The story opens with an introduction to Professor Lidenbrock, a professor of mineralogy at a prestigious institution in Germany. The novel’s narrator is the professor’s nephew, Axel, who lives with him. The professor is an “eccentric man” who has an unfortunate problem of getting tangled up in his speech while pronouncing challenging scientific words, then cursing angrily as a result, something the townsfolk ridicule him for. Regardless, Axel assures the reader that his uncle is an “authentic scholar.” The professor has arrived home early and in an apparent rush for an unknown reason, unsettling his cook, Martha, who scurries away. He goes to his study and calls for his nephew, who meets him there.
Axel describes his uncle’s study as a “real museum.” He recalls how, as a child, he chose to admire the contents of the study rather than play with boys his own age. The professor shows Axel a very rare book that he had found in a shop that day. A “fanatical book collector,” he is extremely excited with his find, reporting to Axel that it is a copy of the original work of a famous twelfth-century Icelandic author. As he begins to decipher it, a sheet of old paper slips from the volume to the floor. The professor picks it up and begins poring over it, noting that it is written in the same runes as the book. Martha enters to announce dinner. The professor orders her away, but Axel follows her out. As Axel is finishing his meal, the professor calls for him once more, and Axel hurries back to the study.
As soon as Axel arrives, he is put to work helping his uncle decipher the code. The professor translates a series of letters to Axel, who writes them on a paper. They make no sense. The professor continues with the next step of his feverish task, and Axel slips into a daydream while gazing upon a portrait of Grauben, a young woman who is a “ward” of Lidenbrock but who is currently away. He confesses to the reader that he and Grauben love each other dearly and are secretly engaged, something his uncle, Axel feels, would simply not understand. Professor Lidenbrock soon appears to have solved the code and revealed the message of the paper. Instead of being excited, however, the professor seems angry and strikes the table. “It makes no sense!” he shouts. Lidenbrock jumps up abruptly and storms out the front door of the house.
As the door slams behind the exiting professor, the house shakes—alerting Martha, who is incredulous to learn that the professor has left without touching his dinner. Exasperated, she disappears into the kitchen again, leaving Axel alone. He ponders his situation as he waits for the professor to return, finally picking up the parchment and examining the coded letters himself. In a seeming irony, by glancing at the paper somewhat absentmindedly, Axel is able to make sense of the letters, noticing that the words they create are in several different languages, not just one.
In a flash, the truth of the message appears to Axel and shocks him greatly. It speaks of a journey that seems to be a secret, one he is certain his uncle would want to undertake if he knew of it. Axel seems so shaken by the possibility that he decides the best course of action is to destroy the paper in the fireplace. As he hesitantly approaches the fire with the book and papers in hand, the study door thrusts open, and his uncle...
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Axel hurries to place the document back on the desk before his uncle notices what he was planning. Lidenbrock is still so absorbed in his own thoughts, however, that he does not notice. Instead, he sits in his chair and resumes poring over the document, making frenetic notes and mutterings, still trying to solve the puzzle. Axel feels torn, as he both sympathizes with his uncle and does not want him to see the true message.
This continues through the night and into the next morning. Axel and Martha realize that the professor has locked them in the house, and Martha cannot go to the market to supply them for meals for that day. Fearing that his uncle will leave again and that he and Martha will still be locked in, Axel finally relents and confesses to his uncle that he has found the “key” to the code. His uncle is suspicious but soon realizes that Axel is correct. He is ecstatic and immediately instructs Axel to pack their belongings for the journey they are about to undertake.
Though Axel suspected his uncle’s reaction, it nevertheless sends a shiver through Axel’s whole body. Axel finally reveals to the reader that the journey mentioned in the parchment is a journey through the earth’s crust, down to its center, something he refers to as “madness.” However, he can see that, as he anticipated, his uncle is insistent. Axel decides to try to talk his uncle out of it, but he realizes this can only be done with logic. His uncle respects him, complimenting him as “highly gifted” and inviting him to raise all his concerns as though the two are colleagues, not uncle and nephew.
Axel slowly picks apart the theory and the message itself, asking critical questions about whether the science is valid or whether the paper is simply a joke. The professor, however, replies to all of Axel’s questions and is able to demonstrate that the paper is likely authentic. Lidenbrock believes that they should take the journey and keep it a total secret until after they have returned home.
Axel decides that after all the drama and excitement, he must take a walk to clear his head. He mulls over the events and begins to experience some doubts once again as to whether this is a great opportunity or just madness. As he makes his way through the city, he realizes that he has been subconsciously walking toward the home where Grauben is living with her relatives. He sees her and tells her everything, but to his surprise, she insists that it is a worthy undertaking and one that will make him a great man. Grauben walks with Axel back to the house, where they find Lidenbrock already organizing their supplies and making plans. Axel seems quietly reluctant and fearful, but the others around him—including Grauben, who dutifully packs Axel’s trunk for him—prepare without delay.
The next morning, a carriage arrives to pick them up and begin the journey in earnest. Grauben sees them off, tearing up as she kisses Axel and telling him that when he returns, she shall be his wife. Lidenbrock and Axel wave good-bye to Martha and Grauben as the carriage drives off.