A short time before “Star Wars,” in a galaxy far, far away, the Rebel heroes featured in “Rogue One: A Star Wars Story” took the first step that led to the Death Star’s destruction. No spoilers there, since earthlings first saw that glorious explosion nearly 40 years ago. Be warned, though: Every detail that follows could dilute the surprise factor of what’s coyly being billed as a Jedi-free spinoff, but might more accurately be described as “Star Wars: Episode 3.9,” so perfectly does it set up George Lucas’ 1977 original.

Not only does “Rogue One” overlap ever so slightly with “A New Hope,” but it takes that blockbuster’s biggest weakness — that a small one-man fighter can blow up a battlestation the size of a class-four moon — and actually turns this egregious design flaw into an asset. Now we know why the Death Star has an Achilles’ heel and how that information fell into Princess Leia’s hands. Plus (and here’s the aspect that should send longtime “Star Wars” fans into ecstatic orbit), director Gareth Edwards has finally made the first “Star Wars” movie for grown-ups.

There are no Ewoks or Jar Jar Binks-like characters here, thrown in just to appeal to pre-school-aged audiences. The plot is designed less like a flashy video game, and more like a down-and-dirty war movie (think documentaries about the conflict in Syria, rather than stodgy World War II films). And quite a few of the principal characters die, which would be upsetting for young viewers, but provides fans old enough to remember seeing “Star Wars” in theaters with a heroic sacrifice designed to inspire a “Remember the Alamo!”-style rallying cry when it comes time for the Red Squadron to do its business. With all due respect to comic-book devotees, this is the “Suicide Squad” audiences have been waiting for this year.

If this all sounds dauntingly technical, that’s because it is. At the risk of being sacrilegious, “Star Wars” — with its notions of Jedis and the Force — has become one of the world’s favorite myths, bypassing a number of traditional belief systems in its increasingly cult-like appeal (give it another century, and fandom could well crystallize into worship). As such, any new addition to the canon demands more than just a passing familiarity with the previously established lore, while the slightest infidelity on the filmmakers’ part threatens to upset its followers.

“Rogue One” is loaded with allusions to other films in the franchise, and though that’s fun for the faithful, it also makes this the “Battlefield Earth” of the series: an elaborate, complex-to-the-point-of-confusing space opera that will earn few converts, while appealing primarily to the already-converted. Except, as for-profit “religions” go, there are a whole lot more “Star Wars” followers out there than there are Scientologists. (By sheer coincidence, both movies feature Forest Whitaker in outrageous wig-and-costume combinations.)

In all fairness, while “Rogue One” is complicated, it isn’t any more so than the elaborate trade-and-taxation backstory of “The Phantom Menace.” Beginning on a volcanic-soil planet that looks suspiciously like Iceland, where Imperial-scientist-turned-farmer Galen Erso (Mads Mikkelsen) has retired to raise crops — and his daughter, Jyn — in relative peace, the movie is the second to center on a female protagonist, after last year’s more traditionally fun “The Force Awakens.” The young Jyn is traumatized after seeing her mother murdered and her father taken into custody by white-caped Imperial baddie Orson Krennic (Ben Mendelsohn). Skip forward 20 or so years, and Jyn Erso (Felicity Jones) has gone from playing with dolls to wielding a blaster — allowing the Oscar nominee a kind of take-charge attitude absent from her previous dramatic roles.

In a series of mostly-generic scenes credited to A-list studio writers Chris Weitz and Tony Gilroy (the latter was reportedly pulled in to consult on reshoots), director Edwards cycles between a number of different planets, introducing members of what will become the film’s crew: There’s shifty Rebel spy Cassian Andor (Diego Luna, “Y Tu Mamá También”), who won’t hesitate to shoot someone in the back, and his pragmatic reprogrammed Imperial security droid, K-2SO (voiced by versatile Disney regular Alan Tudyk); there’s scruffy pilot — and Imperial defector — Bodhi Rook (Riz Ahmed, “Four Lions”); there’s milky-eyed monk Chirrut Îmwe (Hong Kong martial arts star Donnie Yen), and his long-haired, Rambo-like bodyguard, Baze Malbus (Chinese auteur Wen Jiang, “Let the Bullets Fly”).

