The set is the editorial office at the People's Herald. Hovstad is writing at the desk. Billing enters with Dr. Stockmann's report. They discuss the doctor's powerful writing and how they hope to use it to attack the government. Aslaksen is in the other room, and they are careful not to let him hear. Hovstad is excited, because if the mayor accepts the doctor's proposal, he will face the fury of the big stockholders, and if he rejects it, he will face the giant Homeowners Association.
Dr. Stockmann enters and tells them about his argument with the mayor. The three are excited to "tear down" the current administration. Aslaksen enters, and they assure him that both the radicals and the moderates will want to support the doctor. The doctor asks him to pay special attention to his report to make sure that are no typos or mistakes. The doctor is deeply moved by their support and encouragement.
After he leaves, Hovstad and Aslaksen agree that Dr. Stockmann will be very useful to them, although for different reasons. Aslaksen is worried that the doctor is not prudent enough, but Hovstad wants to use him as a political firebrand. Changing the subject, Aslaksen mentions that Governor Stensgard sat in Hovstad's editor's chair before him. Billing says something about mixing radical journalism and politics, and Aslaksen reminds Billing that he himself is running for council secretary. Billing assures them that he is only doing it to annoy the establishment.
Aslaksen steps out, and Billing and Hovstad discuss how much they would like to get rid of him. They depend on him because he lets them print on credit. They wonder whether Dr. Stockmann might be able to help finance the paper. He will likely become wealthy, since the rich Morten Kiil will probably remember the Stockmanns in his will. Billing leaves and Petra enters. She had agreed to translate an English story for the paper, but now she refuses, on the grounds that its content is against everything for which the paper stands. The article is about a higher purpose guiding people's actions. Hovstad replies that Billing, who Petra is in some manner courting, thought the piece would be good fodder to keep the paper's simpler readers happy. Petra is shocked to hear that Billing would be so calculating, and Hovstad also mentions Billing's run for secretary. Petra still refuses to do the piece, but she thanks Hovstad for his support of her father. He implies that it makes it easier that she is his daughter, and Petra leaves, disgusted.
The mayor arrives, to Hovstad's surprise. The mayor comments on how nicely the paper is set up. He begins to talk about the doctor's proposal for the baths, but Hovstad plays dumb, until the mayor notices the doctor's report laying on the desk. The mayor tells Hovstad and Aslaksen that if the doctor's plan for the baths goes through, it will mean a huge sacrifice for the town. The expenses will have to come out of a municipal loan, and the baths will have to be shut down for two years. Hovstad and Aslaksen begin to change their minds about supporting Dr. Stockmann. The mayor assures them that the doctor's report is pure fantasy. Suddenly, they see that Dr. Stockmann himself is approaching, and the mayor hides in a side room.
The doctor wants to see the proofs of his article, but they're not yet ready. He expresses to the two men that if any kind of celebration is being planned in his honor, he wants them to put a stop to it. Just as Hovstad is trying to tell the doctor how things really stand, Mrs. Stockmann enters. She has come to tell Dr. Stockmann not to throw away the livelihood of his family by printing his article. The doctor reminds her that he has the solid majority behind him, and she tells him that it's a horrible thing to have behind him. He tells her to go home while he worries about society. Then, he notices the mayor's ceremonial hat and cane lying on a chair. He guesses that the mayor is nearby, puts on the hat, and begins to parade about the office, until the mayor comes out in a fury. The doctor mocks his brother, convinced that he has everyone's support.
Aslaksen and Hovstad tell him they won't print the article. Hovstad won't dare, because the subscribers control the paper and the proposal would ruin the town. The mayor gives Hovstad an official statement he can print to quell any rumors. The doctor resolves to hold a public meeting, but Aslaksen tells him that he won't find an organization to give him a hall.
In the second act, we saw the mayor turn on Dr. Stockmann. When that happened, the doctor still felt confident because he had the People's Herald behind him. In the third act, we see Hovstad and Aslaksen turn against him. The mayor has an easy time convincing them to turn against Dr. Stockmann. It is no surprise that economic arguments and the lack of visible evidence can be used to change Hovstad's mind.
But Ibsen goes further and shows us that Hovstad is simply not a reliable character. We learn that his support of the doctor is partly motivated by his affection for Petra. He even betrays his friend Billing for the sake of getting closer to Petra. Even before the mayor arrives and speaks to Hovstad and Aslaksen, they are discussing how they can use the doctor for their various ends. From the beginning, Hovstad is eager to use the doctor as a way to stimulate some sort of political revolution. When the mayor brings his carefully crafted arguments to men whose integrity is already compromised, they are easily won over to his side.
While the mayor and the doctor remain consistent in their opinions throughout the play, the newspapermen's ideas change. The mayor and the doctor have clear motivations: The mayor wants to stay in power, whereas the doctor is concerned with morality and science but not with economics or politics. The newspapermen, on the other hand, have many motivations, and, therefore, they can't come to a clear conclusion. Hovstad is a leftist radical, but he also wants to keep the paper in business, and he is interested in Petra. Ibsen uses these characters to illustrate how difficult it is to have a clear opinion in modern society. Hovstad can't afford to have a dangerous opinion and is, therefore, helpless when the mayor or the doctor has the upper hand.
Mrs. Stockmann is committed to her husband, but she is also committed to her family. When the doctor endangers the rest of his family by throwing away his job, she doesn't know what to do. She feels that Hovstad is fooling the doctor, and when Hovstad and all the other men turn on her husband, she feels that her husband has been led into a trap. It appears to her that the doctor has consistently tried to do what is best and has been somehow led into a very dangerous position by these men.
A few miscellaneous things should be explained. The mention of "Governor Stensgard" by Aslaksen is an allusion to Ibsen's early play, The League of Youth, in which Stensgard was a central character. Aslaksen was also a minor character in that play.