The play is set in the summer home of the Tyrone family, August 1912. The action begins in the morning, just after breakfast. We learn as the first act unravels that Mary has returned to her family recently after receiving treatment in a sanatorium for morphine addiction. Edmund, meanwhile, has in recent weeks begun to cough very violently, and we learn later on in the play that, as Tyrone and Jamie suspect, he has tuberculosis. Throughout the course of the play, we slowly find out that Mary is still addicted to morphine, much to the disappointment of her family members.
The gradual revelation of these two medical disasters makes up most of the play's plot. In between these discoveries, however, the family constantly revisits old fights and opens old wounds left by the past, which the family members are never unable to forget. Tyrone, for example, is constantly blamed for his own stinginess, which may have led to Mary's morphine addiction when he refused to pay for a good doctor to treat the pain caused by childbirth. Mary, on the other hand, is never able to let go of the past or admit to the painful truth of the present, the truth that she is addicted to morphine and her youngest son has tuberculosis. They all argue over Jamie and Edmund's failure to become successes as their father had always hoped they would become. As the day wears on, the men drink more and more, until they are on the verge of passing out in Act IV.
Most of the plot of the play is repetitious, just as the cycle of an alcoholic is repetitious. The above arguments occur numerous times throughout the four acts and five scenes. All acts are set in the living room, and all scenes but the last occur either just before or just after a meal. Act II, Scene i is set before lunch; scene ii after lunch; and Act III before dinner. Each act focuses on interplay between two specific characters: Act I features Mary and Tyrone; Act II Tyrone and Jamie, and Edmund and Mary; Act III Mary and Jamie; Act IV Tyrone and Edmund, and Edmund and Jamie.
The repetitious plot also helps develop the notion that this day is not remarkable in many ways. Instead, it is one in a long string of similar days for the Tyrones, filled with bitterness, fighting, and an underlying love.