Mrs. Alving is building an orphanage as a memorial to her husband. This edifice is to be dedicated the next day, and her old friend Parson Manders has come to perform the ceremonies. In a private conversation, Mrs. Alving tells the Parson that her husband had been a complete degenerate, and she is using the rest of his money to build the orphanage so that she can leave only her money to her son Oswald, who has just arrived home from years and years abroad.
In a private talk with his mother, Oswald confesses that he has an incurable disease which the doctors think was inherited. Oswald, however, believes his father to have been a perfect man. Mrs. Alving, then, must confess that Mr. Alving had indeed been a degenerated man and that Oswald caught the disease from his father. Oswald knows that he is dying and wants to take the maid as his mistress so that the maid, Regina, will give him poison when he is next struck by the disease. Mrs. Alving then explains that Regina is in reality his half sister. This does not bother Oswald, but Regina refuses to stay. Oswald then tells his mother that she must administer the medicine when the next attack comes. As the play closes, Oswald begins to have his attack and his mother does not know whether to administer the poison or to endure the agony.