Mapping out the plot summary of Long Day's Journey Into Night is a bit challenging, since there really isn't much in the way of plot. The play takes place exclusively in the living room of the Tyrone family's summer home, in August, 1912. Literally nothing happens in this room other than conversations, a lot of drinking, and a couple of blows to Jamie's face. The "action" of the play (such as the boys going into town and the mother taking morphine) all happens offstage. The detailed summary is really the place to go to get a sense of what happens in this play, but if you want the skeleton, here it is:
When the fateful day starts, there's already some tension floating around the Tyrone family. James Tyrone and his two sons, Edmund and Jamie, have all noticed that Mary, James's wife, spent the night before in the guest room where she used to take morphine.
The boys thought Mary had finally kicked her narcotics habit, but they now suspect that she may be back on the drug. Meanwhile, Jamie is home for the summer because he has no job and needs a place to stay, while Edmund, more distressingly, has been feeling very ill.
As the day progresses, problems mount for the Tyrones. It becomes obvious that Mary's back on morphine. She acts defensive at times, and as though she's a teenager again (when she was "happy") at others. Mary spent her youth at a convent planning to be nun; since then, though, she's faced all kinds of tragedies, including the death of her father, as well as the loss of her childhood home, her innocence, her religion, and her second son.
Edmund's nagging ailment proves to be consumption (i.e., tuberculosis), which was serious, bad news in 1912. It is often curable, but Edmund will have to be sent to a sanatorium (a medical facility for treating long-term illness), which is expensive.
Speaking of money, let's turn our attention to James. James doesn't have any life-threatening issues like Mary or Edmund, but he's got this frustrating tendency to be incredibly cheap. Additionally, he and Jamie (and, to a slightly lesser extent, Edmund) are all serious alcohol abusers.
By the time we reach this August afternoon, the boys have all left the house – Edmund to go the doctor to find out about his disease, and James and Jamie to accompany him and then do some drinking.
Jamie stays out and heads to a brothel, but James and Edmund come back after a few drinks. Mary, meanwhile, takes a quick drive into town to pick up some morphine, and then spends the afternoon talking to the house maid, Cathleen, about her childhood. We can assume that she took some more morphine before this conversation.
Once James and Edmund return, Mary engages them in conversation while high; her strange, morphine-influenced behavior drives James to get more whiskey and Edmund to take off entirely. James comes back in time for dinner, but Mary decides she has to go upstairs and sleep instead.
In the final act, Edmund comes back from a walk down the beach (and a stop at the inn for a drink) to find his father in the living room. The two are both drunk and begin to open up to one another about their pasts and their dreams.
It seems that Edmund, a would-be poet, wants to exist in a different reality (or, possibly, to die), while James admits that the experiences of his younger years are what rendered him so resentful of extravagance and laziness (both characteristics he finds in Jamie).
Jamie comes home drunk and with stories of an overweight prostitute, and after mouthing off about Mary, gets a punch in the face from Edmund. Jamie admits that he hasn't been the best son to Mary or brother to Edmund. In fact, secretly, he wants to destroy Edmund because he's jealous and resentful of his own wasted life.
Finally, Mary comes down carrying her old wedding dress, completely high on morphine and acting like the girl she was when she first met James. Jamie makes another crack at her expense and Edmund smacks him. As she has before, Mary recounts how joyful she was at the convent where she grew up and how much she loved James when they met. She's extremely upset about having lost something (we aren't told what), and concludes the play by saying her marriage with James was so happy – for a time.