The scene opens on a two-room apartment in a tenement house in Dublin. There is a picture of the Virgin Mary, below which hangs a bowl containing a floating votive candle. Furnishings and other belongings are sparse, consisting of a dresser, a small bed, a fireplace, a box of coal, an alarm clock, a bath, a table and chairs, a teapot, a frying pan, a few books, and a long-handled shovel.
The act opens with Juno Boyle and her daughter Mary discussing the murder of their neighbor Mrs. Tancred’s son, written up in the morning newspaper. Mary's brother Johnny - thin, pale, and fearful - irritably tells them to stop reading and leaves the room.
Juno asks if her husband, 'Captain' Jack Boyle, has come in yet. She tells Mary he’ll have to go without breakfast if he doesn’t come in soon, for she is afraid he will invite his friend 'Joxer' Daly in to share his tea if she leaves. She complains that he has already worn out his health insurance and will soon be out of unemployment, yet he is always singing. Mary seems unperturbed, tying a ribbon around her head and musing about which color to wear. Her mother complains about her being on strike and thus not contributing to the household, but Mary insists that “a principle’s a principle” (8).
Johnny reenters. He walks with a limp, having been shot in the hip during the Easter Week rebellion, and he has also lost an arm. He is upset that Mary is leaving the house, since he doesn't want to stay home by himself. Juno reminds him that his father will be home soon, but Johnny counters that his father hates to be asked to do anything. He asks if the candle in front of the picture of the Virgin Mary is still lit, and Juno reassures him that it is.
Jerry Devine, a young man, enters and Mary hurries out. He reports that Father Farrell has offered Boyle a job, but Boyle is still out drinking with his friend Joxer. Jerry rushes out to find him and Juno complains that her husband will deliberately miss the job.
Boyle and Joxer can be heard coming up the stairs, with Boyle singing. Juno sits on the bed with draperies hiding her from view of the newcomers. Boyle invites Joxer inside, reassuring him that Juno has left. He grumbles that Juno is always complaining, and Joxer agrees that this is a hard thing to put up with. Boyle offers Joxer a cup of tea. At this point Juno makes her presence known. She sarcastically offers Joxer an egg as well; flustered, he says he is in a hurry. Boyle and Joxer begin talking of visiting the foreman of a job to start working. Juno expresses her disgust for the charade and chastises her husband for his laziness. She complains that everyone calls him “Captain” when he only once went out on the water.
Juno asks Boyle if he saw Jerry. She complains that he was in a pub; Boyle swears he was not. When she urges him to eat breakfast, Boyle proudly counters, “I’ll have no breakfast - yous can keep your breakfast…. I’ve a little spirit left in me still” (15). Jerry reenters and confirms that Boyle was lying, since the foreman in Foley’s pub told him Boyle had left just ten minutes earlier. Rather than confess or apologize, Boyle complains about being watched all the time. Jerry delivers his news - that he can have a job if he goes to Rathmines - and Boyle complains of sudden pain in his legs that would make it hard for him to work.
Boyle goes into the bedroom to change into his work pants and Juno leaves for work. Jerry speaks with Mary, telling her that he will likely be elected secretary of his union and explaining how well he could support her. Mary has no interest and asks him to let her go, shouting when he refuses. Boyle reappears and asks what all the “hillabaloo” (18) is about. Mary and Jerry exit, and Boyle complains about children not caring about their parents anymore.
Despite his bold words, Boyle puts the breakfast sausage on the pan to cook and starts to sing. Steps are heard on the stairs and he hides the pan under the bed, but it is only a man asking if he wants a sewing machine. Boyle continues to cook his breakfast and sing, but is interrupted again by thundering knocking at the street-level door. Johnny fearfully asks who it is. Joxer is afraid to look, but Boyle says it is a man with a trench coat who is going away.
