Sgt. Frog , Vol. 20: An Underwater Alien Adventure Or A Blast From The Past?

Looks like Keroro and his friends aren't the only ones planning to invade Earth. It turns out there's another alien force wreaking havoc with a monstrous new weapon! When Fuyuki, Momoka and the others go to investigate some strange ocean sightings, their island trip doesn't turn out anything like they expected. What will happen when they join forces with the Keronians to fight these strange assailants?