"Man of La Mancha" is a tale within a tale. Miguel de Cervantes is a prisoner during the Spanish Inquisition. He puts on a play for his fellow prisoners; it is the story of Alonso Quijana, an old man who thinks he is a knight and calls himself Don Quixote de La Mancha.
In this scene Don Quixote (Peter O'Toole) serenades the prostitute Aldonza (Sophia Loren), seeing her as the lady Dulcinea. After Don Quixote is led away in confusion, the townsmen reprise the song in a cruel parody. Aldonza rebuffs Don Quixote's admiration, but at the end of the film she has begun to see herself through his eyes and calls herself Dulcinea.
Peter O'Toole won a Golden Globe for his acting in this 1972 role. His singing is dubbed by Simon Gilbert. The music is by Joe Darion and Mitch Leigh.
An English lesson... The "x" in Don Quixote is pronounced as an "h" (kee-hoh-tee), but in the adjective quixotic it is pronounced as an "x" (kwik-sot-ik). I actually learned this from a fourth grader after mispronouncing the adjective for years!