Five people are found dead, drowned in the River Thames. Police investigations led by Inspector Holt reveal that all of them had taken out insurance policies with the insurance broker Dr Foedor Orloff.

Holt questions Orloff and discovers that his company has paid to beneficiaries of the drowning victims, all of whom are now untraceable. Holt leaves to continue his investigations, and Orloff receives his next client, a struggling inventor called Henry Stuart.

Orloff lends him money on the proviso that Stuart takes out a policy with him, and contributes some of his money to an Institute for the Blind, run by his friend, Mr Dearborn, who is blind himself.

Orloff visits Dearborn's institute with Henry, and discovers that he has a daughter. Orloff leaves and Dearborn delivers a sermon to the masses of blind people, supported by his blind helper, Jake.

Meanwhile, Holt receives a visit from Lieutenant O'Reilly of the Chicago police force, in England escorting a forger, Fred Grogan, who has been extradited from America. Hearing of Holt's case, O'Reilly decides to stay in England and study Scotland Yard procedures.

Henry Stuart is found dead on the Thames mud flats, and Holt decides to carry out an autopsy in order to reassure Henry's daughter, Diana Stuart, that his death was an accident. However, the coroner's report shows that he was drowned in tap water - leading to a murder inquiry.

Holt confronts Orloff and finds that he has an insurance policy signed by Stuart - but Holt suspects that it is a forgery and goes to confront Grogan. Unknown to him, he is correct - Grogan had forged the document with Orloff before his arrival.

Grogan is having a bath in his house when Jake comes in and drowns him in the bathtub. Jake leaves just as Holt arrives and discovers the body. Holt decides on a new tactic, and gets Diana to act as a plant. She has developed a friendship with Dr Orloff, who has secured a job for her at Dearborn's Institute, and Holt instructs her to investigate its activities.

Diana finds evidence of her father visiting Dearborn's Institute, and telephones this through to Holt. As she does so, Jake comes into the room and attacks her, but she manages to fight him off. He escapes through the window just as Holt and O'Reilly arrive.

Holt starts a search for Orloff, and sends Diana back to the Institute. There she discovers a piece of jewellery that belonged to her father. She confronts Dearborn and notices that he looks at the piece, revealing himself to be sighted. She accuses him of being a fake, and of hiding Orloff. Instead, Dearborn reveals himself to be Orloff.

Orloff takes her to the tank where he drowns his victims. She witnesses Orloff kill Jake's friend, Dumb Lew, before Jake himself is called to kill her. Diana tells Jake about Lew, and he turns on Orloff. Orloff shoots him, but Jake still manages to throw Orloff out of the room to the Thames mud flats below, killing him. Jake dies as the Police arrive.

The case solved, Holt and Diana embrace, and Lieutenant O'Reilly returns to America.