The book begins with Sophie receiving two messages in her mailbox and a postcard addressed to Hilde Møller Knag.

Eventually, through the philosophy of George Berkeley, Sophie and Alberto discover that they are actually fictional characters in a book written by Albert Knag as a 15th birthday gift for Hilde, his daughter.

Hilde reads the manuscript, but begins to turn against her father after he continues to meddle with Sophie's life by sending fictional characters like Little Red Riding Hood and Ebenezer Scrooge to talk to her.

Alberto helps Sophie fight back against Knag's control by teaching her everything he knows about philosophy, through the Renaissance, Romanticism, and Existentialism, as well as Darwinism and the ideas of Karl Marx.

The party soon descends into chaos as Albert Knag loses control over the world, causing the guests to react with indifference to extraordinary occurrences.

Alberto informs everyone that their world is fictional but the guests react with rage, believing him to be instilling dangerous values in the children.

A woman from Grimms' Fairy Tales gives them food before they prepare to witness Knag's return to Lillesand, Hilde's home.

While at the airport, Knag receives notes from Hilde set up at shops and gateways, instructing him on items to buy.

The book has been translated into several languages, including Portuguese, Persian, Bengali, English, Hindustani (Hindi, Sindhi and Urdu), French, German, Greek, Chinese, and Kurdish.

[citation needed] The game allows players to learn about philosophy as in the book, while adapting the metafiction elements for a virtual world.

Chinese math rock indie band Baby Formula's self-titled album has a track called "Sophie's World (苏菲的世界)".