It is late spring afternoon in front of the Mannon house. The master of the house, Brigadier-General Ezra Mannon, is soon to return from war.
Lavinia, Ezra's severe daughter, has just come, like her mother Christine, from a trip to New York. Seth, the gardener, takes the anguished girl aside. He needs to warn her against her would-be beau, Captain Brant. Before Seth can continue, however, Lavinia's suitor Peter and his sister Hazel, arrive. Lavinia stiffens. If Peter is proposing to her again, he must realize that she cannot marry anyone because Father needs her.
Lavinia asks Seth to resume his story. Seth asks if she has not noticed that Brant looks just like her all the other male Mannons. He believes that Brant is the child of David Mannon and Marie Brantôme, a Canuck nurse, a couple expelled from the house for fear of public disgrace.
Suddenly Brant himself enters from the drive. Calculatingly Lavinia derides the memory of Brant's mother. Brant explodes and reveals his heritage. Lavinia's grandfather loved his mother and jealously cast his brother out of the family. Brant has sworn vengeance.
A moment later, Lavinia appears inside her father's study. Christine enters indignantly, wondering why Lavinia has summoned her. Lavinia reveals that she followed her to New York and saw her kissing Brant. Christine defiantly tells Lavinia that she has long hated Ezra and that Lavinia was born of her disgust. She loves her brother Orin because he always seemed hers alone.
Lavinia coldly explains that she intends to keep her mother's secret for Ezra's sake. Christine must only promise to never see Brant again. Laughingly Christine accuses her daughter of wanting Brant herself. Lavinia has always schemed to steal her place. Christine agrees to Lavinia's terms. Later she proposes to Brant that they poison Ezra and attribute his death to his heart trouble.
One week later, Lavinia stands stiffly at the top of the front stairs with Christine. Suddenly Ezra enters and stops stiffly before his house. Lavinia rushes forward and embraces him.
Once she and Ezra alone, Christine assures her that he has nothing to suspect with regards to Brant. Ezra impulsively kisses her hand. The war has made him realize that they must overcome the wall between them. Calculatingly Christine assures him that all is well. They kiss.
Toward daybreak in Ezra's bedroom, Christine slips out from the bed. Mannon's bitterly rebukes her. He knows the house is not his and that Christine awaits his death to be free. Christine deliberately taunts that she has indeed become Brant's mistress. Mannon rises in fury, threatening her murder, and then falls back in agony, begging for his medicine. Christine retrieves a box from her room and gives him the poison.
Mannon realizes her treachery and calls Lavinia for help. Lavinia rushes to her father. With his dying effort, Ezra indicts his wife: "She's guilty—not medicine!" he gasps and then dies. Her strength gone, Christine collapses in a faint.
Peter, Lavinia, and Orin arrive at the house. Orin disappointedly complains of Christine's absence. He jealously asks Lavinia about what she wrote him regarding Brant. Lavinia warns him against believing Christine's lies.
Suddenly Christine hurries out, reproaching Peter for leaving Orin alone. Mother and son embrace jubilantly. Suspiciously Orin asks Christine about Brant. Christine explains that Lavinia has gone mad and begun to accuse her of the impossible. Orin sits at Christine's feet and recounts his wonderful dreams about her and the South Sea Islands. The Islands represented all the war was not: peace, warmth, and security, or Christina herself. Lavinia reappears and coldly calls Orin to see their father's body.
In the study, Orin tells Lavinia that Christine has already warned him of her madness. Calculatingly Lavinia insists that Orin certainly cannot let their mother's paramour escape. She proposes that they watch Christine until she goes to meet Brant herself. Orin agrees.
The night after Ezra's funeral, Brant's clipper ship appears at a wharf in East Boston. Christine meets Brant on the deck, and they retire to the cabin to speak in private. Lavinia and an enraged Orin listen from the deck. The lovers decide to flee east and seek out their Blessed Islands. Fearing the hour, they painffully bid each other farewell. When Brant returns, Orin shoots him and ransacks the room to make it seem that Brant has been robbed.
The following night Christine paces the drive before the Mannon house. Orin and Lavinia appear, revealing that they killed Brant. Christine collapses. Orin knees beside her pleadingly, promising that he will make her happy, that they can leave Lavinia at home and go abroad together. Lavinia orders Orin into the house. He obeys.
Christine glares at her daughter with savage hatred and marches into the house. Lavinia determinedly turns her back on the house, standing like a sentinel. A shot is heard from Ezra's study. Lavinia stammers: "It is justice!"
A year later, Lavinia and Orin return from their trip East. Lavinia's body has lost its military stiffness and she resembles her mother perfectly. Orin has grown dreadfully thin and bears the statue-like attitude of his father.
In the sitting room, Orin grimly remarks that Lavinia's has stolen Christine's soul. Death has set her free to become her. Peter enters from the rear and gasps, thinking he has seen Christine's ghost. Lavinia approaches him eagerly. Orin jealously mocks his sister, accusing her of becoming a true romantic during their time in the Islands.
A month later, Orin works intently at a manuscript in the Mannon study. Lavinia knocks sharply at the locked door. With forced casualness, she asks Peter what he is doing. Orin insists that they must atone for Mother's death. As the last male Mannon, he has written a history of the family crimes, from Abe's onward. Lavinia is the most interesting criminal of all. She only became pretty like Mother on Brant's Islands, with the natives staring at her with desire.
When Orin accuses her of sleeping with one of them, she assumes Christine's taunting voice. Reacting like Ezra, Orin grasps his sister's throat, threatening her murder. He has taken Father's place and she Mother's.
A moment later, Hazel and Peter appear in the sitting room. Orin enters, insisting that he see Hazel alone. He gives her a sealed envelope, enjoining her to keep it safe from his sister. She should only open it if something happens to him or if Lavinia tries to marry Peter. Lavinia enters from the hall. Hazel moves to leave, trying to keep Orin's envelope hidden behind her back. Rushing to Orin, Lavinia beseeches him to make her surrender it. Orin complies.
Orin tells his sister she can never see Peter again. A "distorted look of desire" comes into his face. Lavinia stares at him in horror, saying, "For God's sake—! No! You're insane! You can't mean—!" Lavinia wishes his death. Startled, Orin realizes that his death would be another act of justice. Mother is speaking through Lavinia.
Peter appears in the doorway. Unnaturally casual, Orin remarks that he was about to go clean his pistol and exits. Lavinia throws herself into Peter's arms. A muffled shot is heard.
Three days later, Lavinia appears dressed in deep mourning. A resolute Hazel arrives and insists that Lavinia not marry Peter. The Mannon secrets will prevent their happiness. She already has told Peter of Orin's envelope.
Peter arrives, and the pair pledges their love anew. Started by the bitterness in his voice, Lavinia desperately flings herself into his arms crying, "Take me, Adam!" Horrified, Lavinia orders Peter home.
Lavinia cackles that she is bound to the Mannon dead. Since there is no one left to punish her, she must punish herself—she must entomb herself in the house with the ancestors.
In Mourning Becomes Electra you write: "Oedipus was the Theban king who unwittingly killed his father and MURDERED his mother." [Emphasis mine].
It should read: "Oedipus...MARRIED his mother!"
(Oedipus' mother Jocasta did commit suicide after learning her lover was her son. Oedipus however did NOT "murder" her.)