Documentaries have become so chic this season that it's all too easy to forget that sometimes it takes great fiction to tell a certain kind of truth.

"Maria Full of Grace" is a gripping, deeply moving film, one that has the added distinction of marking the exceptionally promising debuts of its writer-director and lead actress. What's more, the film unfolds with an urgency and authenticity that many of even its most noble nonfiction contemporaries lack. "Maria Full of Grace" seems to have been sent from the cinematic heavens to remind viewers of the honesty and sobering realism of which movies are still capable.

Catalina Sandino Moreno portrays Maria Alvarez, a 17-year-old girl who works on a flower plantation in a small town in Colombia. Work on the plantation is monotonous, physically punishing and low-paid, and when Maria becomes ill one day on the flower-trimming line, her boss makes it clear that he's more concerned with the roses than her well-being. Disgusted, the high-spirited Maria quits and is soon faced with looking for a new way to support herself as well as her mother, sister and baby nephew.

When a new acquaintance tells her about an easy way to make money, smuggling drugs to America, Maria agrees to give it a try. Soon, she is swallowing more than 60 latex-encased pellets of powder that she will evacuate once she arrives in New Jersey. On her maiden voyage, however, something goes terribly wrong, and soon Maria is alone on the streets of New York, on the lam from her drug-dealer bosses and unsure of her own increasingly precarious future.

Like 2002's "Dirty Pretty Things," "Maria Full of Grace" is structured and paced to exploit the story's inherent suspense. Especially during the harrowing scenes of Maria's preparation for her first trip to America, and the gut-wrenching plane ride itself, the film has the taut nerviness of the most riveting thriller. But also like "Dirty Pretty Things," in which a lowly cabdriver in London served as a guide through that city's otherwise invisible immigrant community, "Maria Full of Grace" pulls back the veil on Maria's largely unseen world.

Traveling from her tiny village to Bogota to New Jersey to Queens, Maria is an assured, if occasionally terrified, guide through communities that most Americans know nothing about, even if we wouldn't have our fresh roses or clean houses or recreational drugs without them.

But if "Maria Full of Grace" has an undeniable political subtext, it never succumbs to flat-footed didacticism. Writer-director Joshua Marston has created an organic story that seems every bit as spontaneous as his lead character's own impulsive choices. For all of the pathos and almost epic courage of Maria's journey, it never seems the least bit contrived or juiced-up for dramatic impact.

Marston presumably achieved the film's amazing naturalism over the course of a three-week rehearsal and improvisation period with the actors, some of whom were untrained. That preparation has held him in good stead, with "Maria Full of Grace" brimming with potent performances. In addition to affecting turns from Yenny Paola Vega and Guilied Lopez, who play Maria's fellow "mules," the film is given a surprising sense of gravitas by Orlando Tobon (a real-life community activist who plays his own fictional counterpart) and Patricia Rae, who delivers the movie's most moving soliloquy on the mixture of grief, longing, pride and bravery that define the immigrant experience.

In the end, though, "Maria Full of Grace" belongs utterly and completely to Moreno, who takes hold of the camera from the very first scene and never lets it go. Cast while still an acting student in Colombia, this self-possessed beauty carries the movie with supreme confidence, managing to keep the audience on Maria's side even when she makes painfully self-destructive choices. In this captivating, unforgettable performance, Moreno lives up to the movie's title, never letting viewers forget the inherent moral worth of a young woman they would otherwise think of with disdain or condescension, if they thought of her at all.

Maria Full of Grace (101 minutes, in Spanish with subtitles, at Landmark's E Street Cinema and Bethesda Row, Cinema Arts Fairfax and Loews Shirlington) is rated R for scenes involving drug use and language.

Catalina Sandino Moreno as a poor Colombian girl trying to smuggle heroin in "Maria Full of Grace."As Maria, Catalina Sandino Moreno (with Wilson Guerrero) grabs the audience's hearts.