A pirate movie is shot on Shark Island and the Three ??? visit the movie set. Together with Peter´s dad, who is supervising the special effects, they spend a couple of days on the desert island to guard the equipment. But strange things happen. Peter´s dad goes missing and suddenly a real shark is circling around the island. The three detectives start their search. A secret underground tunnel leads the friends straight into the depth of the ocean and into a world that seems to be upside down.

In Rocky Beach, California, live the three detectives: Justus Jonas, Peter Shaw and Bob Andrews. Together, they are The Three ??? and there is nothing they don't take on!

The three friends witness exciting detective stories, solve puzzles no matter how tricky, and can always rely on each other.