The scene still takes place in Mrs. Alving's home, but it is night time. By now the fire is out, the entire orphanage burned to the ground. While Mrs. Alving has gone to fetch Oswald, Regina and Manders receive Engstrand. "God help us all," he says piously and clucking sympathetically says that the prayer meeting caused the fire. Whispering that "Now we've got the old fool, my girl," he tells Manders, the only one carrying a candle, that he saw the pastor snuff the light and toss the burning wick among the shavings. The distraught reverend is beside himself. The worst aspect of this matter, he says, will be the attacks and slanderous accusations of the newspapers. By this time Mrs. Alving has returned. She considers the fire merely as a business loss; as to the property and the remaining capital in the bank, Manders may use it as he likes. He thinks he may still turn the estate into "some useful community enterprise" and Engstrand is hopeful for his support of the seamen's home. Gloomily, Manders answers that he must first await the published results of the inquiry into the cause of the fire. Offering himself as "an angel of salvation," Engstrand says he will himself answer to the charge. Relieved and breathless, Manders eagerly grasps his hand. "You are one in a thousand," he declares. "You shall have assistance in the matter of your sailors' home, you may rely upon that."
United in friendship, Engstrand and Manders prepare to leave together. Announcing to Mrs. Alving that his enterprise shall be called "The Alving Home," the carpenter concludes, "And if I can carry my own ideas about it, I shall make it worthy of bearing the late Mr. Alving's name." The double entendre is unmistakable to everyone except Manders.
Oswald returns so depressed that Regina is suspicious he may be ill. Mrs. Alving now prepares to tell them both what she started to divulge in the previous scene. What Oswald told her about the joy of life suddenly sheds new light upon everything in her own life, she tells them, for his father, so full of "irrepressible energy and exuberant spirits" in his young days "gave me a holiday feeling just to look at him." Then this boy had to settle in a second-rate town which had none of the joy of life to offer him but only dissipations:
He had to come out here and live an aimless life; he had only an official post. He had no work worth devoting his whole mind to; he had nothing more than official routine to attend to. He had not one single companion capable of appreciating what the joy of life meant; nothing but idlers and tipplers — and so the inevitable happened.
What was the inevitable, asks Oswald, and his mother answers that he had himself described how he would degenerate at home. "Do you mean by that Father — ?" and she nods:
Your poor father never found any outlet for the overmastering joy of life that was in him. And I brought no holiday spirit into his home either. I had been taught about duty and that sort of thing that I believed in so long here. Everything seemed to turn upon duty — my duty or his duty — and I am afraid that I made your poor father's home unbearable to him, Oswald.
Then why did she not write him the truth in her letters, demands the son, and she can only say she never regarded it as something a child should know about. "Your father was a lost man before ever you were born," says Mrs. Alving, and all these years she has kept in mind that Regina "had as good a right in this house — as my own boy had." To their bewilderment she answers quietly, "Yes, now you both know."
"So Mother was one of that sort too," Regina muses. Then she announces her desire to leave them to make good use of her youth before it is wasted. With Oswald sick, she does not wish to spend her life looking after an invalid for "I have the joy of life in me too, Mrs. Alving." From now on she shall make her home in the "Alving Home." Mother and son are alone onstage.
"Let us have a little chat," says Oswald beckoning her to sit beside him. Before he divulges the truth about his fatigue and inability to work he warns her she mustn't scream. The illness itself is hereditary, he continues, and "it lies here (touching his forehead) waiting. At any moment, it may break out." She stifles a cry. At the time he had a serious attack in Paris, Oswald goes on, the doctor told him he would never recover from another one. The disease is a lingering one — the doctor likened it to a "softening of the brain" — and it will leave him hopeless as a vegetable.
Showing his mother a dozen morphia tablets, Oswald says he needed Regina's strength and courage to administer "this last helping hand." Now that Regina is gone, however, his mother must swear that she will give him them herself when it is necessary. Mrs. Alving screams and tries to dash out for the doctor, but Oswald reaches the door first and locks it. "Have you a mother's heart and can bear to see me suffering this unspeakable terror?" he cries out. Trying to control herself, Mrs. Alving trembles violently. "Here is my hand on it," she says.
Outside day is breaking. Oswald is seated quietly in an armchair near the lamp. Cautiously bending over him, Mrs. Alving straightens up, relieved:
It has only been a dreadful fancy of yours, Oswald [she chatters] . . . But now you will get some rest, at home with your own mother, my darling boy . . . There now, the attack is over. You see how easily it passed off . . . And look, Oswald, what a lovely day we are going to have. Now you will be able to see your home properly.
She rises and puts out the lamp. In the sunrise the glaciers and peaks in the distance are bathed in bright morning light. Oswald, with his back toward the window, suddenly speaks. "Mother give me the sun." Regarding him with amazement she quavers, "What did you say?" Dully, Oswald repeats, "The sun — the sun." She screams his name. As before, he only says, "The sun — the sun." She beats her head with her hands. "I can't bear it! Never!" she screams. Then, passing her hands over his coat, she searches for the packet of pills. "Where has he got it? Here!" Then she cries, "No, no no! — Yes! — No, no!" Mrs. Alving stares at her son in speechless terror. Oswald remains motionless. "The sun — the sun," he repeats monotonously, and the curtain falls.
As in a Greek tragedy, the protagonist's "tragic flaw" involves not only himself, but his children, in the consequences of guilt. In this act Mrs. Alving receives the full penalty for her guilt of substituting a sense of duty for the "joy of life." Her submission to ancient social standards destroys the creative mind of her artist son and similarly destroys Regina's blooming womanhood. The "ghosts" of heredity reappear as Oswald succumbs to syphilitic paresis and as Regina goes to find her future in a brothel. Mrs. Alving can only administer the final stroke — the mortal dose of morphia — to complete the destruction of Oswald she had so unwittingly begun.
With a dramatic flourish, Ibsen uses the environment as an ironic "objective correlative" to underscore the tragedy. As the dawn breaks over a spectacular mountain landscape, Oswald is thrust into the unending darkness of his lingering doom. The long awaited sunshine, so badly needed by Oswald to continue his painting, arrives only to illuminate catastrophe. By the same token, the light of truth has come too late for Mrs. Alving to avoid the consequences of her lifelong deceit.