Plot summary

Scotland, 1880: Oliver Lindenbrook (James Mason), a distinguished professor at the University of Edinburgh, has just been granted knighthood for service to the Crown in the field of geology. When he arrives at his lecture hall on the day of the announcement, he is surprised by a gathering of staff and students. They present him with an elaborate decorative inkwell and the Dean declares a holiday in celebration of the occasion. As the others leave, one student, Alec McKuen (Pat Boone), remains behind. He offers Lindenbrook an extra token, a rock he acquired in a curiosity shop in Glasgow: a lump of lava, strangely heavy, although lava is usually very light. Pleased with the offering, Lindenbrook invites him to dinner at his home that evening. As Alec leaves, Lindenbrook turns his attention back to the unusual rock.Alec arrives at Lindenbrook's house that evening to find that the professor never came home, and his niece, Jenny (Diane Baker), is both angry and worried. Alec and Jenny go to his laboratory at the University, and find him and his assistant, Paisley (Ben Wright), feverishly experimenting on that same piece of lava. Lindenbrook shows Alec that he has chipped away a fragment of the lava and uncovered a man-made object inside, marked with three grooves or notches. He is about to melt off the rest of the lava to discover what it might be. Tired and distracted, Paisley pours in too much fuel at that moment, and the stove explodes. In the aftermath, Lindenbrook finds that the lava has been blasted free of the object, and it is revealed to be a surveyor's plumb bob with a few Nordic words painfully scratched on it. Lindenbrook and Alec recognize the signature with wonder: Arne Saknussem, a brilliant but eccentric Icelandic scientist, who had disappeared many years earlier while exploring the volcanoes of his homeland.Two weeks later, the Dean and the Rector of the University (Alan Napier and Alan Caillou) call at Lindenbrook's home to ask why he's neglecting his teaching duties. He tells them the story of the plumb bob, and shows it to them along with the translation he had made of its message: "I am dying but my life's work must not be lost. Whoever descends into the crater of Sneffels Yokul can reach the center of the earth. At sunrise on the last day of May, the Mountain Scartaris will point the path." Lindenbrook explains that Sneffels Yokul is an extinct volcano in Iceland, and Scartaris a nearby mountain peak. He has written a paper, and sent it to the eminent Professor Goteborg of Sweden for review, and plans to mount an expedition to Iceland as soon as Goteborg confirms its feasibility.But Goteborg's reply is overdue, although Lindenbrook has written twice. Instead, a letter arrives from the University of Sweden saying that Goteborg has vanished without a word of explanation. Lindenbrook immediately suspects that Goteborg intends to mount an expedition of his own, and claim credit for the discovery. He decides to leave for Iceland at once, and Alec decides to accompany him.In Iceland, a visit to Sneffels Yokul reveals that Goteborg has indeed been surveying there. Lindenbrook sends Alec back to town to acquire the equipment and supplies they will need while he makes his own observations of the crater. Goteborg (Ivan Triesault) has been watching their arrival, and, desperate to delay them, arranges for them to be abducted and imprisoned in a remote eider down storage shed. Fortunately, they are quickly discovered there by a tall, young Icelandic man, Hans Belker (Peter Ronson), and his pet duck, Gertrude. He releases them and escorts them back to Reykjavik.It turns out that they are staying at the same hotel as Goteborg. They ask for him at the desk, but the hotel proprietress (Edith Evanson) is uneasy and evasive, saying at first that he is not in, then saying he cannot be disturbed. Lindenbrook hands his business card to her, asking her to put it in Goteborg's mailbox, and they watch as she does so, learning his room number. They knock and shout at Goteborg's door, but he doesn't answer. Hans picks the lock, and they go inside to wait for his return. As they look enviously over the supplies and equipment he has stockpiled there, they also find his lifeless body, neatly laid out, covered by a blanket. And they discover evidence that his death was not a natural one...a few grains of potassium cyanide linger on his beard.Meanwhile, Goteborg's wife, Karla (Arlene Dahl), has arrived from Sweden. When she asks