The story is framed in a flashback narrative, beginning when de Cervantes (O'Toole) is arrested for performing a stage version of Don Quixote and thrown in a dungeon. There, he tells his story to the inmates; the film cuts back and forth between him narrating and re-enacting some of the play with his acting troupe and the fantasy sequence where O'Toole plays Don Quixote and James Coco plays Sancho Panza. Another prisoner in the dungeon is Aldonza (Sophia Loren), whom Cervantes as Quixana/Quixote incorporates into his narrative as the girl of his dreams, Dulcinea. Loren's singing is quite good, probably the best in the cast and I think she gives the performance of the film. As outre and manic as O'Toole is, Loren is brimming with repressed fury, both as Aldonza who wants nothing more than to be left alone to rot, and as Dulcinea who refuses to concede Quixote is anything but insane.
As Cervantes...putting on a show in the dungeon...as Quixote at the end of his tale.
Besides Loren and occasionally O'Toole, there's nothing much to enjoy in the picture. One bright spot is the makeup effects on O'Toole. You can see here that the film does a good job transforming him in his many incarnations. Notice, too, how the Quixote makeup in the dungeon scenes is subtly less sophisticated than the fantasy Quixote makeup. Nice touch that grounds the action in reality and helps differentiate the timeline of the narrative. O'Toole is probably least recognizable in this film than any other I've profiled, in part because of the makeup but also because his performance differs widely from even the other mentally unstable characters he's played.