Summary: Professor George Challenger and native helpers face river rapids and he loses a bag down the river. Back home, he interrupts a popular lecture by Professor Summerlee by bringing forth a bone, not a fossil, of a pterodon's wing. He is mocked by the crowd and one fellow who speaks derisively of notions of taking a rocketship to the moon, but Challenger wants to organize an expedition to Brazil. Sir John Roxton, big game hunter and playboy, will fund half the adventure. Challenger is not enraged at Malone being a newspaper reporter and volunteering to go along.

At an outpost the expedition hires Samuel (not Sambo) as the "lead bearer." Malone has a run-in with a tarantula. They meet Agnes Clooney and her pseudo-uncle, a missionary with whose help we all construct the tiresome "religion vs. science" debate after delivering the line, "Professor Challenger, I presume." He insists that the earth is 6000 years old, not millions of years, and that fossils are evidence of creatures that did not make it to the ark. Summerlee is arrogant but Challenger later reveals that his selection of science as a career disappointed his devout parents, and he berates Summerlee's insensitivity: "Now I don't know if there is a god, but I do know man is no substitute if there isn't."

Agnes will go with the expedition and Uncle asks if it's charity or ambition. Malone runs into a fetish, alternately considered a "cheap indian trick" and "curipuri." Uncle suspiciously joins and a leak in a bag Malone is carrying attracts deadly coral snakes. Was it only Summerlee's insect specimens? They find a cave and gunpowder marks suggesting that the passage to the plateau has been intentionally blocked.

Antagonism breaks out between Summerlee and Challenger, the former saying to the latter, "Perhaps you'd spend less time in the field if you had anything to go home to!" (There's no Mrs. Challenger in this version; instead, Summerlee is made into a family man.) A pteranodon, harpy-like, steals their pig barbecue. Assertions that it was an Amazonian vulture fall flat. They find a way up, but face a treacherous log crossing. Uncle doesn't want Agnes to go. Challenger is petrified but Roxton tells him that crossing is as "easy as falling off a log." Uncle declares, "That place belongs to the devil. It is no part of God's kingdom." So he strands the others on the plateau by knocking off the log (taking the role of Gomez in the book so that we avoid the inherent racism).

After some wandering, Malone fingers his gun and encounters a baby iguanodon he names Figaro. A giant iguanodon appears and Challenger says, "We're perfectly safe. It's an herbivore." Later it appears Roxton has killed Figaro for dinner, but it turns out to have been another iguanodon instead.

They come upon a pteranodon nest and when Summerlee exudes, trouble starts. Summerlee is bitten. Challenger asks Roxton in a private moment about their odds on the plateau, which seem grim since only a "religious lunatic" knows where they are stranded. An allosaur ambivalently attacks at night and Summerlee claims, "It's a creature from hell."

Malone climbs a tree and sees what will be dubbed Lake Gladys. On the way down he encounters apemen. They visit the lake. Roxton shifts gears: "Roight. Dinnah." Challenger: "Warm-blooded, if possible." Agnes and Malone fight about Roxton's flirting before an allosaur attacks them. They fall into a pit. When the dinosaur follows, it lands on a wooden stake.

The apemen capture Challenger and Summerlee, so Roxton arms everyone else. They see plateau indians in canoes land and explore. Agnes makes a connection with them and Roxton demonstrates gunpower so they all join up in an alliance against the apemen. The ape tribe bash the skull of a native and eat him. Challenger and Summerlee, horrified, exchange mutual admirations. When Summerlee is about to be killed, Roxton rescues him and a battle breaks out. Roxton would exterminate all the apes but Challenger stops him: they are a "new species! They must be preserved!" They have rescued the son of the chief native and are shown caves and a bit of enigmatic history -- a devil left and ruined the escape route (obviously Uncle, since explosives were involved). Challenger is fascinated by the ecological balance, Summerlee by the concerns of escape. Roxton flirts through Agnes' tranlations with the daughter of the chief. But "the women of this tribe hunt and fight with the men, until they are married."

Challenger declares the ape-people "the missing link" and insists "the killing must stop" despite the indians' inclinations with their prisoners. Agnes clashes with Roxton on the issue of hunting, with Roxton's assertion of the "cruelty of Nature" seeming like the rubbish that it is. As the weeks go by, a glider plan fails and Agnes smirks at Malone entertaining the native children. The chief's daughter offers Roxton a bucket of blood and the heart of a kill. The ape-people bury a dead infant and Challenger tries to gain their trust to study them more closely. Agnes laughs at Malone's description of Gladys as "tiny" and "delicate," and they take a swim. Summerlee makes an explosive concoction.

An ape draws a dinosaur in the sand and the prisoners start bellowing and, again, the son of the chief, Achille, and Roxton want to kill the apes, but Challenger blocks the proposal. Agnes and Malone see dinosaurs headed toward the village and it turns out that the apes were calling them. One dinosaur breaks the defense stakes while the natives run. Some throw spears while the apes cover themselves in feces. They cover Malone and the dinosaur avoids them. Roxton shoots one dinosaur dead in the throat. The chief spears the other through the face but is thrown. When Roxton is smacked down, it's up to Malone to shoot the dinosaur, which he does. Roxton laughs hysterically.

Summerlee has blown open the cave hole, and when the chief dies and Achille turns on the white men, Challenger grabs a bag with a giant egg in it and we all bid a hasty retreat. Roxton is attacked by an ape-man who takes his knife and is killed by Achille. Roxton and Achille have a stand-off until Roxton passes out, presumably dead. The chief's daughter cries and Achille softens.

"Oh thank God, we're back in the real world," says Summerlee, until we hear Uncle ranting: "The devil made that place." In a struggle with a gun between Summerlee and the insane missionary, Uncle is shot and dies in Agnes' arms. Natives in canoes rescue the others.

The British public cheer the explorers on their return. Gladys is not at the dock though. Malone's boss McArdle wants to capitalize on the discovery. Malone visits Gladys and finds that she's engaged to Mr. Arthur Hare. But "I named a lake after you!" Malone seems to be taking it well, but, says Gladys, "promise you won't do anything rash."

Challenger is scheduled to talk and there are rumors that he'll be awarded the Chair of Zoology at Oxford. He publicly unboxes a pteranodon hatchling and names the species in honor of Summerlee, but the photo flashes scare the creature and it flies about the room, ultimately escaping out a window. Agnes in the meantime is also retreating from the chaos. Malone tells Summerlee they shouldn't reveal what they have found to this lot, and Summerlee tells the crowd it was a trick -- an Amazonian vulture. Challenger agonizes but ultimately agrees that to reveal their discoveries would mean "the death of the plateau." Instead, Summerlee will tell of "unique insect species! Exciting new plant life!"