The entire play takes place in the family room of the Tyrones' summer home. The year is 1912, the time is one August morning, and Mary and James enter after breakfast. We soon learn that Mary has recently returned from treatment at a sanatorium for her morphine addiction. In Act One's opening, we also learn that Edmund has been away traveling, and that recently his health has been deteriorating. He's developed a terrible cough. Jamie and Edmund enter, and James and Tyrone can't seem to resist fighting. A bit of teasing becomes bitter arguing, but Edmund and Mary intercede and calm them down. Edmund tries to tell a humorous story about one of their tenants, but Tyrone doesn't appreciate and Edmund's interpretation of events. Tyrone calls him a socialist and an anarchist, and Edmund, sick of being criticized, goes upstairs coughing. Mary is worried but refuses to hear talk that Edmund might be truly sick. She goes into the kitchen to supervise the help. With her gone, Jamie and Tyrone talk frankly about Edmund: he might have consumption. The two men fight bitterly, going through a series of arguments we will hear many times before the end of the play: Tyrone accuses Jamie of being without direction, and Jamie accuses Tyrone of being miserly. He blames Mary's morphine addiction on his father's bargain hunting and the consequent shoddy medical care. Mary returns, and the two men shut up. They go out to work on the lawn. Edmund comes down, and he tries to talk to Mary. She's concerned about his health, and he's concerned about hers. He tries to talk frankly about her problems with morphine, because he feels she should confront her past. She seems to prefer to avoid the topic. She complains about Tyrone's miserliness, and how because of it she has never had a real home. He goes out to the lawn to lie in the shade while the other two men work, and Mary is left alone.
Act Two, Scene One. Just before lunchtime. Edmund chats with Cathleen, the hired girl. Jamie comes in and sneaks a drink; Mary has been upstairs all morning, and Jamie fears that Mary is taking morphine. Edmund denies it, but when Mary comes downstairs her strange, detached manner confirms Jamie's suspicions. Later, Tyrone enters and sees soon what has happened. Finally, even Edmund can no longer deny that Mary has slipped back into use of the drug.
Act Two, Scene Two. Just after lunch. Mary criticizes Tyrone for being addicted to bad real estate investments. They receive a phone call from Dr. Hardy, and Tyrone takes it. From his manner when he returns, we know that it is not good news. Mary goes upstairs to shoot up again, and the three men start to fight. Edmund goes upstairs to try to speak to her, and while he is gone Tyrone confirms with Jamie that Edmund does, in fact, have consumption. Jamie worries that Tyrone, miser that he is, will send Edmund to a cheap sanatorium. Jamie goes out, waiting for Edmund so that he can accompany him to town. Mary comes down, and the Tyrone parents talk. We learn about their past: she had Edmund in part for the death of an older son, whose name was Eugene. Edmund comes downstairs. He urges his mother to fight the morphine addiction, but she pretends to have no idea what he's talking about. Edmund leaves, and then Tyrone, leaving Mary alone. First she is relieved, and then she is achingly lonely.
Act Three. Half past six in the evening, same day. Mary sits in the family room, waited on by Cathleen; again and again, she treats Cathleen to whiskey. She muses about her youth, and her childhood dreams of being a nun or a concert pianist. Mary also remembers meeting Mr. Tyrone, and how in love she once was. Cathleen is trying to focus, but she is not terribly sharp and she has become a bit drunk. Edmund and Tyrone come home. Mary receives the men happily, but they see quickly that she is lost in the dope. Mary warns Edmund that Jamie wants to make him a failure, like he is. She thinks about their childhood, and worries that Tyrone's habits have started them on the path to alcoholism. Mary reminds Tyrone of the first night when they met. There is a brief, touching moment of tenderness. And then she returns to criticizing him. She then speaks nostalgically about her wedding dress, and how she fussed over it. She doesn't know where the dress is now; it must be in the attic somewhere. Tyrone goes down into the cellar to get more whiskey, and Edmund and Mary are alone. Edmund tries to tell Mary how sick he is, but she refuses to listen. They talk about her problems with morphine, but talking so directly about the past hurts Mary, so they stop. Edmund leaves. Tyrone returns and asks her to have dinner with him. She decides to go upstairs instead, presumably to shoot up yet again.
Act Four. Midnight, that night. Edmund comes home to find his father playing solitaire. The two have the normal quota of fights and drinking, but they also manage to have an intimate, tender conversation. Tyrone explains his stinginess, and he also reveals to Edmund that he ruined his career by staying in an acting job for money. After so many years playing the same part, he lost the talent he'd once had. Edmund understands his father now better than he ever has. He talks to his father about his days sailing, and talks indirectly about his hopes to be a great writer. They hear Jamie coming home drunk, and Tyrone leaves to avoid fighting. Jamie and Edmund have their own conversation, and Jamie confesses something: although he loves Edmund more than anyone else in the world, he wants Edmund to fail. And he'll try to make Edmund fail. Then Jamie passes out, dead drunk. When Tyrone returns, he wakes up, and then they start to fight again. Mary comes downstairs, by now so doped up she can barely recognize them. She is carrying her wedding gown, lost completely in her past. The men watch in horror. She does not even know they are there.