In one of his most daring exploits, Jack goes down under — literally — to investigate the disappearance of a diver off the Great Barrier Reef of Australia. Using everything in his backpack arsenal from anti-puke pills to a morphing suit, an egg-shaped submarine to Disappearing Milk, he exposes a deadly nest of pirates raiding a shipwreck site. Jack has to thwart their schemes, evade their attempts on his life, and escape the primordial hunger of three deadly species: a Komodo dragon, a set of sharks, and the most poisonous predators in the reef, the Irukandji jellyfish. Can he save the treasure and the diver, not to mention himself?