The Heart of the Jungle" is an episode of The Backyardigans from the first season.
At first, Tyrone is seen alone in the backyard yelling like Tarzan. He then greets the viewer and tells them that he is Tarzan the Animal Guy, who can talk to animals. He starts the song "I'm Tarzan". Pablo walks over as the song is still being sung, starting a second part explaining to the viewer that he is Tarzan the Very Strong. Austin then comes over to the two Tarzans, as Tyrone and Pablo explain in song that Austin is Tarzan Who Doesn't Say Too Much.
After the song ends, Uniqua (in a tan hat) walks over carrying something, and introduces herself as Professor Uniqua. Tyrone asks what she has. Uniqua unfolds her hands, revealing a yellow worm-like creature called a Worman. His name is Sherman. Uniqua cannot understand what Sherman is trying to say, because he can only speak in squeaks. Tyrone volunteers to listen to Sherman, because he is Tarzan the Animal Guy (who can talk to animals).
Tyrone tells the rest that he is lost. He wants to find a way back home to the heart of the jungle. Uniqua says that she will take him there. The Tarzans want to help as well, but Uniqua refuses to let them join her quest, because Tarzans aren't "scientific". The Tarzans don't care about science, because they know there could be perils in the jungle. She finally lets them join. So the foursome venture into the jungle as the backyard transforms into it.
Tyrone hears Sherman, and the Worman thinks his home is in the heart of the jungle. So they have to travel deep into the jungle. Uniqua starts to sing the song "Where in the World". They travel deeper into the jungle. Austin has a "conversation" with Uniqua by saying "Quicksand bad." Uniqua thinks that jungles do not carry any quicksand, and she is totally unaware of any muddy puddles.
They all start to sing the song "Into the Thick of It" as they walk around in circles. Then the Tarzans realize that they have seen all the flowers, trees, and leaves before. Then they all realize that they have not gotten any closer to the jungle's heart. Uniqua uses a vine to swing, but her attempt was fruitless. Tyrone tells her that swinging on a vine only works, if she does the Tarzan yell. Her next attempt works, taking Tyrone's advice. The Tarzans follow, swinging off as they sing a reprise of "I’m Tarzan".
As they get closer to the heart of the jungle, they start to feel rain. Uniqua sings the song "It's Drizzling, It's Pouring". The rain gets heavier and they have to all sit in Uniqua's umbrella so they can drift closer on the small stream the rain has created. After a minute, they realize that the rain has stopped. They see a lot of other Wormans crawling on leaves. One Worman spots Sherman, and all the Wormans scream to greet him. Uniqua and the rest congratulate each other on a job well done.
Austin spots a muddy puddle as they are walking. Uniqua does not see it and steps into it. The Tarzans are alarmed and try to help Uniqua out of it as she sinks deeper into the puddle. They all see that it is quicksand. Austin uses a sturdy twig he finds in the bushes to pull her out.
When she gets out of the quicksand, she apologizes to the Tarzans for doubting their thoughts and invites them all over to her house for a snack of pretzels after Austin's stomach growls. The entire cast returns to the backyard as the imaginary rainforest transforms back into its previous form. They sing the end song and enter Uniqua's house. Pablo, Tyrone, and Austin all open the pink house's previously closed door and they all yell like a Tarzan.