William Friedkin's "Bug," based on the play by Steppenwolf's Tracy Letts, is a portrait of escalating horror in modern America, a nightmare that may or may not be taking place in the minds of an emotionally battered couple clasping and rotting together in a dingy desert motel. Physically and psychologically disintegrating before our eyes, they're played by Ashley Judd as Agnes White, and a relative newcomer, the remarkable Michael Shannon, as Peter Evans.

They're both spellbinding; Agnes is a divorcee whose violent jailbird husband Jerry (Harry Connick Jr.) has just been paroled; she takes up with Peter, a drifter from the Gulf War, whose apparent sweetness belies something dark and sinister: the menace of the bugs, of course. Peter claims that aphids have been implanted under his skin by evil government forces, and soon after meeting Agnes, he starts to dig and gouge them from his flesh. (We never really see the aphids, although Peter claims he does.)

"Bug," most of which is set in Agnes' lonely motel rooms, plays like a mixture of Sam Shepard's chamber dramas ("True West," "Fool for Love") and David Cronenberg's anatomical horror movies, such as "Shivers" and "Rabid," those shockers about malign forces growing inside people, erupting out of their skins.

One of my all-time favorite magazine blurbs was for the '50s Galaxy Science Fiction story Theodore Sturgeon's "When You're Smiling": "It's a shock wave of terror, with a jolting, blinding conclusion." That fits "Bug" too. It's a nerve-rending, extremely unpleasant experience but also a compelling one. The film also represents a real resurgence for Friedkin, who became a directorial superstar with his '70s classics "The French Connection" and "The Exorcist" and has been mostly wandering in a critical-commercial wilderness since then.

But "Bug" is a movie with something piercing to say about American life and paranoia--about the real-life horrors of government medical experiments and amoral covert action--and it's brilliantly acted.

Judd starts Agnes off as one of those earthy dames she excels at portraying. Then the actress simply tears the character apart, making her crazier and crazier, turning her apartment into a Howard Hughes-style maze of antiseptic insanity--with plastic covers over everything and purifying fires. Judd is so good we can feel her madness, and we pity her more because it's born partially out of loneliness and love.

Crooner Harry Connick Jr. is a good heavy, a seductive yet brutal guy reeking with narcissistic charisma. And Shannon, who played the stage role of Peter and looks like a weird cross between Ray Liotta and Anthony Perkins, is the main source of that shock wave of terror. Even before Peter begins to seem dangerous, his soft-spoken, seemingly rational explanations of the aphid plot and other horrors chill you to the bone.

What's the movie's truth? That bad dreams can drive us crazy? Or that on some level we're living in a horror movie? Friedkin, who specializes in both gritty realism and nervous fantasy horror, frequently gets both here. He shoots the fear-drenched lovers with a grim Edward Hopperesque starkness and intensity that evokes the horrors within and without.