“Maria Full of Grace,” a riveting, unsparing drama that takes the audience — excuse the atrocious pun — deep into the underbelly of the drug- smuggling trade, excels as both a human-interest story and a consciousness-raising, John Sayles- style exposé. With insight and keen sensitivity to detail, first-time writer/director Joshua Marston tells the story of Maria (Catalina Sandino Moreno), a willful Colombian teenager who resents having to financially support her mother and unmarried older sister.

Early on, Marston fills the screen with symbols of Maria's dissatisfaction — she doesn't like the taste of her mother's food, is bored with her boyfriend and impulsively climbs gutted buildings to get a better view of the world. Maria's problems multiply tenfold with an unexpected pregnancy. Jobless, broke and unwilling to marry her boyfriend, she recklessly accepts an offer from a stranger to work as a “mule,” smuggling heroin into the United States in tightly compressed capsules, which are encased in latex and given to her to swallow.

The gig pays obscenely well — about $6,000 for the trip, enough to put her family on easy street in financially deflated Colombia. The process — from ingestion to retrieval — constitutes the most troubling, nerve-wracking 40 minutes of cinema this year. The pellets are dipped in egg whites to speed their journey down Maria's throat. After swallowing about 60 of them — a sequence that feels ineffably invasive, almost sexual — Maria is placed on a plane with several other mules bound for New York City.

Of course, we anticipate the worst for Maria — surely, morning sickness or some other pregnancy-related malady will tip her hand — but Marston feints the other way and takes the suspense in an entirely surprising, satisfying direction.

Moreno's performance is so appealing that we improbably find ourselves rooting for Maria's safe passage — nervous and bewildered, the character always manages to find a hidden reserve of dignity and strength. Not to be flip, but it's a bit like “The Good Girl” meets “El Norte.” Later, and less grippingly, “Maria Full of Grace” shifts its focus to Maria's new baby and her notional American dream.

The suspense sags, but the performances have resonance: Guilied Lopez is excellent as a Colombian woman living in New York City who becomes a sort of role model for Maria, and Yenny Paola Vega is haunting as Blanca, a stunning young woman who serves as Maria's mule-mentor and ultimately suffers the worst fate possibly meted out by that horrible profession. Through her, we see these girls less as criminals and more as victims of circumstance.