Ariel finds a map to the Island of Prehistoria and decides to ask her father if she can go. Meanwhile, King Triton is too tense to do anything these days because Sebastian is being very annoying. Ariel and her family go on a vacation in the summer pole where it is cold, but relaxing.
Ariel and Flounder see dinosaurs trapped in ice. Flounder was also trapped in ice, but he was then released and caught a cold. Ariel wanted to share the same kindness to the poor creatures. Flounder just wanted some of the dinosaurs to look like the carnivore. Ariel thinks that the meat eating dinosaur can't be that bad (she thinks he's smiling). Ariel releases them. The ice melts and out comes a family of Apatosaurus (mother and baby), a Tyrannosaurus Rex (big male), a Triceratops (huge adult), and a Stegosaurus (large male). The juvenile Apatosaurus cuddles with its mother Apatosaurus. Ariel was happy that the dinosaurs feel so good, but Flounder knows that they're good and hungry.
The Tyrannosaurus Rex becomes hungry. He sees the Stegosaurus and licks his lips and battles the herbivore. The Tyrannosaurus whacks the trident with his tail and the trident falls into a trench and the dinosaur spots Ariel and Flounder. Ariel and Flounder think that the Tyrannosaurus can't swim so he will drown.
However, something worse happens, the Tyrannosaurus becomes land breathing and aquatic. He is able to breathe in water and swim. The Tyrannosaurus, Stegosaurus, and Triceratops are able to breathe in water (the trident blast must have gave the dinosaurs powers after they were released). The Tyrannosaurus, Triceratops, and Stegosaurus all go on a rampage and one of Ariel's sisters tells Triton that Ariel had the trident. Sebastian almost got crushed by the Tyrannosaurus Rex, only to have his left claw damaged. All the merpeople swim, until a family of Humpback Whales arrived. The Stegosaurus and Triceratops were frightened so they leave and go back to land. The Tyrannosaurus fights the bull whale, but he hits the dinosaur with his tail. Ariel thanks the whale for saving everyone.
The Tyrannosaurus had enough so he leaves and heads back to land. King Triton gets his trident and he sees the Apatosaurus, Tyrannosaurs, Triceratops, and Stegosaurus walking back on land. King Triton realized Ariel has found the Lost Land of Prehistoria. Triton melts the ice and the dinosaurs all become happy where there was water and trees on their island home. Ariel takes cares of Flounder and King Triton and the turtle have to deal with Sebastian. Sebastian told King Triton to read him a story and get him more snowcones. Dudley asks King Trition if Sebastian is driving him crazy again, which King Trition confirms. However, King Trition also states he wouldn't have it any other way.
- Seals also appeared in this episode.
- The four main dinosaurs (as seen in Fantasia) that were seen in this episode were Apatosaurus, Tyrannosaurus Rex, Triceratops, and Stegosaurus.
- The fight between the Stegosaurus and Tyrannosaurus Rex was identical to the one from The Rite of Spring segment from Fantasia.