Nancy, Bess, and George are going on a cruise around the Great Lakes for a fantastic end-of-summer vacation! As soon as the girls set foot on the ship, though, they smell something fishy. Their new friend Amber is upset because her boyfriend, Craig, is neglecting her. After a little investigating on her friend's behalf, Nancy learns that Craig is really a detective. He's been busy hunting for a mastermind diamond thief who may well be on the ship. And it turns out Craig can use some help.

Soon Nancy's hunting for clues on land and offshore. Where there's stolen diamonds, though, there's danger -- and Nancy and her friends are soon caught in the thick of it. Will Nancy be able to crack this case before her ship is sunk?