Sophie Amundsen (Sofie Amundsen in the Norwegian version) is a 14-year-old girl who lives in Norway in the year 1990.
The book begins with Sophie receiving two messages in her mailbox and a postcard addressed to Hilde Møller Knag. Afterwards, she receives a packet of papers, part of a course in philosophy.
Sophie, without the knowledge of her mother, becomes the student of an old philosopher, Alberto Knox. Alberto teaches her about the history of philosophy. She gets a substantive and understandable review from the Pre-Socratics to Jean-Paul Sartre. Along with the philosophy lessons, Sophie and Alberto try to outwit the mysterious Albert Knag, who appears to have God-like powers, which Alberto finds quite troubling.
Alberto takes Sophie from the Hellenistic civilization to the rise of Christianity and its interaction with Ancient Greek thought on to the Middle Ages. Over the course of the book, he covers the Renaissance, Baroque, Enlightenment and Romantic periods, with the philosophies that stemmed from them.
Sophie and Alberto's entire world is revealed to be a literary construction by Albert Knag as a present for his daughter, Hilde, on her 15th birthday.
As Albert Knag continues to meddle with Sophie's life, Alberto helps her fight back by teaching her everything he knows about philosophy. Alberto manages to find a plan so that he and Sophie would finally escape Albert's imagination. The "trick" is performed on Sophie's birthday, after Alberto informs Sophie's mother about everything.