Don Quixote is a character in a old book that goes senile from reading too many books on knights and ups convincing himself that he is a knight. He decides to put on a tin armor then goes off fight windmills and making a general joke of himself.
The old book was by Miguel de Cervantes and is considered the national novel of Spain. The musical and movie(s), The Man of La Mancha, are based on it (Don Quixote was the Man of La Mancha).