Lavinia enters the sitting room. To her horror, she finds herself wishing for Orin's suicide. She implores the portraits to show her Orin's salvation.
Seth appears in the doorway, complaining that the maid is complaining of ghosts against. Lavinia leaves to talk to her. The doorbell rings and Seth lets Hazel and Peter in and exits anew. Uneasy, Hazel complains that Lavinia cannot continue keeping Orin shut up since she is a bad influence. Orin must stay with them for a time.
Orin appears, glancing about to check for his sister. He insists on seeing Hazel alone. He goes to the study and returns with a large sealed envelope, enjoining her to keep it safe. She should only open it if something happens to him or if Lavinia tries to marry Peter. Lavinia can have no happiness and she must be punished.
Hazel implores Orin to confide in and come away with her. Orin furtively suggests that he sneak out. Hazel indignantly refuses to engage in such deceptions. Lavinia is heard from the hall, and Orin hastily sits on the couch.
Lavinia starts at seeing the pair alone. Forcing a joking tone, she notes that Hazel is hiding something. Orin comes to Hazel's rescue by informing her sister that he will be moving in with her for a time. Lavinia refuses and Hazel springs up in rage. She loves Orin better than her and he must move. She prepares to leave, trying to keep Orin's envelope hidden behind her back. Lavinia blocks the door, demanding that she relinquish the manuscript. Rushing to Orin, she beseeches him to make her surrender it—she will do anything. Orin complies and instructs Hazel to forget his "rotting ghost." Hazel hurries out.
Orin makes sure his sister understands she can never see Peter again. A "distorted look of desire" comes into his face. Lavinia does not understand how much she had meant to him since they killed Christine. Perhaps he loves her too much. Caressing her hair, he remarks that she seems neither his sister nor mother but some stranger with the same hair—Marie Brantôme perhaps? Lavinia stares at him in horror, saying, "For God's sake—! No! You're insane! You can't mean—!" Orin replies that otherwise he cannot be sure she would not leave. Without "certainty," he would go mad and confess.
With the word "confess," Orin's tone instantly changes. He urges his sister to confess with him anew. Lavinia refuses. Orin calls upon their ancestors to haunt and hound her for a lifetime. Lavinia wishes for his death. Orin realizes that his death would be another act of justice and that Mother is speaking through Lavinia. He will find Mother again on the island of peace that is Death. He will kneel before her and beg for forgiveness and Orin convulses as if vomiting poison. He pushes Lavinia away with brotherly irritation; Mother is waiting.
Peter appears in the doorway. Unnaturally casual, Orin remarks that he was about to go clean his pistol in the study and exits. Lavinia stops herself from following and throws herself into Peter's arms. A muffled shot is heard.
Act III features Orin and Lavinia's final confrontation. Orin will speak the desire that binds them and their ancestors together. The pair's confrontation is preceded by Orin's last opportunity to escape the Mannon household, an escape made available to him by Hazel.
In this brief exchange, Orin entrusts Hazel with his precious manuscript. Interestingly, the manuscript appears here as not so much that which would bring the Mannon line to judgment as keep Lavinia in the Mannon home. As Orin tells Hazel, she is to read it if he dies or show it to Peter on the eve of their wedding if his sister successfully schemes to marry him. As we will see, these stipulations will enable Orin to keep his hold on Lavinia from beyond the grave.
Orin relinquishes the manuscript upon Lavinia's promise to do anything for him. Not content with the promise that she leave Peter, Orin confronts Lavinia, albeit through innuendo, with the incestuous desire that binds them. Orin proposes that they consummate their assumption of Father and Mother's places. For Orin, the consummation of their unholy union binds them together forever. Incestuous, cross-generational desire is displaced onto a sibling relation who members play Mother/Son, Husband/Wife, and Brother/Sister all at once. The conjured specter of Marie Brantôme—the wild, "animal-like" Canuck who wreaks havoc in the Mannon kinship structure—only lamely triangulates this incestuous pair. Though Orin considers Lavinia more a stranger than his sister and mother, his attraction to her, which reproduces the relationship between mother and son, runs in the family.
Lavinia recoils, perhaps as Christine did, at Orin's proposal. Then, at the word "confess," a break of sorts appears in Orin's speech, and he resumes his calls for atonement. Lavinia then wishes for his death, a wish Orin receives as if sent from Mother. Crucially this moment reveals that Orin's atonement is but another scheme to join the mother in the "secret world" of an illicit love affair. Orin clearly yearns for death at Mother's hands, convulsing symptomatically as if vomiting the poison that Christine put in his father's mouth. The punishment Orin so violently insists upon would allow him to go to Mother on Death's "island of peace." His death is not only the penalty for his crime but a means by which to be with Mother for eternity.
Lavinia, on the other hand, desperately clings to the past's repression, a repression now contingent on her raving brother's death. O'Neill makes use of another suspenseful pause between the suicide victim's epiphany and act of suicide. Clinging to Peter, Lavinia attempts to fill his terrible pause with her desperate ravings about their future happiness. With the gunshot, she defies the ancestral portraits to judge her. She has only kept their secrets. Lavinia exits the scene ever the defiant sentry, burying the past behind her crypt-like mask. Orin has left this sentry with a telling curse, calling upon the ancestors she would silence to haunt and hound her for a lifetime. This curse prefigures Lavinia's ultimate fate.