Sophie is a fourteen-year-old girl living in a suburban neighborhood in Norway. She has a secret den inside her garden hedges where she stores her personal belongings. Her routine life changes when she receives an anonymous letter containing a single question, which prompts her to study the history of Western philosophy through a strange correspondence course. This new education shifts her attention away from schoolwork and childhood games as she learns to observe the world critically.

Alberto is an older philosophy teacher who lives in an apartment filled with historical artifacts. Initially communicating only through typed pages left by his dog, he eventually meets Sophie to deliver lectures in period-appropriate costumes. He firmly believes that maintaining a sense of wonder is essential to living a meaningful life and takes it upon himself to protect Sophie from becoming apathetic.

Albert Knag is a United Nations major stationed in Lebanon. He regularly sends postcards to his daughter, but these messages inexplicably end up in Sophie's mailbox or scattered around her town. His correspondences show a deep knowledge of philosophy and an uncanny awareness of Sophie's daily movements, making him a confusing and omnipresent figure.

Hilde is a teenager living in Lillesand, Norway. She awaits her father's return from his military deployment in Lebanon. Hilde exists as a parallel figure to Sophie, connected primarily through the misdirected birthday postcards her father sends. Like Sophie, she possesses a curious mind and an interest in the larger questions of the universe.

Helene is Sophie's mother, who manages the household while her husband works away as an oil tanker captain. She is practical and grounded, leading her to worry when Sophie begins speaking abstractly about philosophy and spending time with an unknown older man. She constantly tries to bridge the growing gap between her routine worldview and her daughter's new perspective.

Joanna is Sophie's classmate and best friend. She provides a contrast to Sophie's new philosophical interests, preferring to talk about normal teenage activities like badminton and playing cards. The two girls walk home from school together and plan social events, though Joanna finds Sophie's sudden changes in behavior confusing.

Hermes is a golden Labrador retriever who acts as a courier. He safely transports thick envelopes of philosophical writings from Alberto's home to Sophie, providing the only physical link between teacher and student before they officially meet face-to-face.