The 1960 version based on the Sir Arthur Conan Doyle novel opens with title and credits shown over various scenes of lava flowing. A Boing 707 in TWA livery lands at London Airport. A newscaster is there to welcome Professor George Edward Challenger (Claude Rains). A crowd of newspaper reporters crowd the stairway of the airplane clamoring for an interview. Front and center is Ed Malone (David Hedison). Challenger makes himself clear, "Anything I have to say about the expedition will be said tonight at the Institute, and not before." Challenger bops Malone on the head with his umbrella, sending the man crashing to the tarmac below. A small dog, Frosty, climbs up on Malone. Jennifer Holmes (Jill St. John) comes to fetch her dog and explains how she will be attending the professor's lecture. She introduces herself. She is the daughter of Malone's boss. Of course, Ed Malone made the newspaper headlines, "Challenger Assaults London newsman."
At the Zoological Institute in London, a crowd gathers. Challenger enters smoking a cigar. He greets Prof. Summerlee (Richard Haydn) who shows his distain by fanning the cigar smoke away from his face. Jennifer enters with Lord John Roxton (Michael Rennie). They take their seats in the front row as the meeting is called to order. Ed Malone is already seated in the front row. Summerlee introduces Challenger and explains that Challenger has returned from the headwaters of the Amazon River with a report, "That he insists will turn the scientific world topsie-turvey." Challenger tells the audience that on a plateau in Brazil he has found plant and animal creatures from prehistoric times, "Live Dinosaurs." The audience murmurs disbelief, and one gentleman in the audience calls Challenger a liar. Unfortunately for Challenger, on his way down the river, the boat overturned and all his evidence and equipment was lost. Challenger proposes another expedition, not to leave later than a month from now and demands that Sommerlee and two others attend. Roxton volunteers and both Challenger and Summerlee agree. Jennifer volunteers, but Challenger flatly states, "There will be no women on my expedition." Ed Malone volunteers, and of course Challenger objects. Stuart Holmes (an uncredited John Graham), the executive VP of Global News, enters the hall and agrees to finance the expedition to the tune of $100,000, but only if Malone accompanies the professor on the expedition. Challenger agrees, noting, "Since I was never a man to resist logic, or cash, I shall ignore my personal feelings and approve Mr. Malone as a member of the expedition." Summerlee reluctantly agrees to both Roxton and Malone. Roxton and Jennifer depart. Malone writes several articles regarding the expedition. The expedition is covered in the newspapers of many countries and many languages.
Deep in the jungle they arrive by float plane at a trading post. There to greet them is Costa (Jay Novello) and, playing the guitar and singing, Manuel Gomez (Fernanco Lamas). Costa is in charge of local arrangements and supplies. Gomez recognizes Roxton. Gomez is the helicopter pilot. Challenger is shocked to see pink luggage and boots on the dock. Jennifer enters and claims them, and then calls to her brother, David (Ray Stricklyn) and announces that they will both accompany the expedition. Challenger tells Jennifer she and her brother can leave on the float plane, but it leaves. The only other way out is on the helicopter. It is a fait accompli, so brother and sister accompany the expedition. Roxton and Jennifer talk that evening. She is still pursuing the man. After she goes to bed, Malone approaches Roxton and has it out with the man. Roxton informs Malone that Jennifer is coming for two reasons: the first is that she is brave, and secondly she wants to marry a title, namely his. He adds that Malone is interested in the girl. It seems everyone has an agenda, including Gomez.
The next morning they fly to a high plateau. Challenger tells them he was at the base of the plateau and on the ledge above he saw a live dinosaur. They land in a clearing surrounded by large, prehistoric trees. Malone takes a picture of Jennifer, instead of Challenger. They set up camp. Summerlee chides Challenger that he hasn't seen any dinosaurs yet. That evening after supper they hear the roar of some great beast. Trees are being toppled as the creature heads their way. The party heads into the jungle. A strange plant grabs Jennifer around the neck. She is rescued as the lizard goes on a rampage. Costa, a highly nervous man, panics and runs into the jungle. Gomez takes off after him. He sees that the helicopter has been destroyed. The end of outside contact now, as the radio was destroyed. The party begins their trek inland. Roxton and Malone encounter a gigantic three-toed footprint. This seems to confirm Challengers dinosaur theory. Jennifer wanders off to catch her dog. Frosty has found a dinosaur and barks at it. Jennifer picks up her dog and then sees the gigantic creature for herself and screams. Challenger now feels vindicated and Summerlee apologizes for his disbelief. Malone takes pictures, and then they run from the roaring dinosaur. Malone and Challenger manage to fall down a hillside and spot a human female (Vitina Marcus) dressed in animal skins. Malone chases after her through a giant tunnel of spider web material. A huge, green tarantula drops down to menace the native girl. Malone shoots and kills the arachnid, then continues his search for the girl. The party reassembles at a new encampment. Malone returns with the girl in tow. She tries to escape, but is stopped. Challenger notes that she is a local Indian indicating that there must be a way up to the plateau from the jungle below. Roxton reproaches Malone for bringing the