Boyle invites Joxer to stay for tea. Joxer is afraid that Juno might return, but Boyle convinces him that if she did, he could climb out the window and hide on the roof. Joxer agrees to stay and the two speak briefly of books, inspired by one of Mary's on the table. Boyle tells Joxer of the job he is going to. Joxer says it is good news, but Boyle reminds him of the pains in his legs. The two complain about Jerry Devine, Father Farrell, and the clergy, with Boyle arguing that it's no way to reward Johnny for his service to his country by making his father work. A coal vendor's voice can be heard selling coal-blocks as Boyle and Joxer reminisce about Boyle's fictitious days on a ship. The two hear footsteps near the door; Boyle frantically tries to hide everything and Joxer rushes to escape out the window, but it is only the coal vendor, asking if they want any coal. Boyle asserts that he's had enough of following Juno's orders: "Today, Joxer, there's goin' to be issued a proclamation be me, establishin' an independent Republic, an' Juno'll have to take an oath of allegiance" (24).
Juno's voice can be heard outside and Joxer throws himself out the window. When Juno enters, Boyle denies her assertions that he and Joxer had been together. She tells him to smarten himself up as a visitor is coming; Boyle assumes the visit has to do with another job.
Juno fusses to tidy the room and Mary enters with Charlie Bentham, a tall, good-looking young man. Boyle and Johnny can be heard arguing humorously as Boyle changes out of his work pants. Juno introduces Johnny to Bentham, boasting of her son's service to Ireland, and then introduces her husband. Bentham explains that Boyle's cousin Mr. Ellison has died, and that he wished to leave his property only to his second cousin, Michael Finnegan of Santry, and to Boyle, his first cousin. He explains that half of the property would be worth between 1500 and 2000 pounds. The entire family is ecstatic. Boyle claims that he is finished consorting with Joxer, who angrily climbs in through the window. The two argue humorously, Joxer exits, and Boyle claims he is a new man, singing emotionally to his wife about how dear she is to him.
O'Casey's plays are "slices of life" rather than intricate stories, with their significance resting upon the main characters. In Act I we get a good sense of Boyle's character. He is exceedingly selfish; Johnny notes that he hates to be asked to do anything, and when Jerry Devine grabs Mary and she cries for help, all Boyle does is complain about the noise. He lies without remorse and invents pains in his legs to avoid having to work. Boyle is set up in opposition to Juno, a pillar of strength. While Boyle tries to escape reality through drinking and fantasies of his former life as a sea captain, Juno faces reality and takes care of her family.
The act also introduces the play's major themes. We see the dehumanizing influence of poverty, the tragic effect of the war on Johnny, and the futile way in which Mary tries to escape the circumstances of her life through education. We can also see the reflection of O'Casey's life in the play's events. O'Casey was deeply influenced by the 1913 Dublin Lockout, in which Dublin employers locked out unionized workers for six months; thus the character of Mary likely reflects his union sympathies. His pessimistic view of the Easter Rising is evident in the character of Johnny, who represents the cost of the rebellion in terms of the human spirit.
The stagecraft is expertly executed to support the play's themes. A clock lies face down on the mantel, symbolizing the way that time stands still for the Boyles. A shovel leans against the dresser, unused due to Boyle's avoidance of work. The mirror and the books by Ibsen on the table represent two opposing forces influencing Mary, her vanity and her desire to better herself through education. Towards the end of the act, we hear Boyle changing clothes practically in front of the audience, an offstage convention brought onto the stage; it is almost as if he is only acting a part in the drama of human life.
The play is tragicomic, containing elements of both comedy and tragedy. Act I is full of comic incidents: the appearance of the sewing machine vendor and the coal vendor, Joxer's hasty retreat to the roof, Juno's attempt to catch Boyle and Joxer, and the men's mock-intellectual discussions about books and the nature of the stars and the moon all lighten the mood and make the audience laugh. At the same time, there is an undercurrent of tragedy reflected in Johnny's injuries, his overwhelming fear when he hears a man knocking in the street, and the poverty of the household.
The language is rich with literary allusions. Juno is compared with the mythical figures of Deirdre of the Sorrows and Cathleen ni Houlihan, while Joxer's speech is full of references from literary and oral traditions. The play also provides an excellent reproduction of Dublin speech. We see words such as "chiselurs" for children, "chassis" for chaos, and "braces" for suspenders, and irregular spellings reflect Irish tenement dwellers' unique pronunciations. There are also comical phrases and malapropisms such as "Antanarctic Ocean," a portmanteau formed by combining the words Antarctica and Arctic Ocean.