With proof that prehistoric creatures may still be living, a group led by Professor George Challenger is assembled to go on an expedition to uncover more. Little do they know who, and what, they're about to encounter deep within the Amazon jungle.

Equipped with enough research material and evidence to return home, the adventurers prepare for their journey back to London. Marguerite makes a duplicitous trade with Chief Jacoba, involving Marguerite and an arranged marriage. In return, the Chief promises that the group will have safe passage off the Lost World plateau, but a series of unfortunate events leaves them back at square one.

The adventurers are captured by a reptilian race in a land where humans are at the bottom of the hierarchy. They come across other enslaved humans and rally them to help fight back, with a spark of hope for freedom.

To retrieve an antidote that will save professor Summerlee from some deadly venom, the explorers must venture into a bee hive that no human has ever made it out of alive. Meanwhile, Challenger and Marguerite keep the ape men away from the tree house with the help of electricity.

Professor Challenger is taken hostage. To save him, Roxton and Marguerite go into the Cave of Fear to retrieve bones for a ritual offering. In the process, they end up revisiting the dark corners of their past.

Summerlee saves a child from the river, convincing the village leader that they're all witches. Veronica flees to warn the others, while Challenger attempts to convince the village council of modern science to prove the action was not witchcraft. Marguerite and Malone get into an argument about his journal, but they put their differences aside to save the others from being burned at the stake.