The scene is Dr. Stockmann's living room; the dining room is visible through a door downstage. Mrs. Stockmann welcomes Billing to her dinner table. He is late, and so the meat is cold. There is a knock at the door; it is her brother-in-law, the mayor. He says he doesn't want to indulge in so much food so late at night. Hovstad, the editor, arrives. He and the mayor greet each other stiffly and begin talking about the baths. They both agree that the new baths are going to be very good for the town. It is mentioned that the baths were originally Dr. Stockmann's idea, a suggestion that upsets the mayor.
Hovstad goes to eat, and soon Dr. Stockmann arrives. With him he brings his two sons, Eilif and Morten, and Captain Horster, another late guest for dinner. He shows him into the dining room before noticing the mayor. The mayor is surprised to see how much the guests eat. The doctor counters by talking about the excitement of watching young people eat--young people who will eventually grow up and improve society. He contrasts them with "old fossils" like himself and the mayor, who is slightly perturbed by these notions. He comforts the mayor by mentioning how happy he is to be living in a city and to have a sturdy income.
The mayor asks Dr. Stockmann about an article he has written for Hovstad's newspaper. The doctor quickly says that he hopes the article will not be printed just yet as it may not be appropriate depending on some developments that the doctor is as yet unsure of. The mayor is aggravated that the doctor will not tell him what he is talking about, and he tells Dr. Stockmann that he should think more of how to function within a society and less as an individual. Angrily, he leaves.
Hovstad, Billing, Horster, and Mrs. Stockmann come in for liqueurs and cigarettes. Hovstad talks of the rocky relationship between the mayor and the People's Herald. Horster says that he is sailing for America soon, and the two newspapermen, Billing and Hovstad, are shocked that he doesn't care that he will miss the upcoming election. Petra enters, tired from teaching her night school classes. She has a letter that Dr. Stockmann has been eagerly looking for. He goes into the study to read it. Meanwhile, Petra and the newspapermen start up a discussion of paganism. Meanwhile, Billing and Hovstad decry the hypocrisy that Petra must go through as a teacher.
Dr. Stockmann comes in waving the letter. He says that no one will be able to call this discovery another one of his delusions. Apparently, the baths, which are viewed as the savior of the town, are polluted. The doctor sent samples from the water to a lab, and now the results are back, in the letter he has received. Milldale, near the source of the baths' water, is full of polluted water that seeps into the baths' pump room. The pollution comes from tanneries and other industry. Dr. Stockmann assures everyone that the problem can be fixed by replacing the water system. The doctor further notes that if the town had followed his advice about how to build the drains in the first place, they would not have had these problems. The group is very enthusiastic and praises the doctor for saving the town.
Many of the characters in An Enemy of the People are very concerned with politics. The mayor is interested in maintaining his position. He is very disturbed when Dr. Stockmann talks of a younger generation growing up to change things. He also seems very insecure, which is no doubt related to the rather competitive spirit shared by him and his brother. The popular opinion that the baths were the idea of Dr. Stockmann enrages the mayor.