AMONG THE MOST BITING and bitter moments in a movie chock-full of them is a scene that comes early on in Richard Linklater’s Fast Food Nation, a fictionalized adaptation of Eric Schlosser’s nonfiction best-seller about our national obsession with hamburgers, french fries, and eminently disposable calories. A marketing executive named Don Henderson (Greg Kinnear), who works for a McDonald’s-like fast-food chain called Mickey’s, has just been asked by his boss (Frank Ertl) to travel to Colorado to find out how fecal matter found its way into the company’s hamburger patties. Don is new to the job and doesn’t quite know what’s expected of him: Do the head honchos at Mickey’s really want to know how their meat gets processed? Or do they just want a corporate spin job? He expresses these worries to his wife (Dana Wheeler-Nicholson), who tries her best to be supportive. But really, she’s only half listening. Because at the same time, she’s also busy surfing the Internet, clicking her way through The Web site is presently having a sale on high-thread-count bedsheets.

You see, it turns out that Don is stuck on an all too familiar treadmill, forced to make ethical compromises so that his wife and kids can go on enjoying a whole lot of expensive stuff that they don’t really need. That neither Don nor his wife nor his boss even thinks to raise an objection—that they’ve all unquestioningly accepted that this is simply the way upper-middle-class life is lived at the dawn of the twenty-first century—well, it would be a tragedy if it weren’t so plainly appalling.

Let it be said straight away: Fast Food Nation (opening nationwide in November) might just be the most exciting movie of Linklater’s career. It’s hardly something we would have expected from the Austin-based director, who shuffles between indie projects (Slacker, Before Sunrise, Before Sunset) and big-studio ones (The School of Rock, The Bad News Bears remake) but whose work always displays a certain soft-spoken ease, an unwillingness to make much of a fuss about anything. Then again, Fast Food Nation isn’t something we would have expected from American filmmaking period. Despite the twin successes, in 2004, of The Passion of the Christ and Fahrenheit 9/11, Hollywood now seems more afraid than ever of alienating even a single movie-goer or of encouraging artists with distinct political points of view. (Last year, Brokeback Mountain only implicitly made a case for gay equality and marriage but was still deemed too polemical by Oscar voters, who gave the Best Picture prize to Crash instead.) The thrill of Linklater’s movie, though, is that it pays no heed to such rules of propriety or box office. Fast Food Nation is reckless, often didactic, sometimes a little loopy. But Linklater has never before displayed such purpose and zeal. He’s determined to make us swallow some very strong medicine, whether we want it or not.

First published in 2001, Schlosser’s studiously researched and reported book includes nearly seventy pages of notes and bibliography and chapter titles like “Why the Fries Taste Good” and “What’s in the Meat?”; you don’t exactly read it and think, “This would make for a rip-roaring melodrama featuring the likes of Bruce Willis, Patricia Arquette, and Kris Kristofferson.” Linklater told me that when he agreed to meet with Schlosser, in 2002, he intended at most to offer the author suggestions on how Fast Food Nation might be transformed into a documentary. Instead, Schlosser sold Linklater on the idea of a fictionalized version of the book, a kind of deep-fried Traffic, with multiple characters and story lines woven together to create a panoramic portrait of the American fast-food industry. We meet a group of illegal Mexican immigrants, including Sylvia (Catalina Sandino Moreno) and her boyfriend, Raul (Wilmer Valderrama), who cross the border and are transported to Cody, Colorado, where many of them end up working at the very same ruthlessly run, bottom-line-obsessed meatpacking plant that supplies Mickey’s with its hamburger patties. We also meet Amber (Ashley Johnson), a bright teenager who works at a local Mickey’s franchise and is slowly waking up to the social injustices all around her. And we follow Don on his stomach-churning investigation, as he uncovers political corruption, corporate deceit, and horrifying health-code violations at every turn.

“I do not mean to suggest that fast food is solely responsible for every social problem now haunting the United States,” Schlosser wrote in his book. Alas, Linklater seems to have had no such reservations. Scene after scene of Fast Food Nation is shot through with unmitigated outrage—about the way illegal workers are exploited, about how sexual harassment continues to go unchecked, about the way lousy products are cynically sold to American consumers, especially about the way the rich get richer while the poor go wanting for health insurance. By the time Ethan Hawke, as Amber’s uncle, shuffles onto the screen to bemoan the proliferation of Wal-Marts, Burger Kings, and Home Depots in Cody, Fast Food Nation seems approximately one step away from turning into a Communist screed. In interviews, Linklater has tried to downplay the movie’s ideological agenda. (“It’s not saying that if you elect a Democrat, all this is going to change,” he told me.) Make no mistake, though. This is a work likely to enrage the Fox News Channel set.

With any justice, it will also get movie-goers and critics to consider Linklater in an entirely new light. Fast Food Nation follows on the heels of Linklater’s A Scanner Darkly, an adaptation of a Philip K. Dick novel about paranoid junkies, created in the same style as the director’s 2001 film Waking Life, with animation grafted on top of live action. (Both movies had their world premieres at this year’s Cannes Film Festival, where Linklater was the first director ever to have two films in the official selection in the same year.) Scanner is certainly fun to look at, for about ten minutes, until it turns into a crushing, incomprehensible bore. Mostly, it serves to remind us that the director has frequently been drawn to the cultish and hipsterish—works designed to appeal exclusively to a certain breed of disaffected Gen Xer. (Remember Dazed and Confused? Tape? His misbegotten adaptation of Eric Bogosian’s play SubUrbia?) Even with his biggest commercial success, the shaggy-dog comedy The School of Rock, and his one certifiable masterpiece, the romantic drama Before Sunset, you sense a diffidence hanging in the air. Almost as if Linklater doesn’t want to call any attention to himself. Almost as if he’s afraid of working on a grander scale.

No longer. Fast Food Nation is the director’s first definitive epic. His first call to arms too. If we keep swallowing the junk blindly, Linklater warns us, our collective stomachache might just turn into an epidemic that threatens to destroy America whole. You don’t necessarily need to agree with the politics being shouted out loud here. But you can’t deny the tidal shift. The slacker has at last found his cause.

Christopher Kelly is the film critic for the Fort Worth Star-Telegram.