Basil Rathbone and Nigel Bruce are hired to safeguard a gem called the "Star of Rhodesia" in this short but neat Sherlock Holmes thriller that moves fast enough to skim over the potholes in the plot. Three murders later, the air-gun killer is revealed, and Dennis Hoey has overdone his comic act as Inspector Lestrade to an irritating degree. Years before, Alan Mowbray (here playing Major Bleek) had appeared in the Clive Brook and Reginald Owen movies about Holmes, but he wrote that Rathbone's version of the sleuth was the best. Well, we knew that, didn't we?

Holmes and Watson board a passenger train bound from London to Edinburgh, to guard the Star of Rhodesia, an enormous diamond worth a fortune belonging to an elderly woman of wealth; but within the first hour of the trip, the woman's son is murdered and the diamond stolen and any of the passengers in their car could be the killer thief.