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. In addition to this, Sophie and Alberto receive postcards addressed to a girl named Hilde from a man named Albert Knag. As time passes Knag begins to hide birthday messages to Hilde in ever more impossible ways, including hiding one inside an unpeeled banana and making Alberto's dog, Hermes, speak.