Fog-bound London is being terrorised by a Jack the Ripper-style murderer known only as the Avenger, whose victims, all blonde young women, appear each Tuesday night. Joe, a police detective, is courting Daisy, the blonde daughter of Mr and Mrs Bunting, and he is visiting the household when they are disturbed by the arrival of a mysterious man, his face half-concealed by a hat and scarf, who enquires about a room advertised to let.

Mrs Bunting shows him the room, which is decorated with portraits of women. The man agrees a price for the room, and asks Mrs Bunting for a simple meal, but when she returns with the food, she finds that he has turned all the paintings to face the wall.

Daisy takes to the new lodger, but Joe, who is now assigned to the Avenger case, begins to grow suspicious. These suspicions grow when, the next Tuesday night, Mrs Bunting is awoken by the sound of the Lodger leaving the house; next morning news arrives of another murder, this time just around the corner from the house.

The police identify a pattern to the Avenger's attacks, and prepare themselves to catch him where they believe he will strike next. Meanwhile Mr and Mrs Bunting grow concerned at Daisy's developing relationship with the lodger, while Joe becomes furious when he finds the two together in the street. With grounds to suspect the lodger, Joe arranges a warrant to search his room. In a bag locked in a cupboard, the police find a gun, a map on which are tracked the Avenger's attacks to date, and a photograph of a young blond woman, who Joe takes to be the lodger's first victim. The lodger protests that the girl is his murdered sister, and that he was searching for the Avenger for his own reasons, but Joe dismisses this defence and arrests him, but the lodger makes struggles and, handcuffed, escapes into the night, with the police and Daisy in pursuit.

Daisy finds the lodger, who explains that he swore to his mother on his death bed that he would pursue the Avenger until he was caught. The two enter a pub, where the lodger attempts to disguise the handcuffs, but nevertheless attracts attention. When Joe enters the same pub after they have left, the other customers overhear his telephone conversation and conclude that the lodger is the Avenger, setting off in pursuit, just as Joe learns that the real Avenger has been captured elsewhere, and the lodger is innocent.

Realising that the lodger is in danger from the mob, Joe sets off, but the lodger has been caught trying to jump over a railing, and the mob has closed in. Joe fights his way through the throng and manages to reach the lodger in time, while the mob finally discover from a newspaper vendor that the Avenger has been caught.

Later, after the lodger has recovered from his ordeal, he and Daisy are united at last