"Bug" is based on a stage play by Tracy Letts (he adapted it for the screen himself), and it shows. It also was shot on an indie-sized budget and lucked out with a wide release from distributor Lionsgate. The film, dishonestly marketed as a sci-fi/horror film about invading bedbugs, stands safely outside of the mainstream. Its tone is unforgivingly bleak. Its setting is almost entirely within the confines of a motel room. Its script is talky. Its pacing is unhurried and deliberate, taking the time to set up character nuances and an atmospheric, sweat-entrenched milieu. It really isn't even a horror movie at all, at least not in the conventional sense. Although director William Friedkin (2003's "The Hunted") isn't entirely successful with "Bug"—the film is a palpable experience, but more admirable than satisfying—what he does bring to the table in spades is a pall of increasing dread that culminates in a nightmarish crescendo not easily forgotten.

Agnes White (Ashley Judd) is a shut-off, broken-down woman who works as a server at a lesbian bar and spends the rest of her time at the desert motel she lives out of. Haunted by terrible memories from her past, she works to ease the pain with a steady diet of drugs and booze. When coworker and friend R.C. (Lynn Collins) invites a customer—the reserved, good-looking Peter (Michael Shannon)—to come hang out with them, Agnes is disinterested. She's had it with men, after all, and has enough to worry about with the knowledge that abusive ex-boyfriend Jerry Goss (Harry Connick Jr.) has been released from prison. And then something happens that takes Agnes by surprise; after R.C. leaves, she and Peter (Michael Shannon) start chatting and hit it off. She invites him to spend the night—on the couch, that is. One day together turns into two, and in a moment where vulnerability and loneliness meets a reciprocal desire for connection, they sleep together. Afterwards, Peter is bitten by a bug in the bed. Agnes is unable to see it at first with her naked eye, but then she does. Before long, both of them are covered in bites and scabs, and no matter what they do they are at a loss to stop the infestation.

"Bug" is about creepy-crawlies as much as "Magnolia" is about frogs and "The Squid and the Whale" is about sea creatures. A psychological portrait of paranoia and the intoxicating influence one person can have over another, the film is a character study first and a bold and nasty little thriller second. For a long time—well into the second act—the film sets itself up as a straight romantic drama, with the troubled Agnes learning, little by little, that there might be another person in the world for her. By the time it becomes glaringly clear that Peter is very mentally ill, it is too late; he has already dragged Agnes into the twisted rabbit hole with him, and everyone else—the caring R.C., the bullying Jerry, the skeptical doctor that Agnes goes to see—ceases to matter.

Ashley Judd (2004's "De-Lovely") is brilliant as the emotionally scarred Agnes. A brave, authentic performance free of vanity and willing to go to some tragically dark places, Judd is a shattering powerhouse from her first scene to her last. In the beginning, one can sense a light in Agnes' eyes; despite her problems, there is still hope for her. The way in which Judd and director William Friedkin depict that flame petering out, even as Agnes experiences true, kindred love for the first time, is the most disturbing element in the picture. The other performances are uniformly strong, too. As Peter, Michael Shannon (2007's "Lucky You") is disturbingly believable, using his newfound power over Agnes to, ultimately, brainwash her. What is intriguing about Peter is that his relationship with Agnes isn't intentionally malicious; he genuinely believes that microscopic bugs are in his blood and under his skin, and Agnes is too weak and impressionable to question it as anything but truth. Harry Connick Jr. (2003's "Basic") is great at playing a despicable sleaze-ball as Agnes' feared ex Jerry, and Lynn Collins (2007's "The Number 23"), vivacious and likable, rounds out the major players as R.C.

If the actors are convincing in their roles, the script does not hold quite the same scrutiny. A suspension of disbelief is required to accept that Agnes would fall so deeply and quickly into Peter's sick world, and the ceaseless dialogue could have afforded a few cuts to tighten the structure. It may be superficial, yes, but one also must wonder where Agnes was keeping her two hundred rolls of tin foil when she and Peter cover the room, top to bottom, in it. "Bug" is a slow-burner, but when it gets down and dirty—a scene involving a tooth and pliers is hard to take—its aim for the jugular is all the more jolting because of that simmering pace. There is additionally something to be said for an ending as courageous and yet hopeless as the one cooked up; had this started life as a studio film, there is no way the conclusion would have been left unscathed. Meanwhile, the incessant buzzing of the ceiling fan, taking on the sound of an approaching helicopter, menacingly looms over the proceedings like an uninvited, possibly dangerous spectator closing in. "Bug" is a difficult watch and how much it actually offers outside of pure discomfort is open for debate. Nonetheless, the film makes no apologies for what it is, and is fascinatingly macabre in the surprising places it takes its audience. Love the movie or hate it, no one could ever accuse it of being cookie-cutter.