A man goes to see the 'Don Giovanni' at the London opera - and disappears without a trace. Three days later he resurfaces in the Mexican desert, still in his formal wear and with a complete loss of memory, and no idea what happened to him. Sir Curtis assigns Stewart to investigate.The team discovers a case of mistaken identity - the real target was an important scientist - but by the time they realize this, the scientist has already been kidnapped. Now the only clue they're left with is the phrase 'turtle in a shell' - a special dish served in the Bahamas, so that's where the Department is headed.Once arrived there, they realize that this case is about far more than kidnapping - the perpetrators have much bigger fish to fry, and the detectives have very little time to prevent that scheme...