“Jodorowsky’s Dune” opens Friday at Sundance Cinemas. PG-13, 1:25, three and a half stars out of four.

“Jodorowsky’s Dune” is a documentary about failure, and “the greatest movie ever made.” So why did it leave me so oddly happy, this story of a beautiful dream that never came true? Perhaps because there’s beauty in the dreaming itself. In the early ’70s Alejandro Jodorowsky was the very definition of a cult director. His psychedelic Western “Il Topo” was a midnight-movie sensation in 1970, and his even more gonzo follow-up “The Holy Mountain” (which features, among many other memorable images, feces made of gold) was a hit in Europe. Jodorowsky could have next made anything he wanted.

He wanted to make “Dune,” an adaptation of Frank Herbert’s classic 1965 sci-fi novel about a desert planet and the battle over the mind-altering drug (called “spice”) found there. He hadn’t actually read “Dune,” but no matter; he had his ambitions set on making a film that wasn’t just the greatest sci-fi movie ever made, but the “coming of a god, an artistic, cinematographical God.” Jodorowsky is now 85, and director Frank Pavich wisely lets him tell the story, his eyes bright, hands gesturing wildly, his face split by a boyish grin of mad delight as he describes his vision.

Jodorowsky was swinging for the cheap seats; he hired artists Chris Foss, H.R. Giger and Jean “Moebius” Girard to create truly fantastical and baroque designs for the film; unlike the cool steel spaceships we’ve come to know, Jodorowsky’s spaceships glowed with color, as if they were made out of black light paintings. He hired Dan O’Bannon, who would later write a little something called “Alien,” to handle the visual effects. And for his cast, he saw Salvador Dali, Orson Welles and Mick Jagger in the lead roles, and such was his passion that he actually talked them on board his insane journey.

“Dune” didn’t just exist in his mind — Jodorowsky and his artists worked up a meticulous shot-by-shot storyboard of his vision, bound up in a book the size of a paving stone. They shopped the book around Hollywood, but in those pre-“Star Wars” days, studios weren’t ready to commit to a sci-fi film so personal in vision but epic in scope. They all passed, and “Dune” never happened. This should be the tragic ending, except that even unmade, “Dune” ended up influencing a generation of sci-fi films; you can see echoes of the designs in “Alien,” “Star Wars,” “Flash Gordon,” even all the way up to 2012’s “Prometheus.”

Maybe none of them were “artist, cinematographical Gods,” but “Jodorowsky’s Dune” makes a powerful case that a good idea, once dreamed up, cannot be un-dreamed. Chastened by the rebuke, Jodorowsky seemed to lose interest in film — until “Dance of Reality,” which played at the 2014 Wisconsin Film Festival, he hadn’t made a film in 23 years. But the documentary finally lets his vision make it to the movie screen, in a way.

The failure isn’t that “Dune” never got made; the failure would have been if Jodorowsky never dreamed it up in the first place.