When you hear that a new movie scrutinizes the hot-button topic of illegal immigration, you probably assume that the movie is about Mexicans slipping across the southern border.  The first surprise of Frozen River, the movie that won the jury prize as best film at this year’s Sundance Film Festival, is that it focuses on a much less familiar part of the immigration saga:  Chinese and Pakistanis being smuggled across the border from Canada.  But this memorable film is not primarily about the immigrants; instead it focuses on the smugglers–two desperate single mothers who form an unlikely business partnership.

First-rate movies establish a distinctive sense of place, and the desolate spaces of upper New York State are beautifully rendered by writer-director Courtney Hunt.  Frozen River is set in lonely small towns and the squalid Mohawk Indian Reservation on the edge of the St. Lawrence River.  This world, rarely exposed on camera, is marvelously detailed, and the characters come to vivid life.  Ray (Melissa Leo) is trying to hold on to her home and keep life bearable for her two sons after her husband takes off.  She accidentally meets Lila (Misty Upham), a Mohawk woman living in a tiny trailer and hoping to regain custody of her baby son.  The two women are suspicious of each other at the outset, but when Ray learns about Lila’s amateur smuggling operation, she recognizes an opportunity to make money that she desperately needs.

Both actresses give outstanding performances.  Leo may be remembered for her supporting roles in 21 Grams and The Three Burials of Melquiades Estrada, but here she carries a movie with authority and searing force.  Upham matches her with a hard-edged depiction of a bruised woman who won’t allow herself to expose any vulnerability.  Charlie McDermott as Ray’s angry teenage son also contributes a sharp portrayal, and Michael O’Keefe has a good cameo as a policeman who’s not entirely unsympathetic to the women’s dilemma.

The heart of the movie, however, is in the relationship of the two women.  Although they approach each other with wariness and resentment, they are ultimately bound together by their commitment to their children.  The ending is beautifully understated, but it’s a moving tribute to the power of the maternal instinct.