After solving two mysteries in The Mystery of the Secret Room, the Anderson twins go on a cruise with their parents. After getting settled into their new quarters on the ship, Shelby learns from an elderly English woman that one of the dancers on the ship has gone missing. Shelby informs Daniel, and they visit the English woman, who then asks Shelby and Daniel if they would be willing to look for the missing dancer. Can Shelby and Daniel find the missing dancer before the end of the cruise?