Act Three of Ghosts by Henrik Ibsen begins with the sad, orange glow from just outside the windows of the stately manor of Mrs. Alving. The light signifies the last remnants of the orphanage that burnt down at the end of Act Two.
Mrs. Alving is worried because her son, Oswald, will not come in. He has been determined to stay outside and watch the blaze. She exits in hopes of retrieving him.
Pastor Manders enters, flustered by Engstrand who pursues him indoors. Engstrand claims that the pastor absent-mindedly snuffed out a candle and tossed it into a pile of wood shavings. Engstrand implies that it was Manders’ careless action which started the fire, ultimately destroying the uninsured orphanage. Pastor Manders does not recall the events, but he does not refute Engstrand either.
Mrs. Alving enters after failing to convince her son to return indoors. She claims that the orphanage’s destruction is “all for the best. That orphanage would have done no one any good.” Her negativity might stem from the fact of that the orphanage was to be named in honor of her philandering husband, Captain Alving (who unbeknownst to the rest of the community was a drunk and a scoundrel).
Engstrand makes a deal with the pastor. He agrees to say nothing to the public about Manders' supposed fault in the blaze. In exchange, Manders will help Engstrand found his Sailors' Home” (an establishment that seems charitable on the surface, but in reality it is simply a profit-making enterprise that caters to sailors’ vices). Manders is grateful to Engstrand, and together they depart.
Oswald returns, seeing the two men leave. He predicts that the next establishment (the sailor’s home) will burn like the orphanage.
Oswald and Regina sit together, and Mrs. Alving reveals the truth about Oswald's father. Captain Alving was unfaithful, and one of his lovers was a servant, Johanna: the mother of Regina. After hearing this news, Regina suddenly understands that she and Oswald are siblings. Her feelings for Oswald -- her entire perception of the world -- are now warped. She is ashamed of her mother and disappointed in the way Mrs. Alving raised her as a servant instead of a “gentleman’s daughter.” Despite Oswald’s importunities, Regina leaves, hoping to catch the same steamer that will carry away Engstrand and Pastor Manders.
Oswald returns to feeling dreadful, in part because of his spoiled relationship with Regina, but also because he regrets that she won’t be able to take care of him should he become an invalid. Oswald explains that his medical condition causes him to lapse into seizures. He fears that the disease will turn him into a complete invalid, that his brain will "soften," leaving him in a vegetative state.
Oswald wants his mother to give him a lethal dose of pills in case his has another devastating attack of the disease. Reluctantly, Mrs. Alving accepts the responsibility. Soon afterwards, Oswald experiences an attack. He repeats "the sun, the sun," and has slipped into another seizure, perhaps the worst one ever.
The play ends with Mrs. Alving frantically trying to find the lethal dose of morphine pills, but as the curtain falls it is undetermined whether of not Mrs. Alving will fulfill the promise she made, and end the life of her ailing son.