Helene Alving is about to dedicate an orphanage she has built in the memory of her dead husband, Captain Alving. She reveals to Pastor Manders that she has kept hidden the negative aspects of her marriage, primarily due to the immoral and unfaithful behavior of her late husband. She has built the orphanage to deplete her husband's wealth so that their son, Oswald, might not inherit anything from him. Pastor Manders had previously advised her to return to her husband despite his philandering, and she followed his advice in the belief that her love for her husband would eventually reform him. But, her husband continued his affairs until his death, and Mrs. Alving did not leave him to protect her son from the taint of scandal, and for fear of being shunned by the community.
During the action of the play, she discovers that her son Oswald (whom she had sent away to avoid his being corrupted by his father) is suffering from syphilis that he inherited from his father.[a] She also discovers that Oswald has fallen in love with Regina Engstrand, Mrs. Alving's maid, which is a serious problem because Regina is revealed to be an illegitimate daughter of Captain Alving, and therefore Oswald is falling in love with his half-sister.
When the sibling relationship is exposed, Regina leaves, and Oswald is in a state of despair and anguish. He asks his mother to help him die by an overdose of morphine in order to end his suffering from his disease, which could put him into a helpless vegetative state. She agrees, but only if it becomes necessary. The play concludes with Mrs. Alving having to confront this decision: whether or not to euthanize her son in accordance with his wishes.