Rebellious 15-year-old Mia (Katie Jarvis) seduces her mother's boyfriend in the British drama Fish Tank. (Mongrel Media)

Mia (Katie Jarvis), the young teen at the centre of Andrea Arnold’s Fish Tank, is perpetually in motion. At the film’s outset, she’s in the grimy parking lot outside her council flat, marching towards a trashy Essex county girl she intends to head-butt. Mia moves with swift determination, her wiry arms ready to take a swing at anyone who gets in her way. It’s the strut of someone who’s angry at the world.

Director Andrea Arnold shows a keen instinct for depicting female sexuality at its most frightening. She isn’t afraid to go to some dark places.

She has good reason to be enraged. At home, nurturing is scarce. Her party-girl mother, Joanne (Kierston Wareing), is rarely around, and when she first appears, we see her kicking Mia to the ground. Slinking around in body-hugging sundresses, Joanne is either hollering at her daughters or entertaining men with booze and reggae music, too drunk to even notice her kids.

Mia’s only respite is hip-hop dancing, which she studies in music videos, then attempts to emulate in an abandoned flat a few floors up from her own. Watching her moves gives us a hint of the teenage-girl emotions roiling inside her. As she crosses her arms across her chest, gangsta-style, it’s the defensive tomboy pose she adopts on the Essex streets. But when she swivels her hips and gets swept up in her own fluid, graceful movements, a flicker of something much more womanly emerges, suggesting all the urges she’s trying to hide beneath her surly veneer.

Mostly, Mia spends her days crashing around — ditching school, getting drunk and looking for trouble. It’s still preferable to staying inside the claustrophobic public housing she lives in, something Arnold deftly communicates through close-ups and tight framing that make Fish Tank feel like it’s taking place inside a tiny square box.

When mom’s latest boyfriend, Connor (Michael Fassbender), arrives on the scene, he brings some much-needed air into Fish Tank. Laid back and gregarious, he’s a welcome change from the unreliable, moody Joanne. He’s not into yelling, and likes to tease and horse around with both daughters. It’s Mia who benefits most from his presence, finding a jokey buddy first, then a warm surrogate dad who praises her, even encourages her to attend a dance audition that could provide a way out of her squalid surroundings.

But Mia is only 15, and stronger feelings start to burble up. When Connor tries on a new cologne, Mia leans in for a sniff, lingering for a beat longer than necessary. During a family excursion to a rural area more idyllic than anything Mia has ever witnessed, she ventures into the water to help Connor catch a fish. The air crackles between them, and their breathing can be heard on the soundtrack. The moment passes, but when Connor gives Mia a piggyback, and she rests her head against his shoulder, it says everything about the confused longings of your average teenage girl.

Arnold never shies away from exploring what’s going on in this complex relationship, and her two lead actors are equally brave in their handling of the unsettling bond that develops between their characters. Fassbender is so subtle throughout, it’s almost impossible to make a definitive judgment about Connor. (With this role and his work in Hunger and Inglourious Basterds, he’s a star in the making.) Meanwhile, Katie Jarvis, a newcomer Arnold discovered on a Tilbury train station platform, gives the kind of fearless performance that can only come from an untrained actor. She isn’t aware enough of the camera yet to censor herself when messy emotions slip out, and this gives Mia an unpredictable, raw quality.

Mia's interest in her mother's boyfriend, Connor (Michael Fassbender), is reciprocated. (Mongrel Media)

Arnold’s film is already earning her comparisons to British kitchen-sink director Ken Loach. While Fish Tank is clearly in the harsh realism mold — one of its most striking scenes unfolds to Nas’s song Life’s a Bitch — there are elements here that suggest there’s more to her style than the depiction of misery. As in her riveting first feature, Red Road, Arnold shows a keen instinct for depicting female sexuality at it most frightening and unruly. She isn’t afraid to go to some dark places. In this instance, she asks the troubling question: what fresh hell ensues if a 15-year-old gazes at a grown man and he dares to gaze back?

Fish Tank isn’t perfect — the recurring metaphor of a grey horse in chains feels heavy-handed, and one key scene near the film’s end nearly goes over the top. But to quibble over minor points in the script feels like nitpicking when Arnold is more interested in capturing moods and emotions. Working with cinematographer Robbie Ryan in a low, natural light and seizing upon images of swirling blackbirds, a cracked car windshield, some tacky palm-tree wallpaper and a fish gasping on a river’s bank, Arnold says more about Mia’s restless moods than a script ever could.

The end result is staggeringly beautiful, but it’s also the kind of small, independent movie that could get lost alongside more mainstream fare. That would be a real shame, because much like Mia herself, Fish Tank is a stunner, hidden amidst the trash, dead grass and muck.

Fish Tank opens in Toronto on Feb. 19.

Lee Ferguson writes about the arts for CBC News.