At their campsite at the base of the plateau, the explorers are shocked when a large rock falls, sent their way by an Apeman perched on top of an overhead ledge. As the crew look up to see their attacker, Challenger spies overhead a Pteranodon killing and eating a young boar which proves that the statements in Maple White's diary are true. Leaving Zambo and Austin at the camp, they cross a chasm onto the plateau by cutting down a tree and using it as a bridge, but it is knocked over by a Brontosaurus, leaving them trapped.

The explorers witness various life-and-death struggles between the prehistoric beasts of the plateau. An Allosaurus attacks a Trachodon, and knocks it into a bog. The Allosaurus then attacks, and is driven off by a Triceratops. Eventually, the Allosaurus makes its way to the campsite and attacks the exploration party. It is finally driven off by Ed who tosses a torch into its mouth.

The explorers then make preparations to live on the plateau potentially indefinitely. Challenger designs a catapult to defend the camp. During a search for Maple White, Roxton finds his remains, confirming his death. It is at this time that Ed confesses his love for Paula and the two are unofficially wed by Summerlee who used to be a minister.

Shortly afterwards, as the paleontologists are observing the Brontosaurus, an Allosaurus attacks it and the Brontosaurus falls off the edge of the plateau, becoming trapped in a mud bank at the base of the plateau. Soon afterwards, a volcano erupts causing a mass stampede among the giant creatures of the lost world. The crew is saved when Paula's pet monkey Jocko climbs up the plateau carrying a rope. The crew use the rope to pull up a rope ladder constructed by Zambo and Austin and then climb down.

They discover the Brontosaurus that had been pushed off the plateau had landed softly in the mud of the river, trapped but still alive, and Challenger manages to bring it back to London, as he wants to put it on display as proof of his story.

However, while being unloaded from the ship it escapes and causes havoc until it reaches Tower Bridge, where its massive weight causes a collapse, and it swims down the River Thames. Challenger is morose as the creature leaves. Ed discovers that the love he left in London has married in his absence, allowing him and Paula to be together. Roxton morosely but gallantly hides his love for Paula as Paula and Ed leave together, while two passersby note: "That's Sir John Roxton – sportsman."

en.wikipedia.org/wiki/The_Lost_World_(1925_film)