ON CERTAIN RARE occasions, a movie offers a scene of such strength and quality that popcorn is abandoned, the jaw drops in admiration and you yearn to become a character on the screen. "Witness," a superb love/adventure story, has two such scenes and a score of other moments that reverberate for hours after the house lights come up.

Scene One: Philadelphia police Captain John Book (Harrison Ford) joins 30 Amish men for a dizzying barn raising under a brilliant blue sky. We see the barn go up piece by perfect piece as the men, muscles slick with sweat, challenge one another in a ballet on the high beams. Then, their duty done, they ride off in the soft glow of a rural Pennsylvania sunset.

Scene Two: Book happens upon the beautiful Amish widow Rachel (Kelly McGillis) as she bathes half-naked in the flickering haze of a kerosene lamp. Unashamed, she turns toward Book, and the two face each other in silent wonder as an electrical storm crackles through the night.

The film's electricity begins long before that, however, when Rachel's son (the angelic Lukas Haas) witnesses a brutal murder in a Philadelphia train station. Young Samuel becomes the target of the corrupt narcotics agents responsible for the killing and must flee the city. Investigating officer Book also knows too much, so the tough city cop goes underground with the boy in Pennsylvania Dutch country.

Although Book ruffles some feathers at first -- an elder catches him dancing to Sam Cooke tunes with Rachel -- he endears himself to the Amish community with expert carpentry and his brusque way of dealing with rude tourists: "Lady if you take my picture with that thing, I'm going to rip your brassiere off and stuff it down your throat."

His idyll ends, however, in an action-packed showdown -- Book and the Amish versus evil narcs with shotguns -- and we are left with one of the most bittersweet endings since "Casablanca."

The acting in "Witness" is superb. Ford gives a fine understated performance as Book. And McGillis is always believable as the beautiful widow. But director Peter Weir ("Gallipoli" "The Year of Living Dangerously") deserves most of the credit, as he artfully depicts the enigmatic world of the Amish and makes "Witness" such a compelling and unusual thriller.

WITNESS (R) -- At area theaters.