The town in which the play is set has built a huge bathing complex that is crucial to the town's economy. Dr. Stockmann has just discovered that the baths' drainage system is seriously contaminated. He alerts several members of the community, including Hovstad and Aslaksen, and receives generous support and thanks for making his discovery in time to save the town. 

The next morning, however, his brother, who is also the town's mayor, tells him that he must retract his statements, for the necessary repairs would be too expensive; additionally, the mayor is not convinced by Dr. Stockmann's findings. The brothers have a fierce argument, but Dr. Stockmann hopes that at least Hovstad's newspaper will support him. However, the mayor convinces Hovstad and Aslaksen to oppose Dr. Stockmann.

The doctor holds a town meeting to give a lecture on the baths, but Aslaksen and the mayor try to keep him from speaking. Dr. Stockmann then begins a long tirade in which he condemns the foundations of the town and the tyranny of the majority. The audience finds his speech incredibly offensive, and the next morning the doctor's home is vandalized. He and his daughter are fired. 

The mayor insinuates that the doctor's actions were merely a scheme to inherit more of Morten Kiil's money, and Kiil himself soon arrives to suggest just such a plan to Dr. Stockmann. However, the doctor refuses all such suggestions and decides to defy authority and remain in town. His family is supportive, and he says that the strongest man is the man who stands alone.