As the novel begins, an almost-15-year-old Norwegian girl named Sophie Amundsen receives a strange series of letters. The letters have been sent to her, but they’re addressed to someone named Hilde Møller Knag. The letters pose difficult philosophical questions, such as “How was the world created?” and “Is there life after death?” Sophie isn’t sure what to make of these questions, but she doesn’t tell anyone that she’s receiving someone else’s mail—not even her best friend, Joanna Ingebritsen, or her Mom.
Sophie continues to receive letters marked for Hilde. The letters offer her (Sophie) a series of lessons in the history of Western philosophy. The first few lessons consist of long letters from a man named Alberto Knox. These letters cover the origins of philosophy from superstition and religion, the rise of natural philosophy in ancient Greece, and the intellectual achievements of Plato and Aristotle. In these letters, Alberto spells out the philosophical questions that philosophers continue to ask themselves today: what is real? how should humans live? what is the world made of? etc. Sophie is particularly impressed with Alberto’s lessons on Plato and Socrates. Socrates was a wise man, Sophie learns—but ironically, this meant that he claimed to know nothing at all about the universe, and insisted that all human beings had an innate capacity to understand science, logic, and morality. As Sophie proceeds with her letters, her mother begins to assume that she’s receiving love letters from some boy at school.
Sophie tries various tricks to track down Alberto Knox, but nothing works. She learns that Alberto delivers the letters with the help of a dog named Hermes. One day, Sophie is able to track Hermes to an abandoned cabin. In the cabin, Sophie finds two paintings, one entitled, “Berkeley,” the other entitled, “Bjerkley.” Sophie also finds a brass mirror in the cabin, in which she thinks she can see another girl. Sophie takes the mirror back to her house. She also begins to find strange items that don’t belong to her, such as a wallet, a gold crucifix, and a scarf.
Sophie continues to receive letters from Alberto. She learns about the teachings of Aristotle, who emphasized the importance of research and careful study of the physical world, the Hellenic philosophers, who built off of Plato and Aristotle without drastically rethinking their ideas, and the dawn of monotheism in the Middle East. As the letters continue, we learn that Sophie’s Dad is a busy man who travels frequently for his work. Sophie also notices news reports about a UN diplomat stationed in Lebanon—she notices that some of her letters have been stamped from Lebanon.
Sophie first meets Alberto Knox in an abandoned church, late at night. There, Knox gives Sophie a thorough history of the Middle Ages. He discusses the complex interplay between the Judeo-Christian and the Greco-Roman traditions, which were merged in the writings of such seminal figures as Thomas Aquinas and Saint Augustine. Sophie learns that Hildegard of Bingen was one of the key thinkers of the Middle Ages. At home, she looks into her mirror and thinks she sees the face of Hilde Møller Knag.
As Sophie proceeds with her lessons with Alberto, who is now permanently stationed in Sophie’s town, a series of wild coincidences begins. Sophie finds money lying on the ground at the exact instant she realizes she’s lost her bus fare, and she begins to see signs announcing that Hilde’s father, whose name is Albert Knag, will return to Scandinavia from Lebanon very soon. Sophie’s Mom asks her if she’d like to have a party in honor of her upcoming birthday, but Sophie says that she’s uninterested. Sophie continues to receive letters from Albert Knag, and realizes that Albert’s daughter Hilde is exactly the same age as her—they were born on the same day.
Alberto teaches Sophie about the history of the Renaissance in the 15th and 16th centuries. During this period, Europe embraced the doctrine of humanism, celebrating man’s concrete, worldly achievements. It was during this time that Martin Luther launched the Protestant reformation, after which the modern sects of Christianity were established. During this lesson, Alberto refers to Sophie as “Hilde,” but then corrects himself.
