Misty Upham, left, and Melissa Leo star in "Frozen River."

When the two women first cross the river together — in the dead of winter, in a rickety Dodge Spirit — Melissa Leo's character, Ray, says what the audience is thinking: "This is so ... stupid."

Well yes, it is, considering that each time she and Misty Upham's Lila drive across the rugged, icy expanse that separates Quebec and the Mohawk Indian Reservation of upstate New York, they have illegal immigrants stowed in the trunk — men and women from China and Pakistan willing to take this risk for the possibility of a better life.

It's a miracle Ray and Lila make it from one side to the other once much less several times without getting caught or, worse, falling through the precarious surface to their deaths. But then, "Frozen River" is a bit of a miracle itself: a small movie with hugely moving performances.

"Frozen River" is so quiet and precise and self-assured, you'd never know it's the feature debut of writer-director Courtney Hunt. Here, she's come up with that rare thing: a film that feels completely original.

Much of its allure originates from the natural, understated work of its actors, namely Leo, long known for her excellent character and supporting roles on TV's "Homicide: Life on the Street" and in such films as "21 Grams." In her hands, the raven-haired Ray is never a caricature of a desperate mom, struggling to support her two kids alone once her no-good husband takes off just before Christmas. She makes us feel like we're watching a real person.

Ray works as a cashier at the local dollar store and is trying to save enough money for the down payment on a new doublewide trailer, which would be a huge step up from the cramped quarters she currently shares with her two sons, 15-year-old T.J. (Charlie McDermott) and 5-year-old Ricky (James Reilly).

Only when Ray catches Lila trying to steal her car from the parking lot at the bingo parlor does her luck look like it might change. Lila, a Mohawk, is a smuggler — first cigarettes, now people. She recruits Ray to help her ferry her illegal cargo across the river between Canada and the reservation.

The two form an unlikely and uneasy alliance — "I don't usually work with whites," Lila says in her typical clipped sullenness — but the money is good, and both women need it. Both are also survivors, which gives them strength and makes them dangerous.

But Hunt makes plenty of room in her script to reveal the softer side of Ray and Lila, to shed some unexpected light on their motivation. She also wraps up "Frozen River" with one of the loveliest endings you'll see — one that offers just the right glimmer of hope without eroding the film's bracing sense of realism.

<b>TITLE: </b> “Frozen River”<b>CAST: </b>Melissa Leo, Misty Upham, Charlie McDermott, James Reilly<b>DIRECTOR: </b>Courtney Hunt<b>RATED: </b>R for some language<b>GRADE:</b> 3½ Stars (out of 5)