James Tyrone is an aging 65-year-old actor who had long ago bought a "vehicle" play for himself and had established a reputation based on this one role with which he had toured for years. Although that "vehicle" had served him well financially, by the time of Long Day's... opening, he is resentful of the fact that his having become so identified with this character has severely limited his scope and opportunities as a classical, especially Shakespearean, actor. He is a wealthy though somewhat miserly man. His money is all tied up in property which he hangs on to in spite of impending financial hardship. His dress and appearance are showing signs of his strained financial circumstances, but he retains many of the mixed affectations of a classical actor in spite of his shabby attire.
His wife Mary has recently returned from treatment for morphine addiction and has put on weight as a result. She is looking much healthier than the family has been accustomed to, and they remark frequently on her improved appearance. However, she still retains the haggard facial features of a long-time addict. In common with many recovering addicts, she is restless and anxious and suffers from insomnia, not made any easier by her husband and children's loud snoring. When Edmund, her younger son, hears her moving around at night and entering the spare bedroom, he becomes very alarmed. It was in this room that his mother used to satisfy her morphine addiction. He questions her about it indirectly. She reassures him that she just went there to get away from her husband's snoring.
In addition to Mary's problems, the whole family is worried about Edmund's constant coughing. The family fears that he might have tuberculosis, and this anxiety adds to the already palpable tension which surrounds the family. They are anxiously awaiting the diagnosis of his condition. Edmund is more concerned about the effect a positive diagnosis might have on his mother than on himself. The constant possibility that she might relapse worries him still further. Once again, he indirectly speaks to his mother about her addiction. He asks her to "promise not to worry yourself sick and to take care of yourself." "Of course I promise you," she protests, but then adds ('with a sad bitterness'), "But I suppose you're remembering I've promised before on my word of honor."
Jamie and Edmund taunt each other about stealing their father's alcohol and watering it down so he won't notice. They speak about Mary's conduct. Jamie berates Edmund for leaving their mother unsupervised. Edmund berates Jamie for being suspicious. Both, however, are deeply worried that their mother's morphine abuse may have resurfaced. Jamie points out to Edmund that they had concealed their mother's addiction from him for ten years. Jamie explains to Edmund that his naiveté about the nature of the disease was understandable but deluded. They discuss the upcoming results of Edmund's tests for tuberculosis, and Jamie tells Edmund to prepare for the worst.
Their mother appears. She is distraught about Edmund's coughing, which he tries to suppress so as not to alarm her, fearing anything that might trigger her addiction again. When Edmund accepts his mother's excuse that she had been upstairs so long because she had been "lying down", Jamie looks at them both contemptuously. Mary notices and starts becoming defensive and belligerent, berating Jamie for his cynicism and disrespect for his parents. Jamie is quick to point out that the only reason he has survived as an actor is through his father's influence in the business.
Mary speaks of her frustration with their summer home, its impermanence and shabbiness, and her husband's indifference to his surroundings. With irony, she alludes to her belief that this air of detachment might be the very reason he has tolerated her addiction for so long. This frightens Edmund, who is trying desperately to hang on to his belief in normality while faced with two emotionally horrific problems at once. Finally, unable to tolerate the way Jamie is looking at her, she asks him angrily why he is doing it. "You know!", he shoots back, and tells her to take a look at her glazed eyes in the mirror.
The third act opens with Mary and Cathleen returning home from their drive to the drugstore where Mary has sent Cathleen in to purchase her morphine prescription. Not wanting to be alone, Mary does not allow Cathleen to go to the kitchen to finish dinner and offers her a drink instead. Mary does most of the talking and discusses her love for fog but her hatred of the foghorn and her husband’s obvious obsession with money. It is obvious that Mary has already taken some of her “prescription.” She talks about her past in a Catholic convent and the promise she once had as a pianist and the fact that it was once thought that she might become a nun. She also makes it clear that while she fell in love with her husband from the time she met him, she had never taken to the theatre crowd. She shows her arthritic hands to Cathleen and explains that the pain in her hands is why she needs her prescription – an explanation which is untrue and transparent to Cathleen. When Mary dozes off under the influence of the morphine, Cathleen exits to prepare dinner. Mary awakes and begins to have bitter memories about how much she loved her life before she met her husband. She also decides that her prayers as a dope fiend are not being heard by the Virgin, but still decides to go upstairs to get more drugs, but before she can do so, her son, Edmund, and her husband, James, return home. Although both men are drunk, they both realize that she is back on morphine although Mary attempts to act as if she is not. Jamie, the other son, has not returned home, but has elected instead to continue drinking and to visit the local whorehouse. After calling Jamie a "hopeless failure" Mary warns that his bad influence will drag his brother down as well. After seeing the condition that Mary is in, her husband expresses the regret that he bothered to come home and attempts to ignore her as she continues her remarks which include blaming him for Jamie’s drinking, noting that the Irish are notably stupid drunks. Then, as so often happens in the play, Mary and James try to get over their animosity and attempt to express their love for one another by remembering happier days. When James goes to the basement to get another bottle of whiskey, Mary continues to talk with her son, Edmund.
When Edmund reveals that he has consumption (tuberculosis), Mary refuses to believe it, and attempts to discredit Dr. Hardy, due to her inability to face the reality and most importantly severity of the situation. She accuses Edmund of attempting to get more attention by blowing everything out of proportion. In retaliation, Edmund reminds his mother that her own father died of tuberculosis, and then, before exiting, he adds how difficult it is to have a "dope fiend for a mother." By herself, Mary admits that she needs more drugs and hopes that someday she will “accidentally” overdose, because she knows that if she did so on purpose, the Virgin would never forgive her. When James comes back with more alcohol he notes that there was evidence that Jamie had attempted to pick the locks to the whiskey cabinet in the cellar as he had done before. Mary ignores this and bursts out that she is afraid that Edmund is going to die. She also confides to James that Edmund does not love her because of her drug problem. When James attempts to console her, Mary again accuses herself for giving birth to Edmund, who appears to have been conceived to replace a baby Mary and James lost before Edmund’s birth. When Cathleen announces dinner, Mary indicates that she is not hungry and is going to lie down. James goes in to dinner all alone, knowing that Mary is really going upstairs to get more drugs.
At midnight, Edmund comes home to find his father playing solitaire. While the two argue and drink, they also have an intimate, tender conversation. James explains his stinginess, and also reveals that he ruined his career by staying in an acting job for money. After so many years playing the same part, he lost his talent for versatility. Edmund talks to his father about sailing and his aspiration to become a great writer one day. They hear Jamie coming home drunk, and James leaves to avoid fighting. Jamie and Edmund converse, and Jamie confesses that although he loves Edmund more than anyone else, he again ambiguously lashes out at his father calling on him to fail. Jamie passes out. When James returns, Jamie wakes up, and they quarrel anew. Mary, lost in her morphine dreams of the past, comes downstairs. Holding her wedding gown, she kneels and prays, with her husband and sons silently watching her.