The setting is again Dr. Stockmann's living room. Mrs. Stockmann gives him a letter. It is the report on the pollution of the baths that he had sent off to his brother the mayor. It has been returned, with a note that the mayor will come by to speak with the doctor. He and Mrs. Stockmann agree that the mayor is probably jealous that Dr. Stockmann made the discovery.
Morten Kiil stops by. He is delighted by the "monkeyshine" that Stockmann has invented and says that he will laugh if the city leaders are stupid enough to believe it. He emphasizes that the "tiny animals" in the water are too little to see. Hovstad enters, and he and the doctor go to speak in private. Hovstad tells the doctor that he hopes to use the information about the pollution of the baths as a starting point for an all-out attack on the city's leadership. He says that the real pollution comes from them. The doctor agrees that conservatism is bad, but he is hesitant to attack the town's leadership, which is made up of the most qualified men, including his own brother.
Aslaksen stops by. He wants to assure Dr. Stockmann that he can count on the support of the Temperance Society and the powerful Homeowners Association. Aslaksen is the chairman of the latter. He wants to stage a moderate demonstration in favor of fixing the baths. Dr. Stockmann does not think this will be necessary, as he is convinced that the baths' board of directors will see that the repairs are necessary. Aslaksen emphasizes that he does not want to upset the town leaders. Dr. Stockmann is quite moved by Aslaksen's support.
After Aslaksen leaves, Hovstad calls him a cowardly, if decent, man. Dr. Stockmann is confused, but he tells Hovstad that if the mayor refuses to make changes to the water system--as unthinkable as this seems to the doctor--Hovstad an print the doctor's entire report in the paper. The editor leaves, and Dr. Stockmann goes to talk with his family. He tells them he is very proud to have the "solid majority" behind him.
The mayor arrives. He is upset that the doctor conducted the investigation without telling him. He is concerned that the report exaggerates the situation. He says that the cost to make the suggested repairs would be very expensive and take two years. He says that he is not convinced that there is a real problem. He goes on to describe how losing the baths would be a catastrophe to the town's economy. He says that the board might be willing to make some changes in a few years.
Dr. Stockmann is outraged. Throughout his speech, he makes amazed interjections. He says that he will not submit to the fraud that the mayor is suggesting. The mayor insists that nothing about the pollution must reach the public, but the doctor tells him that the People's Herald will support him and print a story about it. The mayor responds by talking about what a helpful brother he's been--getting the doctor a job--and he goes on to say that he hoped to gain control of the doctor by employing him. Now, the doctor will lose his job if he does not cooperate. The mayor feels that the doctor is out of control, an embarrassment to himself and to the city. The brothers rehash their argument over who is responsible for the baths. Dr. Stockmann reminds his brother that if his original plan had been followed, there would be no problem. The mayor insists that the doctor merely cannot submit to authority. He demands that the doctor "conduct further studies" and make a public announcement that his findings were false. He asserts that, when acting as an employee, the doctor has no individual rights. At this moment, Petra, who has been listening at the door, bursts in and tells her father that he must stand up for himself. The mayor urges Mrs. Stockmann to try to have some practical influence over her husband.