LONDON - Two promises are fulfilled -- one with more time to spare than the other -- in David Mackenzie's "Starred Up," a wholly prison-set nightmare picture that careers wildly between the punchy and the plain punch-drunk, and fascinates equally in either register. For the hitherto raggedly gifted Scots filmmaker Mackenzie, it's the film that most satisfyingly stitches together his twin impulses toward grit and grace, energizing familiar genre terrain with a coarse but literate ear and violently poetic eye. For his 23-year-old leading man, Jack O'Connell, it's a gratifyingly early arrival, a seemingly bespoke vehicle that jolts his wild, woolly talent into something that looks a lot like stardom. "Starred up" is British penal jargon for the contentious promotion of a juvenile offender promoted to adult status; for a film that consolidates this much raw potential, it seems an oddly appropriate title.

The scarred, sometimes scarring progress of young men to maturity has been an ongoing preoccupation for Mackenzie, previously peaking with the perverse sexual exploits (or exploitation) of Ewan McGregor in 2003's "Young Adam," and bottoming out with, well, the perverse sexual exploits (or exploitation) of Ashton Kutcher in 2009's "Spread." Sex is a more latent concern in "Starred Up" -- occasionally surfacing to riotous, aggressive effect, as it does in many an all-male prison drama -- though its protagonist's evolution through the course of the film is corporeal in a different way. Hand-to-hand combat is the alternative sexual currency in this cage of masculinity gone stir-crazy; life lessons and father-son conflict are on the cards too, but this is as much a story of physical education, of the human body made impenetrable. It's as numbing as it is moving.

Scarily feral 19-year-old Eric (O'Connell) begins the film sturdy enough, the camera taking in his naked, knotted, crudely tattooed form as he arrives for inspection at an unnamed prison in an unspecified region of Britain. It's a risk and a strength of first-time feature writer Jonathan Asser's low-fat screenplay that it spares us a great many details of past and external circumstance, allowing us to reach for best- and worst-case scenarios: we never learn what Eric's youthful crime was, though his hair-trigger temper and flair for grievous bodily harm suggest it was precociously horrific. Nor do we know the backstory of his wiry-looking but equally dangerous dad Neville (Ben Mendelsohn, shorn of his Down Under accent but none of his stoat-like intensity), except the fact that he's been locked up in the same place for the past 14 years -- effectively for as long as his son (whose mother is out of the picture, we're bluntly told) can remember.

It's not exactly a joyful reunion. Eric and Neville are strangers in one sense, and utterly, maddeningly recognizable to each other in another: as one man's alpha egotism and unmanaged anger is increasingly mirrored in the other, it becomes clear how many destructive patterns of behavior have been indirectly passed from father to son, and how irrevocable they may turn out to be -- for all the bureaucracy-impeded efforts of Rupert Friend's dedicated but unidealistic counselor.  

It's a story so stark and spare that some might be tempted to read it as allegory, though neither Asser nor Mackenzie's perspectives suggest these lowlifes -- uncomfortably likeable as they occasionally, if only flickeringly, appear to be -- are anything but their own special, sorry cases. Though even in this hellish, unnatural social context, there's something identifiably affecting about the relationship between Eric and Neville, however tentatively it thaws.

Neville's half-hearted sense of paternal protectiveness gives way as he realizes that his son is both stronger than him, and stronger without him -- that internal wound grounds the film even as it becomes an ugly kind of ballet in pursuit of external ones. There's a higher concentration of fleshy fighting in "Starred Up" than anything I've seen this year, observed by Mackenzie with an awed, aesthetically conscious stillness that recalls the work of Nicolas Winding Refn: the film's tendency toward bristly, ironic wit keeps it from blood-fetish territory, though Michael McDonough's unexpectedly serene camera, bathed in the bleakly bright primary hues of Tom McCullagh's production design, tests the audience's impulse to look. (Kudos, too, to Jake Roberts and Nick Emerson's brisk but often unblinking editing.)

Mackenzie's sharp, wily craft has never been married to more effectively simple material, but if the film runs out of narrative rope with about 20 minutes to go, it's the unpredictability and go-for-broke physicality of O'Connell's performance that lends it emergency fuel, and looms largest in the memory. (It positively boggles the mind that he was initially cast in this year's teen-lit adaptation "Beautiful Creatures" as Ethan Wate, the dorky Southern daydreamer so beautifully played by his replacement, Alden Ehrenreich.)

A veteran of British teen TV series "Skins," O'Connell's scuzzy charisma and chippy swagger has enlivened a handful of B-level Britpics in the past, though his presence has never been so fearsomely concentrated as it is here: it's as hard to look at this snarling, sneering delinquent as it is to look away from him. (When an actor keeps winning scenes against a switched-on Mendelsohn, you know he's got the goods.) He has surprising delicacy, too: a wordless triangular exchange as Eric finally latches on to the level of affection Neville and his cellmate hits just the right note of gently mortified acceptance. It's a scene indicative of everything this tough but textured film gets right: some bruises are born of untempered testosterone, but they're still tender where it counts.