girl back. He explains that they are all in danger now, because her tribe will try to find her. Malone and Roxton get into a fight. Roxton finds a journal, the Burton White diary. It is three years old and Roxton is mentioned, as are diamonds. Gomez is looking for a name. Malone and Jennifer ask Roxton to explain. Roxton tells of the legend going back hundreds of years, "a terror guarding riches." Diamonds were Roxton's reason to finance the last expedition. Santiago was a member of the previous expedition, and Gomez is looking for him. Roxton admits he abandoned the last party. The journal ends with a notation that native tribes kidnapped the members of the expedition. They hear the roar of dinosaurs and bed down for the night. Costa sees the native girl sleeping and has lust in his eyes. She wakes and tries to escape, but David stops her. David tries to communicate with her. He shows her the rifle, and it is apparent she knows how to use it. Gomez sees something in the trees just outside camp. He calls to Malone and Roxton for help. A shot rings out and Summerlee is injured and the girl escapes. Malone finds Gomez injured; claims he was grabbed and knocked out. His rifle is missing. Malone, David and Roxton search for the girl. Malone loses the girl, but Jennifer joins him. They run from an approaching dinosaur. Malone fires his rifle at it, but it has little effect on the gigantic creature. A second dinosaur joins the first and they fight. In the melee, the tail of one knocks Malone and Jennifer over the edge, onto a ledge below. The fight continues until both creatures topple over the edge to their deaths below. When Jennifer and Malone return to camp it is empty and a shambles. The dog wanders back into camp, followed shortly thereafter by her brother, David. The tribe is above and captures the three remaining members of the expedition.
They are led into a large chamber in the volcano. A ceremony is under way. The three new prisoners are led to a chamber to join their comrades. Costa is getting nervous and Challenger's observation that the natives are cannibals makes it worse. The native girl, who has developed an affection for David, enters the chamber from above. She motions for them to follow her. They follow the girl deeper into the volcanic cave system. The drums stop and the chief leads his guards to get the prisoners. He is shocked and angry to find them missing. The girl leads the explorers deeper into the tunnels and leads them to a solitary figure. He is an old white man, with long white hair and beard. The old blind man is Burton White (Ian Wolfe) and he warns that the volcano will explode soon. He tells them he is the only survivor and there is a way out via "the cave of fire." They hear the chief and his hunting party approaching. Burton refuses to join them on their escape. He claims he is too old and tired, but assures Roxton he will be safe, as it is a taboo to kill a blind man.
The girl leads them on their escape. Gomez wants to kill Roxton now, but Costa convinces him later would be better, after they find the diamonds. Their first obstacle is a passage with plant arms that reach out to kill. The torch held by the native girl causes the plant arms to retract. Skeletons flank the passage showing what happens if the plants catch their prey. Next is a mist covered passage between the rib bones of a huge creature. This is the "graveyard of the damned." They run along the spinal vertebrae like stepping stones. It ends at a large pool of lava. They walk along a narrow ledge over the pool. The native girl refuses to go any further. Roxton takes the lead. They hug the wall face as the ledge becomes narrower. Challenger steps on a loose rock and falls over the ledge. He is assisted back up by David and Roxton. A passageway entrance is at the end of the ledge. Gomez hears the natives approaching and lights the brush on fire to slow their approach. The escapees push a boulder across the entrance to buy some time. They see the escape route exit and walk towards it. Costa finds his diamonds and Challenger and Summerlee find a dinosaur egg. They are informed by Gomez, "This is the end of the journey." He tells Roxton to drop his rifle. Costa greedily fills his hat with the uncut diamonds. Gomez explains, "I want your life, Roxton. In exchange for that of a man you sent to his death when you deserted Burton White...I mean Santiago, my brother." Malone throws the hat full of diamonds at Gomez. The commotion wakes a sea creature. It picks up Costa in its mouth and gnaws on the poor man's body. Challenger exclaims, "Eaten alive. Horrible! Horrible!" Malone climbs up to where a dam of lava has been built so that it can be broken. He pulls on the lever, but he has insufficient weight to pull it down. Gomez climbs up to help, then stays to finish the job. The dam breaks and Gomez and the lava tumble into the lake below. The water creature is killed. The remaining members of the party escape and follow the route through the volcano. Volcanic activity increases and the mountain is racked with earthquakes. They exit the cave at the jungle below the plateau. The volcano erupts and Challenger remarks, "My lost world, lost forever." Summerlee is disappointed because they have no proof. But Challenger corrects him, "Oh yes we will." And pulls out the dinosaur egg intact. "I thought this might help." Roxton pulls a handful of diamonds out of his pocket and gives Jennifer a couple. She kisses both Malone and Roxton. A ground tremor causes Summerlee to drop the egg. It breaks open and Challenger proudly announces, "A baby dinosaur. Tyrannosaurus Rex...It will live long enough to grow as big as a house and terrify all London." We close with the survivors surrounding and looking at the baby dinosaur.