Sophie changes her mind and tells her mother she wants a philosophically-themed birthday party. Sophie continues receiving letters from Albert Knag, in which he tells Hilde that he’ll be back from Lebanon very soon. Meanwhile, Sophie covers the history of the Baroque era with Alberto—the rise of empiricism in the U.K. and rationalism in France. This era led directly to the Age of Enlightenment—the period when European intellectuals came to believe in the importance of natural law, inalienable rights, and rigorous self-study. Major philosophers of this time, such as David Hume and John Locke, questioned Christian dogma and tried to replace it with a secular system of thought. It was also around this time that figures like Baruch Spinoza questioned man’s free will, arguing that the world is predetermined, so that liberty is just an illusion caused by our ignorance of causes and effects. As Sophie learns about Spinoza and his peers, she and Alberto find more signs that a powerful god-figure is controlling their world: banners fly through the sky, saying, “HAPPY BIRTHDAY, HILDE!”
We then cut to the perspective of Hilde Møller Knag, a teenaged girl who lives in a home called Bjerkley and, we slowly realize, is reading the story of Sophie and Alberto in a binder that her father, Albert Knag, has sent her from Lebanon. The banners and letters that Sophie reads are, in fact, meant for Hilde to read—just as the mysterious items that Sophie finds in her room are real items that Hilde has misplaced. Hilde reads about Sophie’s lessons in philosophy, and quickly comes to regard Sophie as an almost-real person. She even begins to resent her father for manipulating Sophie and Alberto so callously.
Hilde reads as Sophie learns about Immanuel Kant and other important Enlightenment philosophers. As Sophie and Alberto talk about philosophy together, their lives become stranger and stranger, and fictional characters such as Little Red Riding Hood and Alice (from Alice in Wonderland) intrude on their interactions. Gradually, Sophie and Alberto become aware that Albert Knag is controlling them. They try to think of a way to escape his manipulations, but realize that there’s no way to do so as long as they’re in a book he’s written.
Meanwhile, Alberto proceeds with his lessons for Sophie. He goes over Romanticism, the period of European culture that followed the Enlightenment and defined much of the 19th century. Sophie learns about George Hegel, Søren Kierkegaard, and other important figures of the era. She also learns about “scientific” philosophers like Karl Marx, Sigmund Freud, and Charles Darwin, whose influence extends far past philosophy into real-world science, medicine, politics, and history. Sophie ends her lessons with Alberto by learning about the 20th century Existentialists, like Jean-Paul Sartre and Simone de Beauvoir.
Sophie prepares for her birthday party, and in another world, Hilde prepares for the return of her father. Hilde calls her friends in Copenhagen, the city through which Albert Knag will be flying, and arranges for them to pull an elaborate prank on Albert. At Sophie’s birthday party, the world begins to plunge into chaos—a fight begins to break out, and at the last minute, Sophie and Alberto “vanish into thin air,” escaping their book altogether.
Mysteriously, the story of Sophie and Alberto continues—we’re not told how, or who’s writing it. Sophie and Alberto drive to Oslo, realizing that the physical world is now “frozen” to them. Meanwhile, Albert Knag arrives at the airport in Copenhagen, where he’s baffled to find a series of letters welcoming him home (much like the letters that Sophie found all over her home, wishing Hilde a happy birthday).
Alberto and Sophie drive to Hilde’s hometown, where they witness Hilde reuniting with Albert. At this point, it becomes unclear which parts of the story are which. Hilde greets Albert, and Albert admits that Hilde has spooked him with her elaborate pranks. Hilde explains that she wanted to make Albert feel like one of his own characters—Albert admits that she has done exactly this. Together, Albert and Hilde look up at the stars, discussing philosophy and science. Albert is impressed with the knowledge and wisdom Hilde has absorbed from the book he sent her. Meanwhile, Sophie and Alberto watch Hilde and Albert. Alberto tells Sophie that since the world is frozen, there’s no way Sophie can communicate with Hilde. Nevertheless, Sophie hits Hilde with a heavy branch. Hilde feels a strange “sting” on her face. Albert jokes that Hilde’s been stung by Socrates, but Hilde insists that it was Sophie who stung her.