Mrs. Alving walks toward the dining room but stops in the doorway, seeing Oswald. He is drinking. He says he is drinking so as not to feel the dampness. He has to be told twice that the Pastor has gone down to say prayers at the orphanage. He enters from the dining room, patting his mother and telling her how nice it is to be home. He then begins to complain about the weather and how he can't do any painting without sunlight. He paces and asks permission to sit next to his mother. He tells her that he has something to tell her: he is sick. He sometimes feels a piercing pain and becomes giddy and senseless whenever he tries to work. He went to a doctor in Paris, who, instead of a diagnosis, simply declared, "The sins of the fathers are visited on the children." But Oswald convinced the doctor that this was wrong, showing him his mother's letters, in which she described what a good man Captain Alving was. The doctor then decides that Oswald must simply be over-exerting himself in his enlightened, bohemian lifestyle. Oswald is crushed by the knowledge that he has ruined his own health with his living habits.
Mrs. Alving, meanwhile, is highly agitated, wringing her hands. She repeatedly bemoans the fate of her "boy." He asks for something to drink. His mother asks Regina to bring in the lamp and then some champagne. She is worried that he will leave home again and tells him he can have whatever he wants. He speaks of Regina, saying she is the only one who can help him. Mrs. Alving offers her own assistance, but Oswald says that, although he considers his mother his best friend in the world, he cannot accept her help as he does not want her to have to witness his torment. Mrs. Alving calls for another bottle of champagne. Meanwhile, Oswald describes how he once casually told Regina that she should visit Paris and that now she had her heart set on it. She reminded him of it when he returned and when he saw her looking expectantly at him, he saw that she was full of the "joy of life."
Regina brings the bottle, and Oswald asks her to join them. After asking "madam's" permission, she does. Oswald continues to speak about the joy of life and work, lamenting that people in Norway assume incorrectly that the world is a "vale of misery." He says he always paints with an eye toward happiness, and he feels that in Norway all that is best in him is deteriorating. Suddenly, Mrs. Alving rises, exclaiming that she now understands how everything fits together and that she can explain everything. The Pastor enters and is convinced that Regina must return to Engstrand He asks why she is drinking with the other two, and Oswald says that they may marry. Then, Mrs. Alving declares that she has something to say. The Pastor protests, but she assures him that she will not disillusion anyone. Then, they all notice a bright light and a clamor of voices emanating from outside. Apparently, the orphanage has caught fire. They all rush out. Even while the Pastor exclaims that the fire is an act of judgment upon the sinful Alving house, he also complains bitterly that the orphanage is not insured.
The play almost comes to its climax but is suddenly interrupted by a fire. The fire is one of two sources of light that acts as twisted symbols in this act. The other is the lamp. Mrs. Alving calls for it amid Oswald's complaints that he cannot work in this land without sun or the joy of life. Yet his complaints continue—Mrs. Alving cannot provide enough light to help his current mood, just as she can do little to help him in general. The fire, on the other hand, actually obliterates a falsehood, the fiction of Captain Alving's good reputation. Yet it is still purely destructive, not a source of enlightenment.
Oswald's sickness makes little sense to the modern reader, but perhaps in the late 19th century it was more plausible to blame an illness on a lifestyle. One possible explanation is that the sickness is syphilis, contracted through imprudent sexual relations. At any rate, the sickness's significance lies in its connections to the play's larger theme of hauntings and ghosts; the illness would seem to prove Mrs. Alving's theory that her son is actually haunted by his father.
Oswald's attitude toward his mother is ambivalent. On the one hand, he tries to convince her that he is a loving son. He pats her and calls her his best friend in the world, but he is also obliged to ask her permission to sit next to her. To a certain degree, they are strangers, as the Pastor pointed out earlier. However, Oswald is desperate, and he looks to his mother for help even though he suspects that Regina is a better alternative. In part, his preference for Regina's companionship may merely be an expression of lust for her, a manifestation of his lust for her merely because he is doomed to repeat the sins of his father, but he may also sense that his mother is in many ways confused.