Sgt. Frog, Vol. 20

Forget a land invasion, the new strategy is to conquer the seas! But wait - that's not Sgt. Keroro and his gang threatening fishing boats and kidnapping Natsumi while the kids are on a beach vacation! Just who are these strange new Keronians, and why are they stationed at the bottom of the ocean? Better yet, what are they planning on doing to the Earth?