He sees windmills as giant dragons, peasant girls as damsels in distress, etc. The first half of the story came out in 1605 and the second half followed in 1615, although nowadays they’re published as one volume.
He subsequently decides to become a knight, search for adventures like the ones he’s read about and right the wrongs of the world.
Don Quixote soon recruits a farmer named Sancho Panza to be his squire, but because his perception of reality is askew, this makes for amusing ways he tries to replicate the life of a noble protector.
For those of you unfamiliar with Don Quixote (written by Miguel de Cervantes Saavedra), the story follows the eponymous protagonist whose sanity is bent after reading too many chivalric tales, now believing that they are actually real.