The pint-size soldiers of Gavin Hood's “Ender's Game,” the long-awaited adaptation of Orson Scott Card's YA sci-fi franchise starter, are dab hands at fighting off alien invaders, unreasonable authoritarianism and each other – though we have yet to see how well equipped they are to handle a real-world human rights boycott. In the last few months, the volume of the marketing for this would-be tentpole has been rivalled by the pop-cultural pushback against its creator: not Hood, of course, but co-producer Card, a sophisticated, audience-attuned storyteller who also happens to be a vocally homophobic loon. Would that Summit could write him out of the production and retitle it “Gavin Hood's Ender's Game” – or, indeed, “Lee Daniels' Ender's Game.” But them's the breaks.

 Why open on this purely circumstantial note? The situation is not of the film's making, after all – Hood's adaptation is even at pains to remove whatever evidence of the author's dubious personal beliefs had seeped onto the page. (Sure, it's just a coincidence that Card settled on the term “Buggers” for the alien race threatening to wipe out humanity.) Meanwhile, a boycott is the last thing “Ender's Game,” a carefully constructed, serious-minded commercial entertainment that treats its young audience with an unusual degree of intelligence, deserves.

At the same time, however, there's something inadvertently apt about a campaign this sincerely right-on against a film that could hardly be more righteous itself: “Ender's Game” isn't especially revolutionary in its one-against-the-system politics, but is distinguished – in the ranks of high-concept kid-lit adaptations, at any rate – by its sturdy, inquisitive moral compass.

Burdening children with the consequences of violence is increasingly commonplace in mainstream filmmaking these days – even the clean-scrubbed cherubs of “Harry Potter” were getting pretty tormented by the Deathly Hallows stages, while the extreme parable of “The Hunger Games” took its underage characters into a realm of post-”Lord of the Flies” bloodlust, albeit with PG-13 restrictions. “Ender's Game” sets kids on each other, too, but only as proxies in preparation for very adult warfare.

Interstellar battle looms between humanity and the tactfully renamed, insect-like extraterrestrial Formics – who have already struck once, with severe consequences. For whatever reason, the military is counting on only the planet's most precocious adolescents to defend our turf. The bulk of the film takes place at a kind of vacuum-sealed space-boot camp, where killing age is reached well before kissing age – Whitney Houston presumably had something else in mind when she implored us to teach the children well and let them lead the way.

Hood's brisk script – its pared-down backstory an unusual asset within the genre – leaves pleasingly open the question of why children are being made to fight these battles in the first place. Perhaps it's simply a natural evolution of present-day military practice: are these youngsters significantly less prepared for the worst than the teenagers sent to fight in the Middle East?

Perhaps the rationale of Harrison Ford's crusty commanding officer Graff is that young, malleable minds may be more receptive to the art of war – a receptive twist on the very claim conservative-minded scolds have been making since the advent of video nasties and the Nintendo generation. If so, Graff meets unassuming but stubborn resistance in the shape of Ender Wiggin (Asa Butterfield), the brightest and most strategically cunning of his recruits, and also – hardly coincidentally – the one least inclined to take his designated enemy at face value.

The narrative of “Ender's Game” is thus spare in terms of narrative incident – no attempt is made to hide the fact that this a mere introduction to a more complicated physical and psychological universe – and rich in theoretical detail, as Ender slides through the multiple hoops of his training program while constantly considering just what he's meant to be fighting for. Ideas come before the extravagant set pieces here; the reverse is all too often true of ostensibly similar films.

“Ender's Game” even risks seeming too idea-driven, too preoccupied with hypotheticals. The bulk of the action places Ender and his cohorts in virtual-reality training setups, like a particularly high-tech update of 1990s shopping-mall Laser Quest; it's merely preparation for combat, so where's the peril? The answer, as Card's readers will already know, lies in playing Ender's growing awareness against the private dealings of his adult commanders – with Graff's pragmatism pitted against the more empathetic approach of outnumbered female Major Anderson (Viola Davis, hardly challenged yet, as ever, conjuring nuance from thin air), and the skepticism of Ben Kingsley's Rackham, a Maori-tattooed mentor with an accent pitched unaccountably between Johannesburg and Christchurch. Incrementally, the stakes are raised with each exercise, until an elegantly executed doozy of a final-act twist – one in which all the moral ramifications of video-game violence come home to roost.

For a story fundamentally about Doing The Right Thing – or, at least initially, the quest to find what that thing might be – this is unsentimental, even cruel stuff, lent grace and power by the unmistakable youth of its lead. Not for “Ender's Game” the creepily preternatural young warriors so prevalent in this manner of material: even costume designer Christine Bieselin Clark's uniforms seem designed to emphasize the protagonist's immaturity, the fabric clinging to Butterfield's matchstick limbs, accessorized with safety kneepads in playtime yellow.

Butterfield was a somewhat insipid presence in Martin Scorsese's “Hugo” two years ago, but his calm-eyed, unformed quality is put to far better use here: stern but thoughtful of brow, he's softly sympathetic as a boy required to make major ethical leaps before he's even determined who he is. (The other kids, ranging from Hailee Steinfeld's bland ally to the reliably irritating Moises Arias as a rival leader, seem considerably less special, that's at least partly the point.)

Though its zero-gravity showdowns exude a certain dreamy, balletic wonder, “Ender's Game” is sci-fi that largely evades whizz-bang flash or spectacle – revelling in the pyrotechnics of destruction, after all, would run counter to its very philosophy. Its effects are, well, effective; Sean Haworth and Ben Procter's angular, steel-and-teal production design both sleekly futuristic and starkly utilitarian.

Hood appears to have taken much on board from his botched steering of 2009's grindingly vulgar “X-Men Origins: Wolverine” – he's figured out a way to marry the stoic, even-handed human drama of his earliest South African work (already somewhat compromised in 2005's Oscar-winning “Tsotsi”) to the right-here-right-now demands of contemporary Hollywood genre storytelling, and made by far his best film in the process. In Orson Scott Card's novels, he's happened upon appropriate outsider material: its firm anti-bullying stance and compassionate appreciation of otherness may not square with the author's other beliefs, but it'd be a shame if that external inconsistency discouraged further investment in a franchise with so much on its mind.