Both Scotland Yard and the criminal community of London are trying to discover the identity of "the Squealer". This mysterious fence forces criminals to sell him their wares for a pittance. When some object, he "squeals" to the police. Those who oppose him are ruthlessly killed, preferably by means of the poison of the Black Mamba. Inspector Elford of Scotland Yard investigates and he has plenty of suspects. The trail leads to the strange Mr. Sutton, owner of a zoological store that also carries predators and poisonous snakes. During his investigation, Elford meets Mrs. Mulford, an older lady who tries to help ex-convicts, and Beryl, her niece, who writes crime stories and works as a court reporter. Inspector Elford discovers a similarity between the typeset of a machine also used by Sutton and letters written by the Squealer. To make Sutton confess, he is tricked by Mrs. Mulford in cooperation with the police into drinking what he believes to be poisoned tea.<ref name="Filmportal"></ref><ref name="Magnum"></ref><ref name="Kramp" />