Set in 1912, at the Birlings' large home in the industrial town of Brumley, Arthur Birling, who is a wealthy factory owner and local politician, celebrates his daughter Sheila's engagement to a rival magnate's son, Gerald Croft. Also in attendance are Birling's wife Sybil and their young, alcoholic son Eric (whose drinking problem the family discreetly ignores). Following dinner, Birling lectures them on the importance of self-reliance and looking after one's own, and talks of the bright future that awaits them (which, he believes, will include a place for himself on the next honours list). While the men confabulate, Sheila and Sybil leave the dining room to go into their drawing room.
The evening is interrupted by the arrival of a man calling himself Inspector Goole, who is investigating the suicide of a young woman named Eva Smith. Her diary, the Inspector explains, refers to members of the Birling family. Goole produces a photograph of Smith and shows it to Birling, who acknowledges that she worked in one of his factories. He admits to having dismissed her for leading strike action with most of the female workers demanding equal pay to males. Despite admitting that he left Smith without a job, Birling denies responsibility for her death.
Sheila (having been sent by her mother to bring Birling, Eric and Gerald to the drawing room) is shown the photograph of Smith. She explains that as she was out shopping with her mother, Sheila noticed a nice dress for sale and asked for help with trying on the dress. Smith, having been able to find employment at the shop, was one of the two helpers. Unsatisfied after trying on the dress, Sheila angrily ordered the manager of the department store to fire Smith. Sheila's real motivation, which she ashamedly confesses, was the jealousy that she felt towards Smith perceiving her as prettier than herself, and that the dress would have suited Smith's figure much better than Sheila's. Eric leaves the dining room.
Sybil enters the dining room shortly after. The Inspector then mentions that Smith changed her name to Daisy Renton. Gerald is noticeably startled, and admits to having met a woman by that name in the Palace Bar, where Smith had resorted to prostitution to sustain herself. Seeing that Smith was hungry and struggling to cope financially, Gerald gave her money and arranged for her to move temporarily into a vacant flat belonging to one of Gerald's friends. Under interrogation, Gerald reveals that he began a relationship with Smith over the summer, but parted with her after a few months. Sheila, disheartened, returns her engagement ring to Gerald, who leaves for a walk.
The Inspector turns his attention to Sybil, a keen patron for a charity that helps women in difficult situations, which Smith (who was by then pregnant and destitute) had turned to for help, using the name "Mrs. Birling". Sybil, seeing this as a mockery of herself, convinced the committee to deny her a grant. She hid her anger under the guise that Smith had been irresponsible and suggested that she find the father, despite Smith repeatedly affirming he wouldn't be of any use. Despite vigorous cross-examination from the Inspector, she denies any wrongdoing. Goole then plays his final card, forcing Sybil to lay the blame on the "drunken young man" who got Smith pregnant. It slowly dawns on the rest of the family, except Sybil, that Eric is the young man who impregnated Eva, and the use of "Mrs. Birling" was in relation to him fathering her child.
Eric then enters, and after brief questioning from Goole, breaks down and admits responsibility for the pregnancy, having forced himself on Smith after a drinking spree at the Palace Bar. After finding out that Smith was pregnant, Eric stole £50 (some £7,300 in 2023) from his father's business to support her and their child, but she refused the stolen money and breaks up with Eric. Birling and Sybil are outraged by Eric's behaviour, and the evening dissolves into angry recriminations. The Inspector concludes by reminding the family that actions have consequences and that all people are intertwined in one society, stating that "if men will not learn that lesson, then they will be taught it in fire and blood and anguish."
As the family reminisce over the evening, they begin to question if "Goole" was a real police inspector. Gerald returns after completing his walk, revealing that he had actually gone to talk to a special constable he knew, who 'swore' that he did not know of any police inspector named Goole. To confirm this, Birling makes a phone call to the chief constable, who confirms that there is no Inspector Goole on the force. With a further call to the infirmary confirming that no recent cases of suicide have been reported, the family concludes that the Inspector was a fraud and that they have been the victims of a hoax.
Gerald and the elder Birlings celebrate in relief, but Eric and Sheila continue to rue their and the family's actions. Birling is then rung by the police, who explain that a young woman has just died at the infirmary in a suspected suicide, and that the police are on their way to question the family. The Inspector's identity is left unexplained, but it is clear that the family's confessions over the course of the evening have all been true, and that public disgrace will soon befall them.
