When a pearl with a sinister reputation for causing misfortune to its owners is stolen from a museum by a master criminal because of Sherlock Holmes’ show-boating, he is naturally obliged to find it. Soon, he learns of a series of brutal murders that seemed to have been commited by a malevolent man mountain known only as the Creeper. Now, Holmes must deal with the seemingly overwhelming menace of this man and his boss in order to retrieve the pearl.

Holmes and Watson are recruited in a serpentine fashion to escort the heir to a European throne back to his native country following his father’s assassination. Because the prince has been educated in Great Britain, Holmes persuades him to masquerade as Watson’s nephew Nicolas on a ocean liner bound for Algiers. Unfortunately the ship is filled with red herrings as well as real assassins, and Holmes is challenged to outwit them all and deliver his charge to his destination. Among the suspects are a knife-throwing circus performer, two shadowy archaeologists, a hulking deaf mute, an enigmatic ship’s steward, a chanteuse with a mysterious song case, and a gun-toting British dowager.