If Fish Tank had been made ten years earlier, it would have been directed by Lynne Ramsay and starred Samantha Morton. That's a sizable compliment to both writer-director Andrea Arnold and star Katie Jarvis, though Arnold may not need the additional boost -- her first feature, Red Road, had already won the jury prize at Cannes, and she made it two for two when Fish Tank walked away with the same award. Acting newcomer Jarvis, on the other hand, is an unexpected revelation. She brings the soulful eyes and spitfire attitude of a young Morton, in a more wiry frame, to the central role of Mia, a rebellious teenage girl who's sort of looking for a father figure and sort of looking for a lover in a housing project in lower-class Essex. Mia finds that candidate in her mother's boyfriend, played by the breakout star Michael Fassbender. Amid Mia's chaotic world of physical scrapes and yelling matches, Arnold documents that budding relationship with a tenderness that makes you hope it's the paternal bond she needs, but secretly fear it's the carnal bond she wants. The film's unwavering commitment to truth leaves the former outcome unlikely and the latter a near certainty. Where the relationship goes contains shocking surprises that take your breath away. Jarvis is simply astonishing, etching one of the more unforgettable portraits of chip-on-the-shoulder toughness mixed with teenage vulnerability, and Fassbender has a smoldering chemistry with her that's independent of sexual attraction, born of two talented performers connecting amid a sea of narrative bleakness. Arnold pulls off a difficult feat here, presenting the audience with one disappointing sample of human weakness after another without actually making us feel glum. Perhaps it's just that filmmaking as good as Fish Tank should leave a spring in the step of any serious film fan.