Master sleuth Sherlock Holmes is hired by the wealthy widow Lady Margaret and her son Roland Carstairs to prevent the theft of an enormous diamond known as the 'Star of Rhodesia'. Travelling by train from London to Edinburgh, Holmes guards the priceless jewel with the help of Dr Watson and Inspector Lestrade. But when the diamond disappears and Roland is found murdered, Holmes must deduce the murderer's identity from among the speeding train's many suspicious passengers.