Guy, Mikoto, Leo and Mamoru’s class are attending the launch of a new submarine developed by Leo. It’s christened as the Ryuuguu 9000 by the students. As Leo gives a speech, a disgruntled crew member complains that he should have been the captain of the Ryuuguu. Penchinon appears behind him and tells him to take it. Mamoru sense the Zonder and immediately pages Mikoto. Guy opens a nearby suitcase containing his Ultimate Armor. The Zonder fuses with the submarine and it begins to move. Hana, who was standing on the entrance hatch to pose for photos, falls inside. Penchinon opens the floodgates and lets the Zonder out just as Mamoru manages to catch up and jump inside. Guy tries to follow but the Zonder throws him off with its arms and dives faster than he can swim. Leo tells Guy that Volfogg has already launched in the Multi-Dimensional Intelligence Submarine, which rises right under Guy’s feet. In the Ryuuguu, Mamoru holds Hana close while the Zonder proclaims its intent to reach the deepest part of the ocean. Hana can only cry in terror while Mamoru says that adults can be difficult. Guy and Volfogg chase after the Ryuuguu, with Volfogg noting a navy submarine nearby. The Ryuuguu collides with it and begins assimilation, forcing the crew to abandon ship. Volfogg tells Guy they can’t approach yet or their ship might get assimilated too, so they must wait for orders from 3G. The Multi-Dimensional Intelligence Submarine rises to the surface and deploys life rafts for the navy submarine crew. Taiga wonders where the Zonder is heading and Entouji says that based on the data its destination is the Richard Plateau in the Marianas Trench, a place untouched by mankind and devoid of any light. The victim was a man disqualified from he captain’s position due to the specialist developer being chosen instead, so now he stole the sub and is using it to prove his skills by diving to the deepest possible point.
As EI-11 dives deeper, its body begins to crack and collapse from the pressure. Mamoru’s pager dies out and he wonders if he could purify the Zonder right then and there but relents because they’d all succumb to the crushing pressure if he did. Up on the surface, Penchinon muses that it was a lucky coincidence that Mamoru got stuck inside the submarine because there’ll be no one left to purify the Zonders once the submarine reaches the deepest part. Mamoru’s friends wonder if he and Hana are still safe, and Sueo tells them that 3G will rescue them. Kazou explains the rescue plan to Guy, which involves using the Dividing Driver to protect GaoGaiGar from the crushing water pressure. The catch is that he’ll only have enough power to hold the field for 30 minutes. Guy forms GaoGaiGar while HyoRyu and EnRyu combine into ChoRyuJin. ChoRyuJin uses his cranes to lower the 3G submarine Marine-1 which Guy will use to dive to 6000 feet while 3G tracks EI-11. Marine-1 eventually reaches its pressure limit and explodes, so Guy bursts out and activates the Dividing Driver to continue pursuit. EI-11 reaches 10000 feet, and Hana wonders if mermaids can come to their rescue. Mamoru continues to hold Hana tightly just as he spots GaoGaiGar from a porthole. EI-11 changes into a giant clawed fish and charges Guy. It fires a pair of torpedoes that pass harmlessly around the repression field. Guy punches EI-11 when it tries to penetrate the field and throws the missile it tried to fire at close range right back at it. EI-11 grapples Guy with its claws and gets a face full of Drill Knee for its efforts, followed by getting its body trashed by Broken Magnum. Guy just barely has time to perform Hell and Heaven before the Dividing Driver’s energy runs out. Leo and Hyuuma note that time is up and assume the worst, sending Mikoto screaming in tears. ChoRyuJin notices something and quickly lowers his cranes. Volfogg reports a large energy reaction at the ocean floor. ChoRyuJin manages to pull up GaoGaiGar, who’s carrying Mamoru along with an unconscious Hana and the Zonder victim. Mamoru had managed to purify the core underwater then use his power to protect everyone from the water pressure. Volfogg marvels at Mamoru’s power and is positive it’s of alien origin.
This was certainly a tense episode, though its impact is somewhat dampened by a few issues. For one, it’s not clear if Penchinon intended to do anything with the Zonder other than have its host live out its fantasy of diving to the ocean floor, which would have generally not accomplished much had it not accidentally acquired Mamoru and Hana as passengers. And then you have Mamoru, who on top of being able to fly and purify Zonders can now project a water pressure-resistant barrier big enough to protect a 30 meter robot, though at least we’ll be seeing this power a few more times later on. Volfogg makes another appearance here, but it’s not yet revealed that he and the police car from the previous episode are one and the same.