Some good stuff in this science fiction film directed by Ridley Scott (Alien, Blade Runner), especially Michael Fassbender as the evil gay robot who studiously models his accent and hairstyle on Peter O'Toole in Lawrence of Arabia. But the movie is not terribly scary, and the screenplay (co-written by one of the Lost writers) is a mess.
My guess is that there is about an hour of plot left on the cutting room floor. Scott is at his best as a slow-paced, moody director, so it doesn't work to hand him a complex screenplay that is supposed to tie together the Alien series and maybe Blade Runner too, and launch a new series.Thus, about two-thirds of the way through Prometheus, Idris Elba, as the starship captain, suddenly explains for us a whole bunch of stuff about the origin of the unknown planet that his character couldn't possibly have figured out by that point.
It would have been just as effective to have Elba face the camera and hold up 60 pages of torn-up screenplay in his left hand while reading from a 4x6 index card in his right hand: "After a night of far-ranging and frank discussions among the studio executives, the producers, Sir Ridley, and the accountants, the decision has been made to skip over pp. 93 to 152 of the screenplay and have one character just tell the audience the unsettling discoveries that would have required at least $57.5 million in incremental expenditures to show. Of course, since all characters in Alien spaceship crews notoriously have their own agendas, this raised the issue of whom audience members would find most trustworthy. Since I'm the black guy in the movie and therefore could be assumed not to have a complicated corporate agenda, I was chosen. So, to summarize, this planet is not actually the ..."
Also, the screenplay is aimed way down the IQ scale. For example, when the ship arrives at the unexplored Earth-like planet in a distant solar system in 2093, it immediately descends through clouds to the surface, not picking out a landing spot until they are 3,000 feet up. Have they never heard of orbiting for awhile?
Scott's last movie, 2010's Robin Hood, was kind of a mess, too. (Scott isn't a writer, and it's silly of studios to keep trying to have him launch trilogies, for which he lacks the kind of large-scale plot-architect skills that Joss Whedon brought to the Avengers.) But, Robin Hood was at least a pretty smart mess (Tom Stoppard did the last round of script doctoring), with lots of interesting historical analogies comprising an attack on Blair-Brown New Labour. But, it didn't do that well at the American box office, unsurprisingly, so maybe they decided to dumb down Prometheus to avoid alienating American ticket-buyers.