Season 4

The Thornberrys are preparing for Donnie's ""birthday"" party in Borneo when one crisis after another happens. Grandma Sophie decides to visit the same day Nigel and Marianne must capture a wild orangutan on film or the foundation will cut their funding.

Donnie searches the jungles of Borneo for his ancestry, causing Eliza to worry that he will break ties with the Thornberry clan.

A huge fire strikes while Eliza and Darwin search for Donnie, Nigel and Marianne come across poachers, and Debbie and Grandma Sophie get taken in by a native tribe.

The Thornberrys reunite and find Donnie in a remote village and learn from the medicine woman there the terrible fate that befell his parents.

The Thornberrys are in Africa for the opening of a new chimpanzee sanctuary which will also be attended by Jane Goodall, famous for her work with primates. When Eliza meets her and introduces her to Darwin, Dr. Goodall tells her that she disapproves of domesticated chimps. Will Eliza be able to convince her that Darwin WANTS to stay with the Thornberrys? Meanwhile, when poachers trap Eliza and Darwin, will Dr. Goodall, Nigel and Marianne be able to rescue them?

The Thornberrys visit a tribe of native New Zealanders called the Mahris who are throwing a celebration called a ""Hooey"". As Debbie flirts with a young Mahri, Eliza and Darwin have a fight. When Darwin leaves and Eliza befriends a possum named Penelope who ends up getting her into a hot air balloon that she doesn't know how to steer, will Darwin come to the rescue?