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Imagine swallowing an apricot whole. Then another. And another. Feeling full? Tough. There's 50 more to ingest before you're done. Now imagine they're not apricots at all, but small rubber pellets filled with heroin. If just one of them bursts in your stomach, you're toast. And if any happen to pop out the other end before you reach your destination, bad luck: you'll just have to wash them off and swallow them again. God forbid any go missing either. Because if - - if - - you get past customs, there's some thugs waiting at the airport who won't think twice about slicing you open if they think you're holding out on them.

Welcome to the oh-so-glamorous world of drug trafficking, as viewed from the lowest rung of the narcotics ladder - - the desperate mules who risk life and liberty transporting smack from one America to another. It's a gruelling tale, related by writer/director Joshua Marston with a methodical, documentary realism that leaves nothing whatsoever to the imagination. But it's also a brilliantly dramatic one, as tense and gripping as any thriller and anchored by a terrific central performance from Oscar nominee Catalina Sandino Moreno. Of course, Maria is a criminal. But as debut helmer Marston sketches her dead-end life in rural Colombia with her soul-sapping job and useless boyfriend, we soon see that what would seem an insane risk to most is really the only option if you're poor, unemployed and up the duff. Of course, it's precisely these factors that make her such an easy mark for the gangsters who seduce her with the unattainable promise of a free holiday and more money than she's ever seen. But Maria's no fool and behind the naive exterior Moreno hides a fierce determination and resourcefulness that has us rooting for her every step of the way.

The central plane journey is such a nail-biter the film inevitably loses momentum Stateside. But it remains a fascinating glimpse at the human cost of the war on drugs, and will make you think twice the next time you seek out some Colombian marching powder.