Led by the maverick Professor Challenger [Bob Hoskins], a party of explorers leave behind the smog and gaslight of turn-of-the-century London and search for a forgotten plateau deep in the uncharted regions of the Amazon jungle. Challenger is joined by Professor Summerlee [James Fox], his arch academic rival, and as the expedition progresses the Professors bicker. Edward Malone [Matthew Rhys], a reporter from the Daily Gazette, records their progress, whilst Lord John Roxton [Tom Ward], big-game hunter and adventurer, leads the way, and missionary’s daughter Agnes Cluny [Elaine Cassidy] helps with her knowledge of local customs. Eventually they attain their goal. But just as they reach the summit, they are betrayed by one of their number. The Reverend Theo Kerr [Peter Falk], Agnes’ uncle, destroys the bridge they have built onto the plateau, leaving the group isolated, with no way back down!

The Professors are captured by the apemen, and before they can go to the rescue, Edward, Agnes and Lord Roxton encounter a party of Indians. After a tense stand-off the explorers and the Indians join forces. Roxton leads a daring assault on the apemen camp to rescue the Professors.