Sophie is a girl approaching her 15th birthday. She encounters first some odd philosophical questions and then the first chapter of a written philosophy course in her mailbox. Sophie starts to learn about the historical beginnings of philosophy. At the same time, Sophie is curious who is giving her this course. Sophie also begins to receive postcards addressed to Hilde, a girl also approaching her 15th birthday, from Hilde's father Albert Knag, a Norwegian Major in the UN Force in Lebanon.
Sophie gets a thorough education in Greek and Hellenistic philosophy, but things begin to become strange. The videotape that Sophie gets not only has the philosopher Alberto Knox in Athens at the Acropolis, but someone on the Acropolis is holding up a banner that reads "Hilde." There is actually an appearance by someone claiming to be Plato himself. At this time, Sophie is introduced to her philosophy teacher, Alberto Knox.