Mrs. Alving, widow of the admired and respected Captain Alving, has been living on her husband’s estate with her maid Regina, carrying on her husband’s philanthropic projects, such as a recently completed orphanage. She is aided in this by the primly proper Pastor Manders, whom she once loved and who disapproves of her “freethinking” ideas. Her son Oswald, who has been leading an artist’s life in Paris, returns home for the dedication of the orphanage, which burns to the ground before the ceremonies.
Returning exhausted from the fire, Oswald reveals to his mother that he is suffering from a venereal disease, the origin of which he does not understand. When he declares his intention to marry Regina, Mrs. Alving is forced to reveal what she has previously confessed to pastor Manders: Captain Alving was in reality a dissipated sensualist masquerading under the guise of gentility; all his reputation and philanthropy were the result of her own industry, and Regina is actually his illegitimate daughter. Oswald realizes that his disease is heredity in origin and obtains from his mother her promise to administer a deadly drug to him should be become insane. As the play ends, Oswald’s mind disintegrates completely under a final seizure, and Mrs. Alving must decide whether to give her son the poison as she has promised or let him go on living in a state of idiocy.