20,000 Leaps Under The Sea!

The trip to the beach that Aki promised the children in episode 18 gets put off again due to car trouble, but living with aliens proves to have its advantges, as Natsumi finds out about the Keroro Platoon's high-tech submarine, and begs them to take her to the beach, putting a slight dent in Keroro's newest scheme, to conquer the world through its oceans. Fuyuki comes along, hoping to gather some oceanic research to complete a school project, but winds up meeting a mysterious girl.
The girl's true origins are hinted at as the Keroro Platoon's undersea flag-planting activities are met with substantial opposition—the ancient Nontolma civilisation, already angered by humanity's pollution of its oceans, and looking to make an example to the Keroro Platoon.