Secrets of Scotland Yard is Republic's spin on a plotline first elucidated in the old E. Phillips Oppenheim novel The Great Impersonation. After losing WW I, the German high command, with remarkable foresight, prepares for the next war by planting a spy in the British Admiralty. Edgar Barrier plays the dual role of the German spy and his British twin brother. When one twin is killed, the other assumes his identity. The question: is the surviving brother the "good" one or the bad? It is up to C. Aubrey Smith, cast as Scotland Yard inspector Sir Christopher Belt, to sort out the mystery. Though it owes a great deal to the aforementioned Oppenheim yarn, Secrets of Scotland Yard is actually based on a novel by Denison Clift, who also wrote the screenplay.
~ Hal Erickson, All Movie Guide