At the Birlings' home in April 1912, Arthur Birling – a wealthy factory owner and local politician – and his family are celebrating the engagement of daughter Sheila to Gerald Croft, the son of the owner of one of Birling's competitors, Crofts Limited. Also in attendance is Arthur's wife Sybil and their children Sheila and Eric, both in their early twenties. Eric, the younger, has a drinking problem that is not discreetly ignored and is a major part of the play. After dinner, Arthur speaks about the importance of self-reliance. He talks about his impending knighthood and about how "a man has to look after himself and his family."
A man calling himself 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 members of the Birling family. Inspector 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 then 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, Milwards, 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 starts at the mention of the name and Sheila becomes suspicious. Gerald admits that he met a woman by that name in the Palace 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. An ashamed Gerald exits the room. Sheila acknowledges his nature and credits him for speaking truthfully but also signals that their engagement is over. After Gerald returns, Sheila hands the ring, which Gerald had given her earlier in the evening, back to him.
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 declare that the "drunken young man" who had made Eva pregnant 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 raped Eva before meeting with her several times later and then stole £50 (purchasing power of £5610 in 2018 pounds) from his father's business to help her when she became pregnant. When Eva realised that the money had been stolen, she refused it. Arthur and Sybil are outraged by Eric's actions, and the evening dissolves into angry recriminations.
Goole's questioning revealed that each member of the family had contributed to Eva's despondency and suicide. He reminds the Birlings that actions have consequences and that all people are intertwined in one society. As Goole leaves he warns that "If men will not learn that lesson, then they will be taught it in fire and blood and anguish" - an allusion to the impending World War.
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, Eva Smith may not have commit suicide or even have existed. Placing a second call (this time 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, still 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 with a telephone call, taken by Arthur, who reports that a young woman has died, 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 if news of their involvement in Eva's demise is revealed.