Thirty-three years after his “Alien” introduced moviegoers to the terrors of extra-terrestrial life and the horrors of being named “Sigourney,” and 15 years after “Alien: Resurrection” basically ended the franchise (we do not speak of the two “Alien vs. Predator” movies), Ridley Scott returns to outer space with “Prometheus,” a visually arresting but thematically drab sci-fi thriller that’s set in the same fictional universe.

You could call “Prometheus” a prequel to “Alien,” but it tells a standalone story, and no prior knowledge of the other films is necessary. If you have seen “Alien,” though, you’ll appreciate some “Prometheus” details better. You’ll also notice a certain symmetry between the two films’ plot lines, and either admire this or feel a little cheated by it.

Set mostly in the year 2093 and mostly aboard the title spacecraft, “Prometheus” concerns a small crew of scientists, engineers, and mercenaries at the tail end of their two-year journey toward a distant destination for an as-yet-undisclosed mission. They slept most of the way, of course, in those suspended-animation sleep pods that sci-fi movies are so fond of, watched over by the android David (Michael Fassbender), who is eerily human-like and wishes he were even more so.

Most of the crew is in the dark up to this point about the specifics of their mission, so I’ll let you hear about it when they do. But in general terms, they’re following up on a hunch by archeologists Elizabeth Shaw (Noomi Rapace) and Charlie Holloway (Logan Marshall-Green) that a planet out here might have clues regarding the origin of life on Earth. Aging billionaire industrialist Peter Weyland (Guy Pearce) funded the expedition because he believes in Shaw and Holloway’s theories, while the mission’s haughty commander, icy Meredith Vickers (Charlize Theron), is derisively skeptical. Let’s get there, look around, confirm it was a wild goose chase, and go home, she says. Again, not to spoil anything, but she’s wrong.

Though it’s been decades since Scott tackled anything this special-effects-heavy, he makes full and competent use of the technological advancements that have been made in the meantime, continuity with the now-retro-looking “Alien” be damned. Shot by expert cinematographer Dariusz Wolski (“Dark City,” “Sweeney Todd,” “Alice in Wonderland”), “Prometheus” has a crisp, cold look that’s appropriately otherworldly, often beautiful to behold. The creature effects are seamless. Clearly no expense was spared to make the film look as awe-inspiring as the state of the art will allow.

It’s the screenplay that doesn’t quite measure up, especially in comparison to the extraordinary visuals. Written by Jon Spaihts and Damon Lindelof (“Lost”), the story is about 80 percent solid and 20 percent ridiculous. Shaw and Holloway, who are lovers, have some jarringly mawkish relationship dialogue. The ship’s captain, played by the terrific Idris Elba, is given little to do. Melodrama pops up when it is least expected or needed. You start to realize that for all its satisfying turns and heart-pounding suspense, and for all the lip service it pays to Big Questions, the story is actually quite thin.

But that almost doesn’t matter when you’ve got Michael Fassbender’s mesmerizing android capturing your attention, or Noomi Rapace fretting about what organism she may have come in contact with, or Sean Harris and Rafe Spall’s uncourageous scientist characters stumbling frantically through a situation that we’ve been conditioned to expect will not end well for them. It isn’t until about the midway point that the film becomes truly exciting, but it never fails to be intriguing.

By being set in its timeline and emulating many of its traits, “Prometheus” invites comparison to “Alien” – which is unfortunate, since most things that are compared to “Alien” do not come out ahead. This film isn’t nearly as scary as that one, and there was a gritty charm to the rough 1970s production values that the digital wizardry, marvelous though it is, can’t duplicate. But if Ridley Scott wants to return to sci-fi, “Prometheus” is a strong reintroduction. It will be interesting to see what bursts out of his chest next.

B (2 hrs., 4 min.; R, one F-word, intense images, some moderately strong violence.)