Holmes and Watson investigate a series of bizarre and apparently unconnected murders, and the death of a possible suspect. The trail leads to a society of hypnotists and a mysterious, glamoruos woman. The fiendish Dr Moriarty, though reported hanged in Montevideo, is belived to be involved.

In a gloomy mansion in the west of Scotland the seven members of the Good Comrades Club are dining. One of them receives a curious death threat, an envelope containing seven orange pips. He dies the next night in a car crash. At his funeral another member receives an envelope with six pips, and ten days later his body is recovered from the sea. Sherlock Holmes is called in.

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.