When Sam Goldwyn said his famous line about how filmmakers wanting to send messages should use Western Union, he didn't take into account the possibility of smuggling. On the other hand, John Carlos Frey, the first-time writer, director, and star of The Gatekeeper, has taken the smuggler's craft to heart, wrapping a lesson in the brutal realities of the U.S./Mexican border in an intense thriller that suggests a gripping paperback as reworked by the staff of Harper's. In a setup that owes a debt to The Believer, Frey plays a U.S. border-patrol agent with a moonlighting gig for a far-right vigilante group overseen by incendiary talk-show host J. Patrick McCormack. Viewing illegal Mexican immigrants as an invading horde intent on taking control of America, they enforce the border on their own at night, eventually embarking on a plan to send Frey undercover to bust up an immigrant-transporting racket. But they don't know about the self-loathing that drives Frey, who turns the rage of a difficult childhood against his Mexican mother and the cultural heritage he viciously disowns. As a writer and director, Frey can't be accused of excessive subtlety: At one point, he has his character smash a lawn-ornament Madonna to punctuate a scene. But the same willingness to plunge into luridness and melodrama allows The Gatekeeper to work as a taut suspense film on its shoestring budget. A second half set in a drug lord's slave-labor camp loses sight of the internal conflict that drives the first half, but it still makes its horrors almost palpable, portraying life among people whose first taste of America comes seasoned with abuse. Frey delivers his message, and makes sure his audience will never forget it.