Fred Hale, a newspaper employee, is murdered in mysterious circumstances by Pinkie, the recently appointed leader of a Brighton protection racket. Ida Arnold, who briefly got to know Fred while visiting Brighton for the day, is suspicious when the cause of death is recorded as ‘natural causes’, and she begins her own investigation into what happened. She meets Rose, a Catholic waitress from a poor family who is being courted by Pinkie. Rose is a potential witness to the murder: she has come to realise that Fred Hale did not come into Snow’s, the café where she works, on the day he was killed; that it was in fact one of Pinkie’s henchmen, Spicer, posing as Hale. But desperate to escape her situation by any means necessary, Rose refuses to believe that Pinkie’s interest in her stems purely from his desire to silence a potential witness to his crime.
Spicer decides to retire from the gang and asks Pinkie for a pay off. Afraid that he might ‘squeal’, Pinkie arranges for another, considerably more powerful Brighton gang, Colleoni’s mob, to kill Spicer at the race course; but Pinkie is also attacked. Spicer manages to escape, and so Pinkie pushes him over the banister at their lodgings. His solicitor Prewitt witnesses this, and Pinkie later visits him at home to offer him a sum of money and suggest he leave the country.
Pinkie is physically repulsed by Rose. But he knows that a wife cannot be forced to give evidence against her husband and, panicked by the increasing threat he feels from Ida, he asks Rose to marry him. As she is under age, the marriage has to take place in a registry office; both man and wife are painfully aware that, as Catholics, they are committing ‘mortal sin’.
Cubitt, another member of the gang, loses faith in Pinkie's leadership. He attempts to join Colleoni's mob but is rejected. While offering him consolation, Ida finds out that Pinkie and Rose are married. She visits Rose at Pinkie's lodgings and tells her that he is a murderer. Rose responds that she knows everything Pinkie has done and she doesn’t care. When Pinkie finds out about Ida’s visit, he immediately suspects Rose of betraying him. He decides that the only way to ensure her silence is to persuade her to enter into a suicide pact, although he does not intend to go through with his side of the bargain.
When Dallow, the third member of the gang, learns that Prewitt has been arrested and that Pinkie has driven Rose up to the cliffs, he sets off in pursuit with Ida. Pinkie leads Rose onto the clifftop and almost succeeds in persuading her to shoot herself, but they are interrupted by Dallow and Ida. During a fight with Dallow, Pinkie accidentally spills his bottle of sulfuric acid - 'vitriol' - on his own face. Blinded and in agony he staggers over the cliff edge.
In the final chapter, Rose goes to see a priest and confesses that she thinks she may be pregnant with Pinkie’s child.