A slick young man buys a jewel with a cheque that bounces. He then disappears and both the police and a lawyer try to find him. The lawyer hires a female detective to track down the missing man, but when his body is found in the Thames the case is by no means over.

Young nurse Jill Holden gets involved with an extortion gang targeting Mrs Cheam, one of her patients. When Mrs. Cheam is murdered, Jill becomes a suspect and must clear her name.

British crime film directed by Peter Duffell and starring Bernard Lee, Moira Redmond and John Van Eyssen, loosely based on the 1918 novel "The Man Who Knew" by Edgar Wallace.