Whatever else can be said, surely Starred Up is the year's rawest English-language film. Merciless shorn of everything that minutely resembles dross, it's a lean, muscly film about lean, muscly psychologies, eschewing needlessly embellishing details or even any reasonable exposition in its goal to present the tension and physical of life inside prison. The film's great sin is its lack of originality - it's the kind of movie that one almost inevitably can't help but to compare to a whole host of precursors - but with the intensity of its two central performances, and director David Mackenzie's savage, unpolished camerawork reinforcing that intensity, originality maybe isn't the most important thing in the wide world.

The title, which like much of the film's prison-system argot, is left for us to figure out rather than nicely explained, is a reference to a juvenile offender graduating into the adult prison system, and the young man who gets starred up as the film begins is Eric Love (Jack O'Connell), 19 years old and already a clear veteran of the system. In the virtually wordless opening sequence, we watch him being processed into his new home, flawlessly enacting the choreography required as he steps through security checks and is strip-searched, while keeping an empty expression locked onto his face. In some ways, Starred Up never improves on this moment and never has to: the long-take, wide-angle detachment of Mackenzie and cinematographer Michael McDonough's shot set-ups, coupled with O'Connell's coiled body language, tells us everything about the way that this young man thinks and feels, and the way that life as a prisoner has amplified whatever de-humanised elements landed him in prison to begin with (we never find out what his crime was, because we never need to - it's very much that kind of film, and God bless it).

The remainder of the 105-minute film doesn't go any of the places that one might anticipate: it's not about Eric finding his footing (he pretty much understands the score from day 1), nor is it about him leaning tricks from older, more hardened criminals, nor is it a story of how he found his soul in the darkness. Basically, it's just about how his brain works in this heightened environment, made worse by the presence his father, Neville (Ben Mendelsohn, wonderful as a brute with a deeply buried and fairly undernourished sense of familial affection), an inmate in the same prison. The men have not, clearly, had a terribly close relationship, and here they are, trying in the most tormented way possible to patch things up. In the absence of a conventionally-defined plot, watching the Loves fence with each other, acrimoniously and angrily, gives Starred Up its spine; that, and a side plot with Eric being adopted as the project of the prison counselor, Oliver (Ruper Friend), who badly wants to redeem the boy, though the boy himself patently does not want to be redeemed.

Oliver is an analogue for the film's writer, Jonathan Asser, who held the same position in a prison facility, and with that we start to understand why Starred Up takes the particular shape it does. This is a psychological drama wedded to a docu-realist study of the experience of being a prisoner, exactly the blend that one would expect to come from the mind of a man whose "I was there" testimony would come in the form of psychological study of inmates. Oliver isn't over-stressed in the narrative, though he is one of the only people to anchor a scene that takes place entirely outside of Eric's perspective; but it's unsurprising that the film's author shares his position. This film centers on a cold, bleak figure, one who has shot down any possibility of being reached or rehabilitated, but it still tries very hard to wriggle into his brain and understand him. Without any sentiment or special narrative pleading that he isn't that bad - the events depicted, including multiple vicious beatings in self-defense, suggest that he certainly isn't that good, either - Starred Up presents Eric as being inherently worthy of study simply because he is a human being, one who has been placed into an extreme situation and behaves extremely as a result.

With O'Connell giving an outstanding star-is-born performance in the film's center, it proves to be an extremely good piece of observational cinema. If we never quite understand why Eric acts as he does, the exploration of what still makes for an excellent, complex, and frequently unconventional piece of filmmaking. The film expects us to do a lot of work: entering this world requires paying close attention to small cues to perceive how the prisoners interact, and the filmmakers rely entirely on a limited color palette and flat camera set-ups to imply the emotional circumscription of the world and its inhabitants. It is a film that only ever shows, telling almost exclusively in the sequences surrounding Oliver's treatment program, and that having more to do with bureaucracy than mollifying the audience. Grappling with Starred Up is rough, but because of the attention and effort it takes to engage with it, it proves to be more engaging and vivid in its depictions than most of the prison movies all throughout history.

The one place where the film perhaps comes up short is in answering the age-old question: why am I watching this? In some ways, it feels like a cross between 2008's Hunger and 2009's A Prophet, which between them cover most of the aesthetic and narrative concerns that Starred Up ends up exploring, and both of them come with a broader sense of the implications of what they depict. The psychological fullness of Starred Up is dense and affective, but the film as a whole feels a touch familiar. Still, it's extremely good at what it's doing, with a wiry realism that suggests Sam Fuller and the Dardenne brothers making a film together, and two of the year's best male performances giving depth to the film's fascinating character study. Top-notch prison movies don't come by but every couple of years, and this is a pretty great one: lithe and mean and full of humanity.