On the boat, Challenger shows a sarcastic Summerlee and his expedition members a map, drawn up by a Spanish man called Padre Mendoz who ended up in the remote, uncharted area of Brazil which Challenger claims prehistoric creatures thrive. Most notably, there is a plateau, which would supposedly isolate the inhabitants from the evolutionary mainstream for millions of years. Upon arrival, Roxton begins flirting with Agnes (Cassidy), the niece of Reverend Theo Kerr, a priest who disregards the idea of evolution.
They eventually find the plateau. They cross over a log bridge, which the Reverend suddenly pushes into a deep crevice in an abrupt mood swing, thus leaving them stranded. In the jungle, Edward makes 'friends' with a hypsilophodont, and the stunned group spot an Iguanodon, and then a group of pterosaurs who attack and injure Summerlee.
They retreat to the forest where Edward is scared out of a tree by an ape that, in his words "looked almost human". In the middle of the night, while they are gathered around the campfire whispering about "Who will miss them if they die" they are attacked by a large carnosaur, which is later identified by Summerlee as an allosaur. They reach a lake which Malone discovers and names "Gladys" while Roxton and the professors rest by the beach. Edward and Agnes walk off along the beach until the Allosaur from last night emerges from the forest and drinks from the lake, but soon notices them as they run into the forest. In the cliffhanger ending of the first episode, the carnivore pursues them through the borders of the forest until they all fall into a pit, where the Allosaur is killed after being impaled on two wooden spikes.
After making their escape, they find out that Challenger and Summerlee have been kidnapped by the ape-men. The apes take them to an enclosed sacrificial chamber, where they are placed on a thick sheet of rock which is covered with blood. The sun shines through a crack and the beasts place Summerlee's head in a groove on the plate of rock, about to smash his head with a large stone when Roxton and the group start shooting all the ape-men. Challenger tries to save the creatures, calling them "The missing link between animal and human" upon leaving the animals territory, the group also rescue an Indian chief's son. The tribe recognizes Challenger as Padre Mendoz - the Portuguese man that returned from the plateau and drew up a map of the area. They are also told of how a man, whom they thought was the Devil, came to visit them and then left, sealing off the cave that was their only route out of the plateau. The two groups cooperate very well together, with Challenger sitting by the chief's side, and Roxton marrying the patriarch's daughter. However, the presence of the ape-men disturb the tribes people but, with Professor Challenger's protection, they remain safe from harm and are kept in a wooden cage/construction on the border of the village.
However, two Allosaurs attack the village after weeks of harmony. After causing much death and destruction, the first Allosaur is killed by Roxton and his Elephant gun but the second, larger Allosaur kills the chief. In an act of kindness, Agnes and Malone set the ape-men loose and they flee back into the jungle. One psychotic ape gets its hands on a knife and keeps an eye on Roxton. Edward Malone eventually kills the larger dinosaur, but is a little late to save the chief and several of the Indians. He is critically injured, and dies in the arms of his son, who blames the white intruders. During the attack, Summerlee blows up the debris blocking their only escape route, and after the chief's son assumes command, the outsiders flee while Roxton stays behind to stall them. The ape with the knife stabs Roxton in the torso, injuring him while the son of the chief shoots the mad ape while it stands atop a rocky mound, growling. Outside, the Reverend comes to block the remaining group's way, intending to kill all of them and seal off the cave again, insisting that the plateau is "Not part of God's kingdom", and calling it "the work of the devil". While struggling with Summerlee, he accidentally shoots himself in the chest and dies.
The explorers go back to London, where the juvenile pterosaur that Challenger brings back escapes. Afterwards, Malone and Summerlee urge Challenger to end the whole affair so that the plateau and its inhabitants can exist in peace; the pterosaur is dismissed as an Amazonian vulture, while the articles Edward sent back are passed off as extracts of a novel he is writing. Edward confesses he loves Agnes, who tells him the same, and they kiss in the great hall of the museum where the crew have been exposed as frauds. In the final scene, Roxton is shown alive and well, and still happily married to Muree.