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.