Thornberry Island is the seventeenth episode of The Wild Thornberrys's second season which was first broadcast on September 15, 1999.
The Thornberrys get stranded on an uncharted island, basking at first in all the beauty and the splendor, but discover soon enough that being marooned isn't all it's cracked up to be.
After searching aimlessly for the Cook Islands, the Thornberrys stumble upon a crescent-shaped island which cannot be found on any map. They find no sign of civilisation or animals in the jungle. Marianne finds out that the Cook Islands are nearby and they prepare to enter the Comvee. Nigel attempts to activate the ocean transport mode but instead activates the vehicle's brand new security system by mistake, sealing the Comvee. To make matters worse, the system can only be deactivated from the inside, leaving the Thornberrys stranded on the island.
Believing that the foundation will come to the rescue, the family builds a fire, sets up a makeshift hut and amasses a large stockpile of coconuts. Nigel apologises to Marianne for screwing up but Marianne forgives him, assuring him that they will overcome the odds. That night, Debbie warns Eliza that they could be stuck on the island forever.
A week passes and no one comes to rescue the Thornberrys. One day, after lunch, Eliza catches sight of their parents savouring some coconut slices behind her back. Believing that they are keeping all the coconuts for themselves, a furious Eliza, Donnie and Darwin take some of the coconuts for themselves and set up a secret makeshift hut of their own deep in the jungle near a large waterfall on top of a cavern. They swim in the lake until evening and go back. When asked by Marianne about where they went, Eliza rebuffs the question.
The next morning, Marianne tries to stop the trio from returning to their cavern but to no avail. Nigel and Marianne leave to build a fire and in the process ignore a radio warning about an oncoming monsoon. While Eliza, Donnie, and Darwin relax at their outpost, Debbie takes the radio for herself and relaxes on a pile of rocks. When Nigel and Marianne demand that Debbie return the radio, she refuses and says out of spite that they will wreck the radio just like they did the Comvee.
As the monsoon looms ahead, Eliza and company return and Eliza once again rebuffs Marianne's demands for an explanation. The trio subsequently paint some sediments by the lake on their faces as the storm arrives. Nigel, Marianne and Debbie search for them, and Donnie naughtily screams and begins throwing coconuts at them. Eliza and Darwin follow suit and an infuriated Debbie retaliates. Their fight is cut short by a commercial bizjet, and the family is unable to get the pilots attention. Eliza apologises for her behaviour and explains that she thought Nigel and Marianne wanted some coconuts. An angry Debbie charges towards them but is struck by lightning which destroys the radio. Eliza, Donnie, and Darwin lead the family to their outpost.
However, the roof caves in and the cavern begins to flood. When the Thornberrys are unable to escape, Eliza begins to tell them about her ability to talk to animals, but she goes under before she can finish her sentence. The family manage to swim to shore and find the Comvee unscathed. Nigel tries to override the security system using a lightning rod but the rod topples. Eliza manages to fix it, but gets electrocuted and falls onto a tree trunk. The security system is fried and the Comvee is returned to normal.
Just then a large tidal wave charges towards them. They get in the Comvee and are swept away. Nigel activates the ocean transport mode and they are saved. After the storm, Marianne asks Eliza about her earlier explanation about herself and animals.