After walking home from school with her friend Joanna one day in early May, Sophie Amundsen finds a small letter in her mailbox. It is addressed to her, without a stamp, and it contains only a question—"Who are you?" The letter makes Sophie think. She wonders whether her name matters much, whether her physical appearance makes her who she is. Then Sophie thinks about the fact that contemplating life leads inexorably to thinking of death, and vice versa. She returns to the mailbox and finds another letter, with the question "Where does the world come from?" written inside. Sophie realizes it is a legitimate question and goes to the den, her outdoor hiding place, to ponder. She thinks about the fact that the world is part of the universe, and that that must have come from somewhere. But that means that something must have come from nothing, which she cannot accept. Equally poor is the possibility that the universe has always existed. Even if God created the universe, he himself must have come from somewhere. Then, when Sophie gets the mail, she receives a mysterious postcard. It is from Lebanon, postmarked "UN Battalion", has a Norwegian stamp, and is addressed to Hilde Møller Knag, c/o Sophie Amundsen. The postcard is from Hilde's father, wishing her a happy 15th birthday and telling her he had to send the card through Sophie. Sophie, totally confused, goes through the phone book but does not find Hilde Møller Knag.
Sophie tells no one about the strange letters, and is uninterested in playing with her friend Joanna the next day. After school she rushes home and finds a letter written to her. It contains three pages describing philosophy. The letter suggests that what is most important in life is philosophizing—attempting to understand ourselves and our role in the world. There are not many philosophical questions, but there are many ways to answer each one. Life itself is like a magic trick, and philosophers must always observe it with wonder. After reading the letter, Sophie goes back to the mailbox and finds another one, which stresses the fact that all that is required to be a philosopher is the capacity for wonder. Babies have this capacity, but most people become inured to life and no longer find it wonderful. Philosophers are different from others, and the philosopher writing the letters wants Sophie to never lose her sense of wonder. The letters will comprise a philosophy course for her to take. Sophie tries to have a philosophical discussion that night with her mother, but it only leads to her mother wondering if Sophie has begun taking drugs.
A day later, after school, Sophie finds a letter from her dad, working far away, and then another on philosophy. This letter describes the situation leading up to the beginning of western philosophy. Before the Greek philosophers, people explained life through myths—stories about the gods. But the early Greek philosophers questioned the myths and began looking for other explanations for why the world is the way it is. Sophie thinks about this and realizes that making up stories to explain the workings of nature is not so far-fetched, for she would do the same if she did not already have other explanations.
The first questions that Sophie receives make her think about who she is and where the world came from. These questions are easy to ask and almost impossible to answer, but what is most amazing of all is that people stop asking them. Sophie realizes that she has never really thought about these things before, and when she does she understands that nothing could be more important. It seems that knowing who we really are is necessary for our lives to have meaning and import. Beyond that, we live in the world, and are in constant interaction with nature, yet most people take that interaction for granted and do not stop to consider how the world itself came about. As little children we are tremendously inquisitive, and we wonder about everything, but as life goes on we begin to take certain things for granted even though we do not understand them. Sophie is warned not to let this happen to herself, and when she talks with her mother she realizes that most adults not only do not ask themselves these questions, they think doing so is absurd. But Sophie is taking the course seriously, and she ponders everything that she reads. Although she is not sure of exactly who she is or where the world came from, Sophie is aware of the difficulties inherent in attempting to answer such questions and also the importance of asking them. What is most important in life is asking these philosophical questions and most people do not ask them. In fact, a philosopher has more in common with a child than with most adults. Gaarder seems to think that most people live their lives without actually partaking in the most important part of living. It is thinking that is critical, and not just thinking about practical, everyday affairs. We need to think about life itself, to ask why about everything that we normally take for granted.
Sophie learns that before people started turning to other types of explanations, they made up myths to explain what they could not understand. After reading about this, she thinks that she probably would have done the same thing—when things seem to happen of their own accord it is easy for us to believe that there is some higher power behind their actions. But what is important is to attempt to explain things using our reason rather than making up stories. With our reason we may be able to actually gain an understanding of the world, whereas the myths simply transfer the uncertainty elsewhere. Sophie realizes that the suggestion that God created the world does not really answer anything. Although for some it might solve the issue of where the world came from, Sophie understands that one could simply ask where God came from, and we would be back to the same problem. The philosophical questions are not to be escaped through easy answers but rather to be struggled through, and the implication is that a good life is one that constantly involves battling these issues.