for her husband at the desk, the proprietress shows her into her office, and breaks the news to her of her husband's death. Lindenbrook, coming downstairs to talk to the proprietress, overhears the conversation, and steps in to inform them both that Professor Goteborg was murdered. The proprietress discloses that the last person to see Goteborg alive was Count Saknussem (Thayer David), a descendant of Arne Saknussem, and a well-known scientist in his own right. She goes off to inform the police.Lindenbrook asks Madame Goteborg to sell him the professor's equipment. She is too distraught to discuss the matter, and tries to put him off. A desperate Lindenbrook then demands that she sell him the equipment, blurting out that he has a right to it as her husband had been trying to cheat him of credit for his discovery. Outraged, she flatly refuses. But later, reading her husband's diary, she learns that Lindenbrook told the truth. She agrees to sell him the equipment on condition that he allows her to join the expedition. He is reluctant, but must consent: Goteborg had bought up all the equipment in town, no other is available. And since Hans, who speaks no English, is also going, and Madame Goteborg speaks Icelandic, she will be useful in helping them all to communicate.At the last moment, Hans decides on his own to bring along his duck, Gertrude.Dawn finds them on the rim of the volcanic crater, waiting for Saknussem's sign. A single ray of light from the rising sun peeks through the crags of Scartaris, marking their gateway. The Lindenbrook Expedition sets off...unaware that Count Saknussem (Thayer David) and his servant have been watching--and will follow.They soon discover that Arne Saknussem marked his pathway at regular intervals with his three-notches signature, and they need only look for these to keep on the right path. One night, the Count and his servant creep past them as they sleep. At the next set of notches, the Count chisels new marks in the rock that point a different direction, while his servant conceals the original marks. The Lindenbrook party is led astray by the false marks and Alec is nearly killed in a fall. Backtracking, they discover where and why they went wrong. Now they know they are in a race with a dangerous enemy.They are separated at one of their campsites, a spectacular crystalline rock formation and mineral spring. At one point, off by himself, Alec drops his lantern through a hole in the rock. He climbs in to retrieve it, and when he starts back he misses the correct corridor. Meanwhile, Lindenbrook chisels off a sample of the unique rock formation, and as he does so, the whole wall collapses, and the chamber floods with water. Lindenbrook, Madam Goteborg, and Hans must scramble to escape. They don't notice Alec is not with them until they are out of the flooded chamber.Alec and the rest of the party wander separately for days, looking for each other. Alec has found himself inside several chambers where salt covers the floor. Lindenbrook and his party find a natural rock bridge that had been broken recently by Alec himself when he crossed it. Alec eventually stumbles upon Count Saknussem, whose servant has died of "too much heat, too much burden and too much fear," leaving the Count stranded. The Count tries to force Alec to pick up the servant's burden. Alec refuses, and begins to walk away. Count Saknussem shoots him in the arm with his revolver. The noise brings Lindenbrook, Hans and Madam Goteborg on the run, but the happy reunion is short-lived, with the Count holding a gun on them all. He tells the others to go back, leaving Hans behind to serve him. Lindenbrook pretends to agree, then tricks and disarms him. They debate on what to do with him: They all agree he is guilty of murder and mayhem, but they are all too civilized to do away with him or leave him behind to die. They reluctantly allow him to join their group.They continue on their journey for many weeks, growing ragged and weary, running short of supplies. One by one, their portable crank lighting units begin to fail due to corrosion from the salt deposits. Wondering how Arne Saknussem could have found his way in the dark, the Count shows them all luminescent algae that have existed for millennia in the caverns.One day Alec finds a forest of mushrooms. Some are as large as trees, but there are also smaller, edible mushrooms, and all are relieved and grateful for the dietary change. While the others rest, Count Saknussem goes exploring