The future is looking bright in Dr. Stockmann's hometown on the coast of southern Norway. At his urging, the town has built some Baths, which will bring lots of tourists and more importantly tourist dollars. Trouble arrives when the Doctor discovers that the water of the Baths is teeming with bacteria, which are guaranteed to make everybody sick.
At first, several of the town's leading men like Hovstad, the paper's editor, and Aslaksen, the head of the Householder's Association, support the Doctor and his discovery. However, the Mayor, Dr. Stockmann's brother, stands firmly against it, because the necessary improvements will cost the town tons of cash and will make him look like an idiot. The Mayor swiftly turns the entire town against his brother, and Dr. Stockmann finds himself suddenly in a hostile environment.
Refusing to have the truth be silenced, the Doctor calls a town meeting to read his findings. Once again, though, he is foiled by his crafty brother; the Mayor manipulates the procedures of the meeting so as to keep the Doctor from reading the report. Enraged, Dr. Stockmann launches into a tirade on a new "truth" he's discovered. He announces that the true corruption in the town and the entire country is that all the power lies with the complacent majority, most of whom are too ignorant to know what's best for them.
The Doctor's impassioned speech only serves to turn the entire town against him. His windows are smashed with rocks, he loses his job, and his house. In the final act, he is visited by several people who attempt to get him to retract his words. These corrupt attempts only serve to reinvigorate the Doctor, who determines to start a school to spread knowledge and truth to the poor. By the end of the play, Dr. Stockmann has found strength in being alone.