Steve, Matt, and Jenny board their very first cruise ship prepared to relax, have fun, and eat a lot of food.

But on their first night out at sea, they discover the ship holds a secret. Many years ago, a smuggler hid a very expensive jewel somewhere on board, leaving behind a letter, written in code, as the only clue to its location.

Excited at the possibility of solving another mystery, the trio begins the investigation only to unwittingly draw the attention of three very dangerous men and one mysterious deckhand, all of whom seem interested in finding the diamond for themselves.

With only three days to find the gem or lose it forever, the three kids race against the clock to decipher the clues and discover the gem, while keeping themselves and their parents out of danger.