Fifteen-year-old Sofie Amundsen from Oslo finds mysterious letters in her mailbox one day. They contain questions like "Who are you?", "What is a human being?" and "Where does the world come from?". At first, Sofie is irritated, but the more she reads, the more fascinated she becomes by the adventurous and mysterious content.
The unknown letter writer takes Sofie into the thought world of the great philosophers and tells her the story of Europe, antiquity, the Middle Ages, and the Renaissance. In doing so, she meets many important figures, such as Socrates, Plato, and Aristotle.
The letters become more detailed and take Sofie into the world of philosophy. She learns a lot about human thinking and the different philosophical currents. However, what particularly fascinates her is the question of the meaning of life and how we as humans can find our place in the world.
However, the story suddenly takes an incredible turn: Sofie finds out that she herself is part of a crucial philosophical debate. The letters are actually written by a philosopher who is trying to solve the riddle of life.
Sofie is thrilled by the new insights and suddenly feels much more mature and self-confident. She begins to question her own identity and think about her life and future.
In the end, many questions remain unanswered, but Sofie has learned a lot and is grateful for the exciting experiences she has had through the mysterious letters. She now knows that life is a puzzle to be solved - and she is curious what else awaits her.