As debut features go, writer-director Courtney Hunt's Frozen River is perfectly serviceable, if not particularly exceptional. Laureled at Sundance, it would be a textbook example of a solid little indie movie that gets quietly dumped in the waning days of summer because the distributor can't really think of anything else to do with, except for this: it's graced by two of the strongest performances in any film released in the States so far this year.

The film is about two mothers driven to extremes to protect their children: first up, we meet Ray Eddy (Melissa Leo), whose gambling-addicted husband abandons her and their two boys shortly before Christmas, taking the money that was supposed to go towards upgrading their crappy single-wide mobile home to a less-crappy double-wide; while looking for him in the nearby Akwesasne Mohwak reservation on the New York-Ontario border, she bumps into the widowed Lila Littlewolf (Misty Upham), whose one-year-old baby was abducted by her mother-in-law; since this took place on the reservation itself, Lila has no legal recourse. The younger woman has been looking for a car to kickstart a sort of business smuggling illegal immigrants into the United States through the reservation, across the frozen St. Lawrence River; since Ray just so happens to own a car, it's not terribly far-fetched when the two women, both in dire need of a lot of money quickly, pool their resources to ferry as many people in Ray's trunk as they can manage without getting caught.

I'll give it this, it's not a story I've seen told elsewhere. And it's a pretty decent thriller to boot, considering what I'm sure were less-than-ideal shooting conditions. That said, Hunt seems to be aiming for a slow burn that doesn't really come off very well: instead of the sense that these women are getting deeper and deeper into a situation they can't control, it rather feels like an hour of nothing followed by a half-hour of contrivance. The big problem, I think, is in the particular story beat that shifts the story from one to the other (semi-major spoilers): Ray and Lila pick up a pair of Pakistani immigrants with a mysterious bag, and in her unforeshadowed anti-terorrist furor, Ray tosses the bag out into the snow. Lo and behold, it turns out to have contained the couple's child, and when Ray and Lila go back to find the baby, it's dead - except wait, it's not. There was no way out of this scenario, I fear: kill the kid and the movie veers into unearned miserabilism, keep the kid alive and it zips right on ahead into mawkish clichΓ©. Then there's the end itself, which I will not spoil, other than to marvel at the way it reduces what had 'til now been an ambiguous story about lawbreakers with honest motives into a chipper - but indie-appropriately cynical - celebration of can-do American pluck. It's not an ending honest to the characters or the story, and while it's not so happy as I just made it seem, it's quite a bit more "have your cake and eat it" than Hunt had been trafficking in 'til that point.

So anyway, back to where I started: thank God for the stars. Melissa Leo is maybe not a name you've heard, but it wouldn't be a surprise if you'd seen in her in something, anyway: she was on the genre-defying TV series Homicide: Life on the Street for its first four seasons, and she's guest-starred on several procedural shows. The cinephile might remember her scene-stealing turns in 21 Grams and The Three Burials of Melquiades Estrada. Frozen River is her first starring role, and she's never been better: her interpretation of Ray leaves plenty of room for us to recognise that the character is crude and selfish, even as we can't help but be moved by the obvious depth of affection she feels towards her children. It's the kind of performance that's easy to overlook because it doesn't have any Big Moments, just a fictional life so fully inhabited that it never crosses your mind that you're not watching a real woman.

All the way 'round from Leo's decade-and-a-half of strong character work, Misty Upham is in the first project she's ever done that any normal human being is at all likely to have heard of. If Hollywood is just - and isn't that a naΓ―ve way to begin a sentence - we'll be hearing a lot more from her in the future. Like her co-star, she inhabits the role rather than plays it, and she does so with a sweet fragility that contrasts with her character's violent and erratic behavior. It's absolutely the best kind of coming-out performance, the sort that wedges itself in your mind so that you're even more impressed long after the movie than you are watching it. Best of all is the interplay between the two actresses; their arc from mutual antagonists to pretty much each other's only friend is a more compelling story than the film's actual plot, and most of that is to the credit of Leo and Upham, who let that development slip into their performances without any clear signposts in the script for just where that should happen.

Beyond that, it's a functional, low-key movie; Hunt wisely recognises that her story is a tiny thing that would be damaged with over-the-top stylistic whorls, and so her direction never breaks out conservative adequacy; which is more than a lot of first-timers can claim, of a certainty. But based on this evidence, there's no reason to suppose one way or the other that she's got a formally elegant movie in her. Frozen River is over quickly and efficiently, but without those tremendous performances, it would certainly be easily forgotten. Still, quiet proficiency is a major plus for an indie film in this world of overbaked quirk, and I hesitate to take Hunt's unstrained lack of ambition for granted.