Last of four titles in Basil Rathbone's Holmes films to feature Ian Wolfe in a supporting role.
Holmes:
However, it's a mistake to accept something that's true merely because it's obvious.
The premise of the film is that the only way the prisoner in Dartmoor can communicate with his colleagues outside the prison is through the music boxes. So how does he get the information to them about when and where the boxes are to be sold? This can be explained if the prisoner can pass message outside through an intermediary, but for safety, he doesn't want to directly inform the intermediary about the location of the plates.
This movie's final credit sequence rolled over a scene of Basil Rathbone and Nigel Bruce leaving Dr. Johnston's house. This sequence was later removed by a TV distributor and has been replaced with a THE END frame from one of the earlier Sherlock Holmes films.
The opening credits were also trimmed by the same distributor; the only version the restorers could find that had the film's title on it was from an old broadcast print whose image was blown up to match the size of the rest of the film.
English