For the characters in Ghosts, tomorrow is a big day. They are all convening on Rosenvold, the Alving estate, to dedicate the Captain Alving Memorial Orphanage. Mrs. Alving is the widow of Captain Alving (also called Chamberlain Alving), a gentleman widely respected in his community. Mrs. Alving kept his true behavior – alcoholism, womanizing, and illness – secret for twenty years. She sent her son, Oswald, away at age seven to protect him from the polluting influence of his father, who also had an illegitimate daughter by a servant. This daughter, Regina, was brought up by the carpenter Engstrand and now works in Mrs. Alving's house.
Pastor Manders is a minister in town and the executor of the Alving Estate. He's an old friend of the family. In fact, Mrs. Alving was in love with him and once fled to him from her husband. Pastor Manders sent her home, citing her duty and his need to protect his reputation. Now he has more reason to scold his old flame, as Mrs. Alving has been reading radical books and openly agrees with her son's radical ideas about sex. She tells Pastor Manders the truth about her husband, but still wants to keep it hidden from Oswald.
Oswald has secrets of his own. He's terminally ill with syphilis (the same illness his father had) and has come home to die, hopefully with the help of Regina, whom he intends to marry. Just as Mrs. Alving is about to let them know this can't happen – they don't know they're brother and sister – a fire burns down the orphanage. This is doubly problematic because Pastor Manders insisted, and Mrs. Alving agreed, on going without insurance. Pastor Manders is terrified of the scandal but finds an out in Engstrand, who promises to take the blame for the fire if Manders supports his Sailor's Home (which is really a brothel), with the funds from the Alving estate.
Finally, Mrs. Alving tells Oswald the truth about her husband. He was miserably unhappy, sought pleasures of the flesh, and one of the consequences was Regina. While Regina pursued Oswald as a meal ticket, once she finds out he's sick with syphilis (and her half-brother), she gives up. Oswald and Mrs. Alving are left alone, and Oswald asks his mother to put him to death. She agrees, but doesn't realize it will come so soon. As the play ends, the sun is rising, Oswald is slumped in a vegetative state, and Mrs. Alving stands with the morphine, trying to decide what to do.