In "Witness," Australian director Peter Weir brings his fascination with the clash of cultures to the States, replacing his aborigines and Indonesians with the Amish, religious separatists hidden in a sylvan Pennsylvania of dogmatic nonviolence and bygone ways. A conventional doomed romance stapled onto an even more conventional cop thriller, "Witness" never hangs together; and Weir's sentimental view of Amish life is enough to make you swear off egg noodles for life. Still, the high points make it worth the ride. Showcasing no fewer than four performances, each as shaped and delicate as cut glass, "Witness" can sparkle with a rare kind of movie magic.

Waiting with his widowed mother Rachel (Kelly McGillis) in Philadelphia for a connecting train, Samuel (Lukas Haas), an 8-year-old Amish boy, heads for the men's room, where, peering from his stall, he witnesses a brutal murder. Investigating, Capt. John Book (Har- rison Ford) discovers that the victim was an undercover cop murdered by other cops involved in a French Connection-style drug conspiracy. The corrupt constables give Book a taste of lead; spooked and wounded, he gathers mom and boy and lights out for the sanctuary of Amish country.

Nursing him back to health with linseed oil salves and specially brewed teas, Rachel falls in love with Book, he with her. But he's an "English," a "Yankee," and a whispering campaign grows to have her "shunned." This practice of ostracism is something besides quaint -- there's a certain edge to the screenplay (by Earl W. Wallace and William Kelley), a sense of both the homey virtues and the dangerous backwardness of the closed, inbred Amish community.

Weir, though, discounts the dark side -- he's in love with their simplicity. He daubs the Amish (particularly McGillis) with a snowy white light, and he ignites the women's white kerchiefs till they glow like halos. Every time the story ventures doubts about these people, Weir throws a low-angled Portrait of the Saints on the screen. Movies being what they are, it's these images, not the nuances of the script, that stick with you.

Weir loves the Amish so much, he sometimes loses control -- the movie's production number, in which the men get together to erect a barn while the women embroider quilts, has the jostling, insistent style of a television commercial. And when Rachel and Book finally run in slow-motion to their inevitable clinch, it's a dead ringer for that old "the closer he gets, the better you look" ad. Add to this Maurice Jarre's score, a lot of fake Aaron Copland alternated with weird synthesizer sounds that bludgeon you with how "foreign" and "ethereal" the Amish are.

Wallace and Kelley skillfully weave most of the Amish people's quirks into the story (such trappings of civilization as telephones and radios are forbidden, and even buttons are the Devil's work -- their coats fasten with loops and hooks). Still, there's a textbook air to some of "Witness," the braggadocio of thorough research, with incidents included simply to show off "fun facts" about the milieu.

"Witness" is framed with action sequences. The first, punctuated with the rapt innocence of the child's face as he peers from the toilet stall, has a shocking power; but by the end, Amish pacifism has seeped into Weir's soul -- he winces and turns away from the final shootout. Separated from the love story, in which Book struggles to make friends in a hostile milieu, "Witness' " cop conspiracy subplot is familiar stuff. And because it's such an obvious ploy to get your blood pumping, the disjunction is automatic -- the love story never really informs the thriller.

What saves "Witness" is its cast, and Weir's artistry in orchestrating and modulating their performances. Hoisting up his cheek in a skeptical wink, Ford is a handsome beagle raising a leg to sanctimony; he's the classic American hero and the classic American wise guy rolled into one, and his shtick (with the sober Amish as a tribe of straight men) can be awfully funny. Ford's derisive tough-guy stance is so familiar and pleasing by now, it's actually a liability here -- when he stands up for the Amish by thrashing a local tough who taunts them, you're standing up and cheering before Weir, who disapproves, can get a word in edgewise.

Ford seems to realize this, and he's willing to stretch. If the essence of his persona is his mastery of every situation, Ford looks out of sorts in his Amish exile -- the wink freezes halfway as he realizes that his old riffs sound hollow here. And nothing dissolves Ford's aplomb as much as McGillis. When he looks at her, his eyes boil in his head, and his lip curls like a leaf in a campfire. Love scares him.

McGillis has the same kind of clear, American Girl beauty as Grace Kelly. When her patriarch of a father confronts her with her love for Book, she replies that she's committed no sin -- in her pellucid, cornflower-blue eyes, lust is sanctified. McGillis is so pure, her beauty seems to float off the screen, but the wry, knowing arch of her eyebrows suggests something else beneath the surface. In "Witness," McGillis has to show the audience the slow welling of her heart without ever showing the same thing to the other characters -- if they knew, they'd "shun" her. It's an enormously difficult task that she pulls off effortlessly.

As Rachel's suitor Daniel, ballet star Alexander Godunov gives a sly, engaging performance; lantern-jawed, with tiny shrewd eyes peering out from beneath his straw skimmer and seeing all, Godunov gives us the Amish that Weir would rather forget -- the hard, calculating peasant. And in a film culture where "adorable" kids make you yearn for W.C. Fields throttling Baby LeRoy, Haas is . . . well, adorable. He has a natural presence bereft of self-consciousness, and the kind of puddly eyes that can take the camera for hours; Weir makes the most of him, placing his innocent gaze at the moral center of the movie.

There is a moment in "Witness" when McGillis stumbles upon Ford trying to fix his car; he gets it working well enough so that the radio comes on, playing Sam Cooke's "What a Wonderful World." As he waltzes her in a corny two-step -- such music is forbidden to her -- the brief glimpses of their faces, lit only by a lantern, betray a galaxy of emotion, love and fear and laughter and nerves and a simple joy, and as they embrace each other, the vast darkness of the barn seems to embrace them both. For a moment, the clash of cultures doesn't matter anymore, and Weir touches something almost too beautiful to bear.

Witness, opening today at area theaters, is rated R; it contains graphic violence as well as some nudity, profanity and sexual themes.