For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Dr. Stockmann's life looks pretty good at first. He's surrounded by friends and family. He's brought the chance for great prosperity to the town he lives in, by promoting the idea of build the Baths. Then he makes the unfortunate discovery that the new Baths are contaminated with bacteria. When Mayor Stockmann, the Doctor's brother, refuses to act on Dr. Stockmann's suggestions for fixing the polluted water, Dr. Stockmann becomes determined to expose the truth no matter what.
Act II
The Mayor blocks his brother at every turn. When the Doctor refuses to keep the contamination quiet, the Mayor turns his brother's allies against him. Both Hovstad, the editor of the newspaper, and Aslaksen, the head of the Householders' Association, fall prey to the Mayor's manipulations. The Mayor is even able to keep Dr. Stockmann from reading his report at a town meeting called for that very purpose. Enraged, the Doctor uses the meeting instead to tell the town just what he thinks about them. Dr. Stockmann goes off on a furious tirade about terrible stranglehold that the ignorant majority has on society.
Act III
The whole town has turned against the Doctor by this point. He's all set to move to America, when a series of visitors come by to try and tempt him away from his principles. The Mayor, Aslaksen, Hovstad, and Morten Kiil all put in an appearance and try to corrupt him in some way. In the end through, the Doctor decides to stay in the town he loves and fight for truth and justice.
With the help of his daughter, Petra, he determines to start a school to educate the poor. Soon he'll have a troop of young acolytes with whom to fight the ignorant majority.