The Lost World is shattered by gunfire as trophy hunters try to capture a Troglodyte child. Veronica swings through the jungle to save him, and becomes the hunters' new target. She escapes to the Treehouse to warn her friends just ahead of the hunters. One of the hunters declares that he has been commissioned to locate the Challenger expedition and help them return to England. Despite Veronica's warnings, and Marguerite's suspicions, the hunters are welcomed and set up camp nearby.

The explorers have a strange guest at the Treehouse who impresses Challenger with his knowledge of geology. Meanwhile something attacks Challenger's electrostatic accumulator. The explorers chase after a group of underground dwellers whom they believe to be the culprits. Their guest turns out to be one of them, and the explorers are soon taken hostage. It turns out that the underground dwellers need Challenger's scientific expertise to help them evolve above ground.

During a powerful thunderstorm on the plateau, all five of our heroes experience dreams and visions of their ""lost"" colleague, Summerlee, who had disappeared into a raging waterfall and presumed dead. Malone sets off after this ghost, and Challenger Marguerite and Roxton follow Ned. The three are ambushed by cannibals who fancy Marguerite's arm for an aperitif. Roxton and Challenger break free and save her, and then the three resume their hunt for Malone, only to be trapped by a hungry T-Rex.

Ned meanwhile, has fallen and knocked himself out. He regains consciousness, concussed and thirsty, and follows Summerlee's ghost along a dry riverbed. He stumbles and twists his ankle, but still hobbles on. Summerlee appears to be leading him to the now dry waterfall where he was last seen.