Plot
John Arnold DeMarco is a 21-year-old who dresses like Zorro, with a mask, hat and cape, and claims to be Don Juan. After a passionate affair, he decides to commit suicide. At the site of the billboard of a masked model from which he plans to jump, psychiatrist Jack Mickler dissuades him by posing as Don Octavio de Flores. John is then held for a ten-day review in a mental institution.
Mickler, who is about to retire, insists on doing the evaluation and conducts it without medicating the youth. He listens to John's story as Don Octavio: Don Juan was born in Mexico, he has an affair with his school tutor which ultimately leads to the death of his father in a swordfight. Mickler listens to the story while spotting inconsistencies, such as his Castilian accent, but continues to go along with it.
At home, Mickler's marriage to his wife Marilyn lacks passion. As Mickler notices that John's presence at the institution is having an impact on the staff – both distracting the women and dancing with a male attendant on the lawn – he finds himself being influenced, and starts listening to opera in his house and rekindling the passion with Marilyn.
Eventually Mickler meets with John's grandmother, who tells him that John grew up in Phoenix, Arizona, and that his father, "The Dance King of Astoria", died in a car crash. When Mickler returns to the institution and confronts his patient with this information, John dismisses his grandmother as misanthropic and of making up the backstory.
In response Mickler recounts the story of an insecure young man who fell in love with a woman in a magazine, who then contacted her and was told she never wanted to speak to him again. When John asks what happened to the young man, Mickler tells him he tried to commit suicide.
As the ten days tick down, pressure mounts on Mickler to support the youth's indefinite confinement, a decision which he is skeptical of. John has mentioned that his mother became a nun and remains in the convent to this day, and in a subsequent meeting with John, Mickler suggests that John's mother could have possibly had affairs, to which John responds with violent anger.
John concludes his story, about how he was kept two years in a harem as the lover of the sultaness, before finding true love and being rejected on a remote Greek island by his one true love, Doña Ana. Mickler tells him at the story's end that he believes John is Don Juan DeMarco, the greatest lover the world has ever known. When John asks Mickler who he is, he says "I am Don Octavio de Flores" and that he has seen through all of his masks.
At the end of the ten days, Mickler is able to convince John to take his medication, and makes him understand that not everyone believes he is Don Juan. Right before his retirement, Mickler and John meet with the board, with John in street clothes. He speaks with an American accent, and acknowledges that he was born in Queens, his father was killed in a car accident, and that his mother was unfaithful to his father, so entered a convent in Mexico.
John is released and accompanies Mickler, along with his wife Marilyn, to the remote island of Eros that Don Juan has described where he met his true love Ana. The older couple dance on the beach, while John connects with someone in the distance.
