Marlon Brando plays the psychiatrist assigned to his case. He is the best clinician the hospital has ever had. Don Juan thinks Marlon Brando is Don Octavio de Flores, a Spanish nobleman from the same era as the original Don Juan. Marlon Brando plays a very open, warm-fuzzy character, very different from any other role I've seen him in. He is 10 days from retirement. This will be his last patient.
Marlon is intrigued and charmed by the Don Juan's fantasy world and wants to explore it. Marlon's boss, the head of the hospital wants Don Juan put on medication right away. Marlon refuses because he's enjoying the Don Juan's fantasies too much. In their sessions Don Juan talks about his life, all of which is an elaborate fantasy, but all the while he is preaching the romance of seduction, honor and the beauty of women to Marlon.
Don Juan has a Don Quixote like effect on most every one around him. The female nurses fall all over him. Best of all, Marlon Brando adopts Don Juan's philosophy and starts, "Don Juaning," his wife, played by Faye Dunaway. This is all quite a surprise to her. Their long-cold relationship comes alive. He takes her to romantic dinners, buys her diamond earrings, buys flowers, and on and on. He tells his wife of 32 years that he wants to her to know her! He says he wants to know what she was thinking and feeling all those years he was thinking about himself. She laughs. She says she thought he'd never ask. Marlon Brando starts to consider that Don Juan Demarco is the real Don Juan.
It is hinted that if Don Juan is put on medication, he will end up staying in the hospital for the rest of his life. Marlon Brando confidently feels he will have him out in 10 days. Marlon only gives him his medicine right before he has to go before the judge who will decide his fate. Brought back to reality before the judge, by the drug he comes across as close enough to normal, and the judge releases him, to Marlon's pleasure and the militant disapproval of the head of the hospital.
As Don Juan is being released, Marlo Brando announces to himself, in his mind, "I am Don Octavio de Flores the world's greatest psychiatrist! I have cured 1,000 patients."
All three fly to the Greek island of Eros for a vacation. In his mind, to the viewer, he announces, "Yes, Don Juan was incurable. He was infected with incurable Romanticism and, and it is very contagious." Marlon and Faye dance to Mariachi music on the beach.
This is the most unlikely bunch of producers, actors and actresses to make a movie like this.