Right before her 15th birthday, Sofie receives a card asking: "Who are you?" In the commotion, she forgets to write an essay assignment. Sofie receives more cards with questions from Alberto Knox. In school, the teacher found Sophie's philosophical essay magnificent.
Who wrote the essay and who is Alberto Knox?
Sofie gets a VCR tape in which Alberto Knox takes her on a trip to ancient Greece and introduces Socrates and Plato. Alberto proposes to teach Sofie more about philosophy, and Sofie agrees. She follows the messenger to a cabin in the woods.
In the cabin Sofie sees a girl (Hilde) in the mirror that looks just like her, but then the girl disappears.
Sofie goes to the movies with her friends, however she sees a different movie, in which she is introduced to the cynic Diogenes.
Sofie takes her best friend Jorunn to the house of Alberto Knox, and they discover many postcards addressed to Hilde.
Later Alberto calls and asks Sophie to pass by. He says he has been commissioned by the Major to teach philosophy to Sofie. The journey continues as they explore the Middle Ages, with Hildegard von Bingen, Thomas Aquinas.
Back home, Sofie finds a cross with initials H.M.K ...
After an explosion in the chamber of the philosopher Sophie and Alberto enter the Renaissance, the rebirth of Ancient Greek ideas. They meet Shakespeare, Gutenberg, Copernicus, Leonardo da Vinci and Michelangelo.
Sofie and Alberto get to the "soul", as interpreted by Descartes (Cogito ergo sum) and Berkeley (all is the power of God). They get in direct contact with the Major and are told their time will end at the last page.
Sophie decides to deceive the Major.
Sofie test her plan to deceive the Major and it seems to work. She walks around with a red shaded glasses; there is a difference between what you perceive and how it really is.
God is dead, according to Nietzsche.
Alberto and Sofie attain Romanticism, a period during which the focus was arts and nature.
But also the period of the Russian Revolution, Hegel's idea of thesis, antithesis and synthesis forms the basis of Marxism.
Finally, Alberto and Sofie end up with Freud and psychoanalysis.
Sofie executes her plan to escape. Georg starts a distraction on her birthday party, Alberto and Sophie escape and pass through the mirror. They end up in Hilde's world but can not make any contact. Sophie finally realizes what she is ...
an immortal idea.