TV.com

Harold explains to the kids about something called D-13, a kind of plankton that can make fish grow 4 times their size when they eat it. So that explained the huge sharktooth, meaning the D-13 has reached the ocean.
Sheena finds an wrecked boat, and concludes that there should be a lifeboat nearby to get them home. While Sheena and Uncle Harold search for it, Billy goes on a search of his own, and is kidnapped by a green creature.
Sheena finds a canoe, and they are ready to leave, but they can't find Billy. Sheena goes looking for him, but runs into Uncle Harold. They spot a giant lizard and crab. They run to safety, and then Harold is pulled underground. Sheena jumps in after him, and sees more super-forbidden>Harold created an antidote to return the fish people to normal, and Sheena asked what happened to Dr. Ritter, and Uncle Harold said he didn't know, and that Ritter might return, but he really had become a fish person himself!