Oswald Alving has returned home to visit his mother on one of the occasional visits he has made since leaving home as a young boy. He was sent away to prevent him from becoming morally contaminated by his father, Captain Alving, who subsequently died of syphilis. This time, however, he intends to stay and marry the maid, Regine; he is unaware that Regine is his half-sister, sired by the profligate Captain Alving.
Parson Manders, the mother's former lover, also visits and reprimands Mrs. Alving for not living a more conventional life and rearing her son. In the play's climax, Oswald reveals that he, too, is suffering from syphilis and will inevitably develop dementia. To make up for the past and to prove her love, Oswald asks his mother to give him a fatal dose of morphine when signs of dementia appear. At the end of the play it is not clear what she will do.
Dr. Thomas Stockmann, a public-minded doctor in a small town famous for its public baths, discovers that the water supply for the baths is contaminated and has probably been the cause of some illness among the tourists who are the town's economic lifeblood. In his effort to clean up the water supply, Dr. Stockmann runs into political cowards, sold-out journalists, shortsighted armchair economists, and a benighted citizenry. His own principled idealism exacerbates the conflict. The well-meaning doctor is publicly labeled an enemy of the people, and he and his family are all but driven out of the town he was trying to save.
