Fish Tank boasts one of the most leading, metaphorically unsubtle, and wholly appropriate titles of any film in recent months: perhaps primed by the opening credits, it takes a stronger-willed viewer than I to require more than a few shots to marvel that, by God, it is rather like watching a poor girl trapped in an invisible, polyhedral cage. To make sure we get it, writer-director Andrea Arnold tips her hand a little bit, and early on there's a close-up of a hamster (or gerbil: it's a domestic rodent, anyway) in its clear plastic hamster house, which almost precisely fills the frame - an unambiguous equation of the visual space of the movie with confinement, and while we've already picked up on that, subconsciously or not, I'm amused and pleased that Arnold feels the need to cover her bases like that. Don't worry, Madame Director, it's quite obvious what you're doing, but it's a swell shot anyway. Though I might point out that hamsters are not fish.

I'm inclined to silently look away from this and other moments when Arnold hits her subtext just a bit too hard, on the grounds that Fish Tank is in one key respect a tremendously non-standard movie: it was composed in the 1.33:1 aspect ratio (which is strictly speaking the traditional television ratio, and not quite the 1.37:1 of "full frame" movies, though the difference is virtually impossible to spot with the naked eye), instantly distancing itself from virtually every other theatrically-released project of the last several decades. Arnold has implied in interviews that she did this because that ratio is inherently more claustrophobic than others, which offended me as an aspect ratio snob; but in the particular case of Fish Tank (and really, that's what we're here to consider), she's absolutely right. This is an immaculately-composed film, a box to keep the protagonist trapped in her tiny world, one of the best examples I've seen in a while of a director using the frame to smother any implication of off-screen space. There is no getting out of this fish tank; that is the tragedy of it, or would be if the main character regarded herself as a tragic figure.

The formal precision of Fish Tank isn't just a matter for visually-attentive cinephiles to notice and approve of with knowing erudition - it's a key element of the film's meaning, even if you only pick it up subliminally (which, in this case, you certainly would). That close, cramped world is all that 15-year-old Mia (Katie Jarvis) knows at the beginning of the story, and she wants to get the hell out, but she's young and confused enough that her every attempt to change her life is tentative and halting, mired in her emerging awareness of her own sexuality. She has no other models, really: one of the first things we see in the film is her watching a group of girls her age, maybe a couple of years older, dancing to show off for some boys (the dancing, naturally, looks rather more like pantomimed intercourse than anything else). Nor does her mother Joanne (Kierston Wareing) offer any better example: she's just taken up with a man named Connor (Michael Fassbender), and the couple makes no effort to hide their largely physical relationship from Mia or her younger sister Tyler (Rebecca Griffiths), having noisy sex with the bedroom door half-open, and the like.

There's a plot to give away, and I'll refrain from doing it, but really Fish Tank is in its soul a character study: watching Mia struggle with her rapidly evolving self-identity as she tries to find some tiny measure of happiness, whether it comes in the form of practicing for a dance competition, or crushing hard on Connor (as well she might: after proving his talents beyond a shadow of a doubt in Hunger and Inglourious Basterds, Fassbender now gets to show off that he's gorgeous, too), despite also kind of viewing the older man as a brother/father figure. Certainly, he's the only person in the movie who's especially nice to her, ever, and it cannot help this poor, messed-up girl that his kindness is obviously based at least somewhat in his physical attraction to her.

Jarvis had neither experience nor formal training when she was cast, and one must wonder if she's just playing a version of herself, as certainly seems likely (she comes from exactly the same background, and one of the scenes in the film was taken from a moment in her own life). And then in the next breath, one admits that it doesn't matter. This is a stunning performance, better than hundreds of professionals have ever been able to muster after years in the business: unsentimental and refusing to beg for sympathy on Mia's behalf, but at the same time so earnest and real, leading us in to see the frightened adolescent hiding beneath the feigned bluster, violence and vulgarity of adulthood, that we can't help it: we care about her, deeply. It is necessary that Jarvis should be the key element of the cast, although everyone is good (even Griffiths, failing entirely to be a precious little brat-child), and Fassbender in particular gives another in his chain of miraculous performances that should be triggering his major period of stardom right about...

If there is a flaw with Arnold's miraculous study of adolescence, it's that, like I suggested up top, she doesn't quite trust herself, and so there are a few too many symbols, executed with the wincing clumsiness endemic to cinematic symbolism (including, I am sad to report, the very last shot). The worst is the heavy-handed use of a chained white horse that Mia keeps trying to free - can you begin to imagine that that might represent? - and generally, Arnold is on surer footing when she uses visual metaphors, like the oppressive use of red light in one scene, yellow in another (you'll understand the whys for both if you see it), or a real corker of a tracking shot near the end that watches Mia in a series of disjointed mirrors, fragmenting and coming back together as she walks away from what she thought was her future. At any rate, the story doesn't need all of these obvious signifiers, and it takes away a bit from the effectiveness of the film that the director keeps popping up to insist on it. As though her outstanding framing of the magnificent performances she pulled out of the actors didn't do that already.

It is both uncomfortable and even terrifying, but Fish Tank is a rewarding and vastly rich movie; putting honest, raw, pained humanity before our eyes without apology. In this it is a direct contrast to a certain pair of movies to which it bears more than superficial resemblance, both competing for the Best Picture Oscar; 'twould be déclassé to name them, but I think it points out a certain truth: there are movies that want you to like them, and they are given popular acclaim and mainstream acceptance, and there are movies that don't really exist for you at all, and this makes them much less sexy - but if you can find them, the rewards are all the greater for it. Fish Tank hasn't gone under the radar, exactly (it has picked up a chunk of festival awards, and a BAFTA), but the relative invisibility of its tiny U.S. release proves enough which category it belongs to. It's just too good for its own good.