With fear drifting out of the fog and terror lurking in every port, master sleuth Sherlock Holmes (Basil Rathbone) and his trusty assistant Dr. Watson (Nigel Bruce) are instructed to escort soon-to-be-king Nikolas back home to Algiers after the assassination of his father. Travelling aboard a transatlantic oceanliner with Nikolas disguised as Watson's nephew, Holmes must keep close watch on his young charge as onboard assassins attempt to kidnap the young monarch and subvert the throne.