While swimming in the ocean, Wilykit gets swallowed by a giant green robotic frog. The robot then starts pursuing the horrified Wilykat who was also swimming with her. Wilykat manages to outrun the metal behemoth by riding a Unicornand runs back to the Cats Lair to fetch Lion-O. When the two arrive back at the scene they discover that Wilykit is safe and that the robot, called Hercules, is controlled by the friendly Dr. Dometone, who actually has come to ask the ThunderCats for help.
At Cats Lair, Dr. Dometone explains to the ThunderCats that he is the director and principal guardian of the Great Oceanic Plug, a device created to seal a giant fissure in the ocean floor and prevent all the water from draining into the earth's core. But recently, the plug had been under attack from the alien Scrape, who wants to drain the oceans to obtain a mineral found on the ocean floors. This resident of planet Blue Plunder pilots a giant electric eel shaped robot and has already succeeded in destroying Samson, one of the two giant robots guarding the plug. The ThunderCats agree to help and Lion-O and the Thunderkittens ride with Dr. Dometone in "Herky" to the plug while the rest of the Cats stay behind to work on an emergency rig.
Inside the Great Oceanic Plug, the crew are out of food and supplies and out of desperation one of them decides to go out for help in a small propulsion sub. However, Scrape's Giant Eel who was hidden nearby, lunges at the sub and devours it whole. Scrape then pilots the eel straight into the plug via the observation deck.
Herky arrives and its occupants see the Giant Eel sticking half way out of the dome of the plug. Lion-O and Wilykat don Shark suits and swim out with a grounding cable which would neutralize the Giant Eel's electric charge. Scrape, meanwhile, notices Herky and drives his Giant Eel straight towards it. The eel wraps itself around Herky and is about to deliver its lethal charge when Lion-O swims up behind the eel and throws the grounding cable on it.
With the eel put out of commission, Scrape jumps into the propulsion sub that he had swallowed earlier and drives out of the eel. Wilykat tries to stop the sub but Scrape captures him. Seeing this, Lion-O calls the other ThunderCats. Meanwhile, Scrape has infiltrated the dome of the Great Ocean Plug and opens the plug. However, Panthro, who has arrived in the ThunderTank with Tygra and Cheetara, encloses the dome in a compressed air bubble to prevent Scrape from succeeding. Tygra then activates the vacuum suction and Scrape gets sucked out of the dome and into the ThunderTank.