Mutated sea-creatures are being discovered at a rising rate, and the Planeteers set out to learn why - but Gi and the others are unaware that in her anger of putting up with Suchi while enhancing the Ecosub's controls, she lost a gear for the speed-increasing control. They only find this out after they've already taken off to investigate the water-contamination that is the result of Sly Sludge's illegally pouring waste in the ocean. Things get out of hand when he sees them before they see him, so takes a reporter and young fisherman aware of the crisis as hostages in case his old enemies return. But the Planeteers have their own problems to deal with, like escaping a large clam that accidentally traps them when the sub crashes through Sludge's barge, and still have to get out in time to stop a large mutant squid from attacking a sea-coast.