After his mother is attacked by a two-headed monster, Burgy, an orphan baby dinosaur leaves his underground home and surfaces to the Earth, only to be found by two children who desperately try to save him from a greedy circus owner and an unscrupulous animal dealer.
There is a hidden undiscovered world deep under the surface of the Earth, consisting of huge underground caves and vast lakes, inhabited by never-before-seen creatures, including peaceful dinosaur-like animals and a two-headed monster, Orga, a product of constant pollution from the surface. A mother dinosaur and her baby called Burgy share a tranquil moment by a lake until they are attacked by the aggressive Orga. In order to save her baby, the mother tries to fight Orga but is soon lost under the surface of the lake. Desperate, Burgy finds himself alone and is tricked by two Nudas, funny looking creatures with wings, that his mother has gone up, where nobody has ever been before.
Eager to find his mother, Burgy manages to climb up through a secret passage to the surface of the Earth and soon finds himself surrounded by a world he’s never seen before. Two children, Rea and Sandy find Burgy and immediately realize that the poor baby dinosaur desperately needs their help. However, in the meantime, alarmed creatures of the underworld send two Nudas up to the surface in order to bring Burgy back, thus keeping their world a secret. Rea and Sandy escape their homes and as the children are gone, their parents start to look for them. Great Excelsior, a greedy circus owner, gets his hands on Burgy and tries to exhibit him as part of his freak show. Soon an unscrupulous animal dealer Dorsini sees Burgy and decides to steal him from Great Excelsior.
While the parents end up in the underworld during their search for their kids and are forced to fight Orga, up on the surface there is an enormous search taking place to catch Burgy involving Rea and Sandy, two Nudas, Great Excelsior and his assistant, animal dealer Dorsini and two thieves he hired to steal the baby dinosaur. At the end Burgy and his mother are reunited again while the children and their parents vow to keep the underworld as a secret.