Not many directors today compare their films to a drug experience. The idea was more fashionable in 1975, when Alejandro Jodorowsky was planning his version of the sci-fi novel Dune. As he recalls in Jodorowsky’s Dune, a documentary about that meticulously prepared and never-shot epic, he intended it to be a “sacred” experience that would also “give LSD hallucinations without the drug.”

Now 85, the Chilean-born Jodorowsky is a charismatic, white-haired presence on screen, a major reason for the documentary’s allure. You can see how his genial insistence on the visionary greatness of his Dune—intergalactic battles, mythic creatures that presaged those in Star Wars, a mystical theme, Salvador Dali as Shaddam Corrino IV, the emperor of the galaxy—could have persuaded a core of talented collaborators to join him in Paris to get started.

But a bigger reason for the new film’s appeal is that its real subject is not Jodorowsky but that rare, enticing item: the never-made masterpiece, the mad obsession that nearly came to life. Jodorowsky’s version of Dune is one of the legendary unrealized films, along with Stanley Kubrick’s long-planned but never started Napoleon, Orson Welles’s lost cut of The Magnificent Ambersons, and Terry Gilliam’s once-and-future The Man Who Killed Don Quixote. We can imagine the greatness of these films without having to measure the reality against the myth.

Jodorowsky’s Dune, a first feature by Frank Pavich, offers a tantalizing glimpse at the extravagant, wildly expensive movie the director had in mind. At the time, Jodorowsky was best known for two eccentric, surreal works: El Topo, a mystically themed Western often considered the first midnight movie, and The Holy Mountain, even more imagistic and dream-like. (Kanye West has said Holy Mountain influenced his Yeezus tour.) Obviously, Jodorowsky has no use for conventional narrative. He even says in the documentary that he never read Dune; someone told him it was fantastic.

Yet he created a complex saga in which each planet has its own ruler, backed by a different band on the soundtrack—Pink Floyd was to represent Caladan, the planet ruled by Duke Leto Atreides, who would have been played by David Carradine. Jodorowsky may have been obsessed, but he was also shrewd. Working with the French producer Michel Seydoux (a droll presence in the documentary), he put together a team of people who would individually go on to work on films like Star Wars and Alien. As he tells it, he signed a reluctant Orson Welles to play the blimp-like Baron Vladimir Harkonnen by promising to hire the gourmet chef from his favorite restaurant in Paris for the shoot. Dali insisted on being the highest-paid actor in the world, which Seydoux negotiated as $100,000 a screen minute; Dali never realized how few minutes he was meant to appear in the film.

Jodorowsky storyboarded the entire movie—which he said might be many hours long—and compiled a giant production book for the Hollywood studios, with photographs, sketches, and costumes. Some of those storyboards are animated in Jodorowsky’s Dune, so you can see how ambitious the project was, with all its creatures and planets, and how impossibly expensive it would have been in those days before CGI (not to mention the cost of those French chefs).

Everyone in Hollywood turned it down, and the only time Jodorowsky seems angry in Pavich’s film is when he talks about how the studios were trying to destroy his dream. Well, don’t they always? That anger tells you why he was never going to be a studio guy, and he was probably right to walk away rather than compromise. When David Lynch, another true visionary, finally directed a version of Dune nearly a decade later, it turned out to be one of Hollywood’s famous flops. Lynch himself says it was a mistake to have made the film without having final cut.

But would Jodorowsky’s Dune have been any better? Take a look at El Topo and The Holy Mountain and you might wonder. The glimpses of those films in the documentary are dazzling, and in small amounts Jodorowsky’s artistry is undeniably brilliant. But to sit through an entire Jodorowsky film requires an extremely high tolerance for surrealism (or maybe LSD).

Jodorowsky moved on, and recently directed an autobiographical film, The Dance of Reality (opening in May). Today, he says, “Dune is in the world like a dream.” But few dreams die so definitively. Until recently, all that existed of Kubrick’s biographical Napoleon, which he started planning right after 2001: A Space Odyssey, was a production book more than 1,000 pages long. And like Jodorowsky, Kubrick also had the delusion that some Hollywood studio would back his vision. “It’s impossible to tell you what I’m going to do except to say that I expect to make the best movie ever made,” he wrote in the draft of a letter to a studio executive. No wonder that didn’t work. But this month, Steven Spielberg announced that he plans to turn Kubrick’s screenplay of Napoleon into a miniseries.

Gilliam’s spin on Don Quixote actually did begin filming, and the attempt was captured in the 2002 documentary Lost in La Mancha, a chronicle of a shoot in which everything that could have gone wrong did: funding fell through, it rained in the desert, and Jean Rochefort, who was to play Don Quixote, had such terrible medical problems that he couldn’t ride a horse. For pure entertainment, it’s hard to imagine Gilliam’s film rivaling the tragic documentary. But every few years since the project was shut down, some publication breathlessly and prematurely announces that production is about to start again.

Gilliam has learned a few things about mad obsessive movies, though. As he said in an interview with Indiewire last December when asked about fresh rumors that the film was a go, “We’ll see. My default position at the end of each film is that it’s back to Quixote.” If it does get made, he said, “It’ll be a disappointment. People’s expectations will be so high, and it will be just another film. But I just want to get it out of my system.” He’s willing to gamble on destroying a myth.