"Don Quixote de la Mancha" is a novel by the Spanish author Miguel Cervantes. It is generally considered to be the first novel (in 1605 and 1615, see below). Originally written in Spanish, the story has been translated to many languages, including English.
The novel Don Quixote actually consists of two parts: the first was published in 1605 and the second in 1615 (a year before his death). Between the first and second parts, a "fake" Don Quixote account was published by a Cervantes imposter. For this reason, Part II contains several references to an imposter, and Part II ends with the death of Don Quixote (so no other imposter could experiment with Cervantes' character).
The plot covers the journeys and adventures of Don Quixote and his squire, Sancho Panza. Don Quixote is an ordinary Spaniard who's obsessed with stories of knight errantry. His friends and family think him crazy when he decides to become a knight errant himself, and to wander Spain on his thin horse Rocinante, righting wrongs and protecting the oppressed..
Don Quixote is visibly crazy to most people. He believes ordinary inns to be enchanted castles, and their peasant girls to be beautiful princesses. He mistakes windmills for oppressive giants sent by evil enchanters. He imagines a neighboring peasant to be Dulcinea del Toboso, the beautiful maiden to whom he's pledged love and fidelity. Sancho Panza, his simple squire, believes his master to be a bit crazy. But both master and squire undergo complex change and development throughout the story.
Master and squire have numerous adventures, often causing more harm than good in spite of their noble intentions.
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Many people may be more familiar with the musical The Man of la Mancha than with the book itself. If they read the book, they would be in for some surprises:e.g. Dulcinea, or Aldonza Lorenzo, that is one of the main characters of the play, doesn't appear even once in the book! She is only reffered to, as Don Quixote's platonic love, his lady, and the person to whom Don Quixote dedicate his deeds. This compulsion to insert a "love interest" into vehicles that get along without them in the original is typical for American fiction, film, and theater.