew Harrison Brady’s arrival. He is to act as a prosecutor in Cates’ trial. E.K. Hornbeck, a journalist from the Baltimore Herald, arrives as well. Upon his arrival, he notices a caged monkey and compares its eagerness to take a penny to the greed of human beings. The entire town rejoices Brady’s appearance and are seen are carrying anti-evolutionist banners. Hornbeck stands in the back and observes the scene.
Brady is asked to give a speech and he is stated an honorary colonel in the state militia. While having brunch all together, the district attorney, Davenport, says that he is delighted to be working for Brady. Rachel tries to defend Cates and Brady takes her aside to talk with her.
Hornbeck says that the Baltimore Herald has sent Henry Drummond to act as defender for Cates. This alarms the mayor and Reverend Brown, Rachel’s father. They think of ways to prevent him from participating but Brady insists that defeating him in front of everyone would prove to be a greater spectacle. After everyone leaves, Hornbeck lets Rachel know of his support for Cates by showing him his approving article. Rachel expresses her anxiety about the public turning against Cates. Hornbeck tells her that the public stopped supporting Brady when they learned to form opinions of their own.
At the storefront, Drummond arrives. “Hello Devil, welcome to Hell", says Hornbeck. The prosecution, consisting of Brady and Davenport, and the defense, Drummond, interview several locals to hire them as jurors. A man named Bannister is hired because he says he attends church and does not read Darwin’s works of evolution, as he is illiterate. Brady asks everyone in the courtroom to remove their jackets and he makes a snide remark about Drummond’s purple suspenders, to which Drummond retorts back smartly. Brady appoints a man named Dunlap as a juror after he says he believes in God and Brady. Drummond insists on questioning him and he points out the unfairness in granting Brady the tile of Colonel. The mayor grants him the same title temporarily. The next man to be interviewed is named Sillers, and he is asked about his religious beliefs. He says that he does not have any children, and Brady asks him that if he did have any, what would he do if his hypothetical child had a “Godless teacher” at school. Drummond objects to this question and it is withheld. Sillers is appointed as a juror as well. Drummond asks him about his religious practices, and he says that his wife takes care of the religious matters in the family. Drummond asks him about Charles Darwin to which Sillers relies that he is merely a store worker. The judge declares the end of jury selection. Brady starts to object and brings up Drummond’s past trickery. Drummond demands for a “Read your Darwin" banner in place of the “Read your Bible" banner. Rachel implores Drummond to call off the trial. Cates says that he will stand, no matter what. Rachel admits that she might have to testify against him. Cates is alarmed and expresses his fear of crucifixion if Rachel revealed their conversations. Drummond and Rachel have a conversation, and Rachel talks about how she is scared of her father since her childhood. She says that Brady is not as intimidating as her father. Drummond says that Cates is a good person and asks for Rachel’s support in the case.
Brady walks along the courthouse lawn followed by a horde of reporters, excluding Hornbeck. One of them asks him about Drummond, to which Brady replies that they used to be friends before, but even if his own brother challenged the works of the Bible, he would stand up to him. Brady notices Hornbeck and tells him about his biased reporting. Hornbeck says that he writes as a critic. Brady then invites him to the prayer meeting. Drummond is invited by glares as he enters the meeting. The reverend tells the story of God’s creation of the world to the crowd. Rachel enters the meeting. Reverend’s communication with the crowd becomes more and more wild. He asks them to curse out the jailed man and advises them to pray for Cates’ condemnation to eternal damnation. Rachel asks her father to stop and the reverend says that the Lord would punish those who want to forgive Cates.
Brady interrupts the sermon and reminds the crowd of the Bible messages, which encourage forgiveness. As the crowd leaves, Brady approaches Drummond and reminds him of their past friendship. Drummond says that perhaps it was Brady who left him by standing still. Brady is astonished at this reply.
The trial continues and Howard is on the stand. He is interrogated about Cates’ teachings and asked about whether he ever mentioned God. Brady starts mocking evolutionists. Drummond keeps asking Howard about his own opinions on evolution and the prosecution objects to it. Drummond says that he has his own right to think. Howard is let go from the witness stand. Next, Rachel is called to the stand. Brady asks her about Cates’ religious beliefs. She says that Cates stopped attending church when a young boy drowned in the river and Reverend Brown said he would be damned because he was not baptized. Cates starts shouting about his disdain against religion, and the judge demands order.
The trial resumes, and Brady asks Rachel to recall their private conservations about religion. She hesitates and then Brady quotes Cates’ words, where he had said that man created God and marriage was equal to animal breeding. Rachel objects, claiming to have been misquoted and misunderstood. She is then dismissed. Drummond then calls Brady to the stand, which is deemed strange by the judge but, nevertheless, he allows it. Drummond then asks Brady about whether he has read the writings of Darwin. When Brady says no, Drummond asks him how he could dismiss something he does not know. He further interrogates him about the scientific plausibility of phenomenon such as Jonah and the whale, Joshua stopping the sun, and the like. Brady simply says that these are miracles brought upon by God. Drummond asks Brady to state the difference between a man and a sponge. When Brady says that the difference is the will of God, Drummond says that Cates wants the same God-given right as the sponge. This reply receives applaud from the crowd. Drummond then brings a rock to Brady and asks him its estimated age. When he says it could be ten million years old, Brady says it is impossible as 4004 B.C. was the year of creation. Drummond again debunks his theory saying that the concept of twenty-four-hour day might not have existed back then. Brady is infuriated at this humiliation. He keeps spewing content from the Bible and Drummond keeps dismissing everything, to the crowd’s delight. The judge calls off the trial.
The following day is the verdict. Hornbeck is present and bows before Brady in a taunting manner. A microphone is placed and Drummond is warned not to utter the word “God". Cates is announced guilty, which receives a polarized reaction from the crowd. Cates is granted the right to speak and he says that he will continue to oppose the law, which he claims to be “unjust". The judge sentences him to a punishment of $100 fine. Brady’s request to read a statement is accepted, but the courtroom is filled with constant cacophony. The reporters interrupt Brady and his speaking time is cut short. Thus, Brady collapses and is carried out. Faint murmurs of a supposed presidential speech are heard from his mouth as he is carried out, a fact that amuses Hornbeck. Cates asks Drummond if he was the winner and Drummond says he has achieved a moral victory and that Hornbeck and the Baltimore Herald have paid $500 for his bail. Rachel enters the scene and announces that she is leaving her father. She hands Cates a copy of Darwin and apologizes to Drummond for her closed-mindedness. The judge then arrives and tells them that Brady has passed away. Drummond sympathizes but Hornbeck starts mocking him crudely and Drummond defends him. Hornbeck says that Brady will be forgotten and then, he leaves. Cates and Rachel step out together, leaving behind the copy of Darwin. Drummond picks it up and picks up the court’s copy of the Bible on his other hand, balancing the two. With heavy thoughts clouding his mind, he leaves the court.