Ramon Karadis is a powerful blackmailer. The police don't like him.
When he is found dead in his locked room, Superintendent Meredith (Bernard Lee) is stumped. How was he killed and how did the murderer leave a locked room.
Before that. Karadis had set up John Lexman for murder. Lexman pleads his innocence and Karadis had conveniently hidden all the witnesses.
Superintendent Meredith was sympathetic to Lexman's pleas of innocence even though he has been convicted. Then Lexman escapes from jail.
This is an Edgar Wallace mystery in two halves. The first part is all about Lexman and the audience feels sympathetic about his plight.
The murder of Karadis comes later although it is obvious to the audience who was responsible. It was a case of how it was done.
Bernard Lee is very effective here.