beyond the forest. He comes back long enough to order Hans to fell some of the giant mushrooms and build a raft, then leaves again. Lindenbrook follows him, and finds that Saknussem has discovered a vast underground ocean. To continue their journey, they must cross this sea -- hence, the raft. When they try to launch it, however, they are attacked by giant fin-backed lizards. Defending Madam Goteborg, Hans manages to kill one of them with a spear. The others immediately turn on it to devour it, and while they are feeding, the party escapes onto the subterranean sea.After days on the water, they suddenly encounter a magnetic force so powerful it rips away everything made of metal...even Madam Goteborg's wedding ring and Hans's gold tooth. They have reached the center of the Earth, equidistant from both magnetic poles. But they're caught up in a maelstrom, and it wrecks their frail craft. They make it to another beach exhausted, no food or equipment, with only the rags on their backs, and collapse, exhausted. They decide to sleep before they try to plan their next move. Gertrude wriggles free of Hans's arms and strolls down the beach. The Count, also wakeful, sees her and begins to follow. When Hans wakes, he follows Gertrude's tracks, calling for her. He finds a lone white feather at the mouth of a cave...he finds more inside, stained with blood. Furious, he begins to climb toward the Count, who is using a rock to scratch something on a rock wall. He tries to strangle the Count, but Lindenbrook and Alec drag him away. Angry and frightened, the Count rages at them all, accusing them of being ungrateful fools. As he rants, he steps backward and stumbles into the wall on which he was pounding earlier. It collapses on top of him, killing him.The others move up to the place where he fell, and see in the chamber beyond the ruins of an ancient city, which Lindenbrook speculates may have been part of the Lost City of Atlantis. Exploring, they find the skeleton of Arne Saknussem...with a broken leg to explain why he never returned. His only remaining possession is a knapsack full of gunpowder. Madam Goteborg notes that his hand seems to be pointing, and they turn to see the opening of a volcanic shaft, with a tremendous updraft, indicating it could be a direct path to the earth's surface. But it is blocked by a massive boulder. Lindenbrook suggests they use the gunpowder in Arne Saknussem's knapsack to dislodge it. They forage through the city, and find flint and iron with which to make fire, and a few crude tools. They set their charge and twist together a fuse. Lindenbrook lights it, and they take shelter in a bowl-shaped altar stone. At first, the high wind from the shaft snuffs the fuse. When Lindebrook returns from relighting it, he is pinned down by the long tone of a giant lizard. Alec frees him, just as the fuse reaches the power charge. The force of the blast does dislodge the boulder, but also triggers a volcanic eruption. The altar stone (fortunately made from an ancient form of asbestos) is borne up by the hot lava into the shaft, and propelled at a terrific speed to the surface. They are ejected from a dead volcano in the Lipari Islands, now activated by the blast, into the Mediterranean Sea, where all but Alec are rescued by fishermen. Alec, it develops, was thrown early from the altar stone, and is rescued (trouser-less!) from a tree by a group of nuns.Back in Edinburgh, the expedition is greeted enthusiastically by a crowd of students at the University. Lindenbrook thanks them, but says that all his notes and samples were lost and he would never embarrass the University by claiming credit for an accomplishment he can't prove.Alec is there, in a wheelchair with a cast on his leg, and Jenny by his side. It seems that he was in such a hurry to leave the church after their wedding that he fell and broke his leg. Nevertheless, they are clearly happy. Hans is also there, but leaving shortly for Iceland. Lindenbrook offers his heartfelt thanks, and Hans replies in a few laboriously-learned English words that if the professor ever decides to go down there again, Hans will go, too. They shake hands and say goodbye. Madam Goteborg tells Lindenbrook she is leaving, as well, to return to Sweden. Lindenbrook protests, saying she must stay and help him with his memoirs. As they argue about the terms on which she will stay, they both finally realize that they care for each other, and want to be together. They kiss, and the crowd cheers.