While Pidge is out on patrol, he spots some children playing near the water. When he stops to investigate he realizes they're looking at tadpoles swimming around. Unaware of their sinister nature, he scoops a load up for the kids in his helmet (so they can watch the tadpoles turn into frogs) and gives the kids a ride home in his Lion. The kids' father is just leaving as they arrive and it turns out he's the captain of an Arusian navy ship, the Altea. Pidge also takes a helmetful back to the Castle as a gift for the Princess.
Overnight, the innocent looking tadpoles morph into fierce, three-eyed amphibias nightmares. The Altea, now at sea, is also attacked by the vicious frogs and they manage to signal an s.o.s. When the Voltron Force realizes it's the Altea under attack by frogs, Pidge puts two and two together. Once the VF take care of the frog situation at the Castle, Pidge goes to make sure the kids and their mother are okay. They're almost cornered until Pidge uses a barrel of gas to blow up/ignite the offending critters.
Meanwhile, the rest of the team goes to the aid of the Altea. The frogs are disposed of easy enough and the Altea saved. But of course, that was just the lure. Lotor then launches the real robeast. It grabs Pidge and takes him underwater. After Pidge breaks free, Voltron is formed and pursues the Robeast ever-deeper into the ocean. Pidge's Lion suffers a blow that has it taking on water. Learning of this, Lotor orders the attack concentrated on Pidge's Lion. Pidge suffers two more serious blows before the Robeast grabs Voltron and drags him 1,000 phathoms deep. Water pressure is their main concern and they have to equalize the pressure quickly or be crushed. With that crisis over, the team surfaces, forms the blazing sword...yada yada.