Be warned: Within its first few minutes, "Under the Same Moon" will reach out, grab your heart and squeeze, hard.

This often wrenchingly moving film — about a 9-year-old boy searching for his mother across the U.S.-Mexico border — exerts immediate, irresistible power, combining an affecting story, indelible characters, urgent topical relevance and superbly calibrated sentimentality. As this absorbing story wends through the harrowing realities of modern-day immigration, the audience's only recourse is to hope ever more fervently that "Under the Same Moon" won't break the hearts it has so swiftly and thoroughly captured.

Thanks to the uncommonly shrewd judgment of screenwriter Ligiah Villalobos and director Patricia Riggen, both newcomers, "Under the Same Moon" never feels like rank exploitation, even as it steadily aims for the emotional jugular. They keep their story gratifyingly simple: Rosario (Kate del Castillo) has been living in Los Angeles for five years, working as a maid and sending money back to her son Carlitos (Adrian Alonso), who lives with his grandmother in Mexico. A sudden turn of events inspires the bright, resourceful Carlitos to travel to California to find Rosario, even though he possesses only a mailing address and a vague notion of where she calls him every week from a pay phone. "Under the Same Moon" follows the boy for an event-filled week during which he crosses illegally into the United States and along the way encounters a series of strangers who aid, abet and betray him.

Deftly cutting between Carlitos' journey and Rosario's life in L.A. — where she must cope with imperious bosses, a romantically interested suitor and a governor who recently ruled she won't be allowed to drive legally, consigning her to hours on the bus — the filmmakers present a vivid, almost palpable image of the lives of undocumented workers who sacrifice everything to escape grinding poverty and offer their children something more. "No one chooses to live this way without a good reason," observes one of Carlitos' traveling companions. That simple truth takes on unforgettable life in a story that traverses tense border checkpoints, chemical-laden tomato farms, restaurant kitchens and L.A.'s faceless, imposing metropolis.

In addition to smoothly advancing Rosario's and Carlitos' parallel stories during the week's tick-tock, Riggen has done a terrific job of casting, assembling an ensemble of unfamiliar actors who deserve to get their breaks here. Alonso, as the sturdy yet vulnerable Carlitos, delivers a turn that recalls silent film star Jackie Coogan in its winsome somberness. When the youngster befriends a reluctant Samaritan, played by Eugenio Derbez with scowling diffidence, it's as if the Tramp and the Kid were beamed to 21st-century Ciudad de Juarez.

As hugely appealing as Alonso is in his role as chief heartstring-puller, del Castillo proves to be yet another dazzling discovery, an actress of smoldering beauty who projects a serene air of authenticity whether she's dusting a glass end table, embarking on that endless commute, or dancing with a suitor at an impromptu party. (Among many things, Riggen also deserves credit for assembling a fabulous soundtrack of mostly conjunto ballads; the band Los Tigres del Norte even shows up for a playful cameo.)

If "Under the Same Moon" dramatizes a serious, even potentially tragic story, the tone never succumbs to unremitting grimness. Instead, the filmmakers regularly leaven their story with little unexpected grace notes of humor and tender irony. The sight of Alonso and Derbez dancing and singing at the top of their lungs in a coffee shop kitchen offers proof alone that the filmmakers appreciate the joys of life far more than its maudlin twists of fate.

Which isn't to say they don't ratchet up those twists to an almost excruciating degree. The fact that "Under the Same Moon" has been crafted with such an adroit, sensitive touch should reassure viewers that, as it approaches its utterly gripping climax, their hearts are in good hands, too.