Roland Carstairs enlists the help of Sherlock Holmes to stop the robbery of the Star of Rhodesia, a giant diamond owned by Carstairs' mother, Lady Margaret. Holmes has a strong feeling that the diamond will be taken away during a train trip from London to Edinburgh.  Holmes promptly interchanges diamonds with Lady Margaret while in her chamber. Problems arise when Roland is killed and the faux diamond is taken away. Holmes must get to the bottom of the case and soon finds out that Colonel Sebastian is related to the murder.