Desafío a Scotland Yard: La vela doblada

The mysterious Ramon Karadis (Francis de Wolff) whose friend John Lexman (David Knight) is being blackmailed. Karadis advises Lexman to meet the blackmailer and provides him with a revolver just in case of any funny business. The meeting goes wrong and the blackmailer is shot dead. Things look bleak for Lexman but Superintendent Meredith (Bernard Lee) of Scotland Yard begins to suspect that Karadis has some greater involvement in the case than it would initially appear. Karadis later gets shot in a room with no windows and a steel lined door which locks only from the inside.

Fairly good story of the locked room story which comes across a little pedestrian in this programmer and doesn't engage a lot, however it boasts a good cast and the ending is good, though the killer is rather obvious.