Two scuba divers off the coast of Monte Carlo find an old treasure chest then something attacks and grabs one of them. At Torrington Academy Martin pulls a series of practical jokes on Diana and claims she's no fun so she vows to prove him wrong. Called to Centre they avoid eating MOM's inedible cooking and take the first warpgate to Monte Carlo. Diana interviews (and develops a crush on) Stefan the remaining diver who claims he has no idea what attacked them of why. Something big and unseen attacks the wharf and drags off a boat and Martin & the gang follow. They go diving in the area the diver disappeared and Java meet ""Rover"" his old dolphin-like pet from his caveman days. Diana finds an ancient coin where the treasure chest used to be at the start of the show and they enter an underwater cave but something carries off Diana. Java and Martin follow and track her signal to a giant sea monster which swallows them whole.