At the Birlings' home in 1912, Arthur Birling, a wealthy mill owner and local politician, and his family are celebrating the engagement of daughter Sheila to Gerald Croft, the son of Birling's competitor. In attendance is Arthur's wife Sybil and their adult children Sheila and Eric. Eric, the youngest, has a drinking problem that is discreetly ignored. After dinner, Arthur speaks about the importance of self-reliance. A man, he says, must "make his own way" and protect his own interests.
Inspector Goole arrives, interrupting the evening and explaining that a woman called Eva Smith has killed herself by drinking strong disinfectant. He implies that she has left a diary naming names, including members of the Birling family. Goole produces a photograph of Eva and shows it to Arthur, who acknowledges that she worked in one of his mills. He admits that he dismissed her from Birling & Co. 18 months ago for her involvement in an abortive workers' strike. He denies responsibility for her death.
Sheila enters the room and is drawn into the discussion. After prompting from Goole, she admits to recognising Eva as well. She confesses that Eva served her in a department store and Sheila contrived to have her fired for an imagined slight. She admits that Eva's behaviour had been blameless and that the firing was motivated solely by Sheila's jealousy and spite towards a pretty working-class woman.
Sybil enters the room and Goole continues his interrogation, revealing that Eva was also known as Daisy Renton. Gerald startles at the mention of the name and Sheila becomes suspicious. Gerald admits that he met a woman by that name in a theatre bar. He gave her money and arranged to see her again. Goole reveals that Gerald had installed Eva as his mistress, and gave her money and promises of continued support before ending the relationship. Arthur and Sybil are horrified. As an ashamed Gerald exits the room, Sheila acknowledges his nature and credits him for speaking truthfully but also signals that their engagement is over.
Goole identifies Sybil as the head of a women's charity to which Eva had turned for help. Despite Sybil's haughty responses, she eventually admits that Eva, pregnant and destitute, had asked the committee for financial aid. Sybil had convinced the committee that the girl was a liar and that her application should be denied. Despite vigorous cross-examination from Goole, Sybil denies any wrongdoing. Sheila begs her mother not to continue, but Goole plays his final card, making Sybil admit that the "drunken young man" should give a "public confession, accepting all the blame". Eric enters the room, and after brief questioning from Goole, he breaks down, admitting that he drunkenly forced Eva to have sex and stole £50 from his father's business to pay her off when she became pregnant. Arthur and Sybil are upset by this, and the evening dissolves into angry recriminations.
The implication resulting from Goole's questioning is that each of the people there that evening had contributed to Eva's despondence and suicide. He reminds the Birlings that actions have consequences, and that all people are intertwined in one society, saying, "If men will not learn that lesson, then they will be taught it in fire and blood and anguish." Goole then leaves.
Gerald returns, telling the family that there may be no "Inspector Goole" on the police force. Arthur makes a call to the Chief Constable, who confirms this. Gerald points out that as Goole was lying about being a policeman, there may be no dead girl. Placing a second call to the local infirmary, Gerald determines that no recent cases of suicide have been reported. The elder Birlings and Gerald celebrate, with Arthur dismissing the evening's events as "moonshine" and "bluffing". The younger Birlings, however, realise the error of their ways and promise to change. Gerald is keen to resume his engagement to Sheila, but she is reluctant, since he still admitted to having had an affair.
The play ends abruptly with a telephone call, taken by Arthur, who reports that the body of a young woman has been found, a suspected case of suicide by disinfectant, and that the local police are on their way to question the Birlings. The true identity of Goole is never explained, but it is clear that the family's confessions over the course of the evening are true, and that they will be disgraced publicly when news of their involvement in Eva's demise is revealed.