Melissa Leo owns “Frozen River.” The film dwells in that uncomfortable place where long bets are about to go bad, and Ms. Leo’s unrelenting focus carries the viewer through it all, her determination maintaining the unrealistic hope that things will work out in her favor.

The accomplished actress, who earned a devoted following for her turn as Sergeant Kay Howard on the television series “Homicide,” has failed to gain traction in mainstream films, but that may change with her performance in director Courtney Hunt’s festival favorite, which arrives on New York screens Friday. Her unassuming expertise in navigating the lead role here has been recognized and should be rewarded soon.

Ms. Hunt has created a nuanced drama with a grip that tightens as the minutes tick by. The filmmaker took the Grand Jury Prize for best feature at this year’s Sundance Film Festival for this dissection of the tenuous lives of two immigrant smugglers on the New York-Canada border. The film refrains from the broader arguments surrounding the debate on immigration, focusing instead on the choices made by these desperate women.

In lesser hands, the film could have wandered into movie-of-the-week territory. Ray Eddy (Ms. Leo) is a single mother robbed of her savings by her husband. Her need to make ends meet leads her to the high-earning, high-risk task of smuggling Canadian Indian immigrants across the border. Working with a Mohawk woman named Lila (Misty Upham), who is also a single mother, Ray begins to lead immigrants across the porous Canadian border over the ice-locked St. Lawrence River into rural upstate New York — to a place she would love to leave.

From the opening shot, in which Ray holds back tears in a disintegrating fuzzy pink robe on the morning after her husband has bailed with the money earmarked for their new trailer, Ms. Leo takes hold of the screen. The range of emotion and depth of character that she achieves in the most minute detail is enthralling. Her character is strong while shattered, at once terrifying and endearing. Bare of makeup, her deeply creased face betrays the burden of her hardship before she even speaks.

Ms. Leo’s selfless approach allows the character of Ray to come into full focus. The actress’s spare performance tactfully avoids the weepy push points that typify most single-mother dramas. Her Ray is strung out, bigoted, tough, and robust, but almost always endearing.

Ms. Leo’s nuanced performance provides a framework for the other characters. When Ray first encounters Lila, who lost custody of her newborn when her husband died in a failed smuggling run, her luck briefly turns from bleak to hopeful. After an initial confrontation, Ray convinces Lila to include her in the smuggling venture until she can get the money for her double-wide. She may not condone the methods or the results, but Ray appreciates the potential earnings involved. Similarly, Lila is both comforted and trapped by the laws on the reservation.

While Ray is off with Lila, her sons are left to their own devices. James Reilly plays Ray’s youngest son with youthful exuberance, while Charlie McDermott’s turn as the rebellious 15-year-old T.J. reflects the hardheaded resourcefulness of Ray, only in adolescent relief. T.J. may not always make the right decision, but he always tries to.

As the story progresses, the similarities among all the characters become clearer. Everyone is placing bets they can’t afford to lose.

Putting their morality on hold for the sake of profit, Ray and Lila help bring Chinese and Pakistani immigrants across the solid lake, ignoring what will happen to them once they reach their destination. Their survival instincts acutely tuned, the women might just get what they need. But Ray’s stranglehold approach to negotiation has its flaws. While the prospect of success, mingled with the false security of Ray’s ivory skin and Lila’s attachment to the reservation, keeps them moving forward, it can all be gone with one bad break.

Ms. Hunt has done an excellent job of melding the tendentious hopes of her heroines. As we watch them struggle to get out from under their various burdens, “Frozen River” achieves a gripping narrative, exposing the consequences of clinging to hope at all costs.

mkeane@nysun.com