While all of these eccentrically named Rebels fall within well-established character types, the ensemble is diverse enough that viewers will surely gravitate toward their own favorites. K-2SO gets the best lines, though the blind Chirrut earned the loudest applause at the film’s premiere, every time he switched from mild-mannered Buddhist monk mode to his impressive Zatoichi-esque fighting style (as a result, “Rogue One” should be huge in China). The film has a fair amount of boilerplate plot to get through before the mission itself can get underway, and apart from its elegantly stark prologue, feels eye-crossingly hard to follow for nearly its first hour, finally picking up once Jyn is ordered to locate her father.

Jyn’s dad has become the lead engineer on the Death Star since she last saw him, and he holds the key to its destruction — and we all know how that turns out. Still, for this batch of characters (none of whom have been so much as referenced in the subsequent episodes), the subsequent mission to steal the blueprints and beam them to the Rebels will be a Pyrrhic victory at best. In their effort to second-guess the film’s secrets, “Star Wars” obsessives have been citing Mon Mothma’s line, “Many Bothans died to bring us this information,” from “Return of the Jedi,” though in that case, the Senator was talking about the Death Star II (there are no Bothans in “Rogue One”).

Fans may know what comes next, but most of the Rebel history in “Rogue One” is being revealed for the first time (its clever connective plot hatched by “Star Wars” insider — and ILM visual effects supervisor — John Knoll himself). Still, the sequences that met with the most raucous approval at the world premiere screening were those featuring actions or characters audiences recognized from earlier films (a couple of whom are convincing computer-rendered performances, since the original actors have either died or aged too much to reprise their roles).

So the faithful enthusiastically approve, but does that make “Rogue One” a good movie? Or is it merely exploiting its connection to a well-loved and widely known phenomenon, the way that “Ben-Hur” integrated Jesus cameos into a less compelling parallel narrative? The answer is neither one nor the other, or a little of both. But at least it’s not the crass cash-grab skeptics may have feared. If this is your first “Star Wars” movie, you will be compelled to immediately follow it with “A New Hope,” since the film doesn’t end so much as abruptly roll credits after delivering a long-delayed payoff no other 2016 release can possibly rival.

The film’s tangential approach is precisely why Edwards was such a perfect choice to direct. While “Godzilla” proved that he could handle a blockbuster of this scale (and “Rogue One” feels every bit as big as “The Force Awakens”), it’s actually Edwards’ low-budget debut, “Monsters,” that suggested what’s so effective about this spinoff — that film spoke of an alien invasion from the perspective of a couple dealing with other concerns, playing a familiar genre from a fresh angle. The same could be said for “Rogue One,” which is an effective war movie in its own right, but focuses on the kind of characters who tend to get a single scene or line in the other seven films. It’s a reminder of the hilarious “Star Wars” debate in Kevin Smith’s “Clerks,” in which fans consider the fate of all the contractors hired to rebuild the Death Star — suggesting a level of interest in every character in the “Star Wars” universe, no matter how minor.

That said, “Rogue One” could fairly be accused of “speciesism,” since it features disappointingly few aliens. There’s actually quite a bit of the “Star Wars” formula missing here: no Jedis, no light sabers (until Darth Vader finally shows up), no iconic villain (Mendelsohn’s sneering, John Hurt-ful performance suggests a mid-rank Nazi functionary), and no chemistry, despite a half-hearted attempt to suggest Jyn and Cassian could be a couple.

Still, between epic battles featuring scores of familiar spaceships and the genuine thrill of hearing composer Michael Giacchino riff on John Williams’ classic score, there’s no denying that the film belongs to the creative universe Lucas established. This is the rebellion as it is experienced in the trenches. Younger audiences will be bored, confused, or both. But for the original generation of “Star Wars” fans who weren’t sure what to make of episodes one, two, and three, “Rogue One” is the prequel they’ve always wanted.