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.
Aspiring young Scottish politician John Shand enters into an unusual agreement with the wealthy Wylie family -- if they fund his education, he must marry their daughter, Maggie. Staying true to his word, John weds Maggie and begins a successful career, thanks largely to his savvy wife. The couple's relationship is placed in jeopardy when John faces temptation in the form of the lovely aristocrat Lady Sybil Tenterden.
