The Thornberrys are preparing for Donnie's "birthday" party in Borneo (They don't know his real birthday, but celebrate the day they found him) 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. This is no easy task since poachers are lurking about and Marianne breaks her wrist climbing down a mountain. But to make matters even worse, Donnie runs off into the jungle after Eliza gets angry with him. As Eliza and Darwin search for Donnie in the rainforest, they run across a baby orangutan that needs to find her mother as Donnie is reunited with his "family", an orangutan and her son. Meanwhile, Debbie and Grandma Sophie get taken in by a native tribe and Nigel and Marianne scare off poachers. Finally, the Thornberrys reunite after a huge fire and find Donnie in a remote village and learn from the medicine woman there the terrible fate that befell Donnie's real parents.