Pollution is leaking into the sea and Ariel, Flounder, Sebastian, and Scuttle decide to investigate. They come to an island which is inhabited by a mad scientist in a dark & scary castle. While Ariel, Flounder, and Scuttle take to the sewers, Sebastian stays on shore, thinking he's safe. Unfortunately it's Sebastian who is captured by Doctor Vial, a wannabe Dr Frankenstein who does crazy crab experiments, creating giant henchmen Franken-Crabs. Dr. Vial's experiment on Sebastian goes wrong and Sebastian is found and hidden by Vial's assistant/indentured servant/slave, an emaciated (and probably drugged) boy named Daniel. Daniel just wants a friend to play with and Sebastian tries to convince him that he doesn't need to stay in a bad situation just because his family is poor. Dr. Vial recaptures Sebastian & ties him to a gurney, while Ariel & Flounder get stuck in the sewers and almost get attacked by the Franken-Crabs and the mutant green ooze, until they escape. Scuttle saves Sebastian, but Sebastian still wants to save Daniel, so he talks about throwing a monkey wrench into the works, which Ariel does when she throws a wrench into Dr. Vial's hydropower water wheel cogs. The water wheel blows up, ruining Dr. Vial's latest experiment, and giving Daniel the opportunity to escape.