On a windy night, Fordyce enters a deserted and mysterious old house, number seventeen. At the top of the stairs he discovers a man's body, and a vagrant, Ben. As Fordyce is investigating the house, Ben searches the body and finds a pair of handcuffs and a gun, which he pockets. They are startled by the shadow of someone crawling along the roof. A girl, Rose Ackroyd, falls through the skylight. She is the daughter of the dead man, Ackroyd, who lived next door. She reveals that after her father received a telegram he disappeared from their house. Following him, she discovered a card on the window ledge, with "No 17, 12.30" written on it. The telegram is from a detective, Barton, warning Ackroyd that he has traced the Suffolk necklace to Sheldrake and expects him to make his getaway from Number Seventeen tonight.

At 12.30, the doorbell rings and Fordyce answers it while Ben discovers the corpse has disappeared. A couple, Brant and Nora, arrive, ostensibly to view Number Seventeen, which is to let. Fordyce lets them in, along with another man, who claims to be Brant's nephew. They all go upstairs. Threatened, Ben pulls the gun, but Brant draws his own. Ben fires and hits Fordyce in the wrist. The wound is dressed by Nora, who is a deaf-mute. Brant shepherds Ben, Fordyce and Rose into a room, where they are searched and the card and telegram found.

On the landing, Brant and his 'nephew', who have not met before, talk, and discover that they are both there to meet Sheldrake and make an escape to the continent in a goods train from the depot beneath the house. They show each other their tickets: Brant has the card which he retrieved from Rose with the telegram, and his nephew an identical one. The lights go out and another shot is fired as Ben attempts to flee. Brant and his nephew lock Ben in the bathroom where he is attacked by a man, who retrieves a diamond necklace from the cistern. Feigning unconsciousness, Ben picks his pocket and takes the necklace.

Sheldrake appears, bloody and bruised. He demands to see the others' cards, and together the criminals tie Fordyce and Rose to the banisters at the top of the stairs. He indicates the escape route via a bedroom to the other three, but locks the door behind them and reveals that he is Ackroyd, and unties his daughter and Fordyce. When he unlocks the bathroom to release Ben, the real Sheldrake emerges, and he and Ackroyd fight. Ben intervenes, only to knock Ackroyd out. Sheldrake locks Ben and Ackroyd in the bathroom, and releases the other three, who again tie up Rose and Fordyce. Nora returns to free them: she is not really a deaf-mute and uneasy about the company she keeps. She follows Brant and his nephew, who force her to go to the continent with them. Rose remains in the house to nurse her father, and Ben and Fordyce give chase to the fleeing criminals, who board an empty carriage in a train bound for Germany via a train ferry.

Ben manages to board the train, but Fordyce does not. The criminals are spotted by the guard and the three men go to the back of the train to deal with him, leaving Nora alone. Ben extricates himself from a wagon full of tonic wine and reaches the carriage. Meanwhile, outside, Fordyce commandeers a bus and follows the train to the coast. In the guard's van, Brant and his nephew demand the diamonds from Sheldrake, who no longer has them. Sheldrake denounces Brant's nephew as a policeman, and the nephew announces that he is in fact Detective Barton. Barton returns to the empty carriage to search Nora for the diamonds, but Ben intervenes and offers Barton the necklace. As Brant and Sheldrake arrive at the carriage and Barton takes off again the necklace is dropped. Barton gives them the slip and returns to the carriage but the necklace is gone, and he handcuffs Nora to the carriage.

Brant and Sheldrake are spotted by the train driver and shoot him and then dispose of the fireman, but they are unable to stop the engine. The runaway train crashes into the ferry, detaching it from its moorings and sinking the train. Fordyce, who has arrived at the quayside, dives in and rescues Nora. In the harbourmaster's office, Barton congratulates himself that he has apprehended Nora if not retrieved the diamonds. Fordyce begs to differ: he unmasks Barton as Henry Doyle, who the police have been chasing, knowing that he would follow Sheldrake and the Suffolk necklace. Moreover, Fordyce reveals that he himself is Detective Barton. He asks Nora to accompany him to breakfast. Delighted, Ben opens his coat to show that he is wearing the diamond necklace.