This is one that I meant to cover earlier, but things just got in the way. Let's rectify this oversight. This is Jodorowsky's Dune...
The film is a Documentary which talks to (all of the living) people behind Alejandro Jodorowky's attempt to make a feature-length version of 'Dune'...back in the early 1970s. You see the amazing Concept Art, hear all of the neat ideas and get a major look at what could have been.
You get to see how visually-inspiring the film could have been, how unique (and great) the Casting was/would have been and just how ahead of its time it was going to be.
Bear in mind that this was in the works *before* Star Wars, so this would have likely-been the Sci-Fi follow-up to 2001: A Space Odyssey and skewed how future works of the Genre would have become.
Behind all of the disappointment about it not getting made (no SPOILER, right?), you get to see what the film that never was set in motion, be it the Production Casting behind Alien and other such works.
If you are one of those people that thinks that creativity is dead in the Film Industry, you need to see this. While the film ultimately never got made, you can be inspired by the amazing creativity and effort of those who attempted it.