Helen Alving is about to dedicate an orphanage she has built in the memory of her dead husband, Captain Alving.
She reveals to her spiritual advisor, Pastor Manders, that she has hidden the evils of her marriage, and 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.
However her husband's philandering continued until his death, and Mrs. Alving was unable to leave him prior 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 so that he would not be corrupted by his father) is suffering from inherited syphilis, and - even worse - has fallen in love with Regina Engstrand, Mrs. Alving's maid, who is revealed to be an illegitimate daughter of Captain Alving, and thereby Oswald's own half-sister.
The play concludes with Mrs. Alving having to decide whether or not to euthanize her son Oswald in accordance with his wishes. Her choice is left unknown.
