When the King of a small but friendly nation is assassinated, Seaview is assigned to take the young Crown Prince back home to assume his throne. Neither Nelson (who gets turned out of his own cabin) nor Crane are used to handling children. Help arrives in the form of a mysterious flute-playing old man who befriends the boy after he is picked up at sea.

A mad art connoisseur seizes control of the Seaview and plans to pirate Leonardo da Vinci's Mona Lisa from a French Ship bound for the World's Fair.