Witness” is newly available in a bare-bones Blu-ray from Warner Bros; it is also available via Amazon Instant.

There is not a wasted or unnecessary shot in Peter Weir’s “Witness,” or a superfluous line of dialogue. Like the great barn-raising scene late in the second act, the film’s construction is both efficient and unhurried, functional and beautiful. Like the barn, it is a noble but wistful landmark; there is something defiantly out of step about it, even in the Hollywood landscape of 1985, and certainly today.

The barn has a number of levels, and so does the movie. “Witness” is at once a compelling thriller, a smoldering love story, a thoughtful study in comparative cultures, and a respectful exploration of religious community and nonviolence. It is about belonging and not-belonging, and crossing boundaries between one and the other. It is about being in a world but not of it, being an alien in another world, and coming to feel alienated in one’s own world.

It is about attraction between a man and a woman (Harrison Ford and Kelly McGillis) separated by their fundamental life commitments, about the allure of the forbidden and unobtainable; it is also about a brief but powerful connection between a fatherless young boy and a childless man. Among the main cast are elderly and juvenile, male and female, pious and secular, urban and rural, honest and corrupt, jaded and innocent, peaceful and violent. If it’s not quite the world in microcosm, it’s a big chunk of it.

“Witness” starts among the Amish in Lancaster County, Pennsylvania, amid golden cornfields and rippling grass, whitewashed farm houses and black horse-drawn buggies. Aside from an opening title identifying the year as 1984, there is little in the opening scene, a funeral for a young farmer leaving behind a widow and a young boy, to tell us we are not watching a period piece; not until later do we get a striking shot of an immense semi-trailer and a line of cars creeping along behind an Amish buggy and a trotting horse: two worlds momentarily juxtaposed, yet with nothing in common but the road under their wheels.

In his first Hollywood film, the Australian filmmaker Weir (“Picnic at Hanging Rock”) doesn’t hold the audience’s hand or explain what we can gather for ourselves. The first words of dialogue — from a man who may be a minister at a household gathering that is evidently a funeral — are in German. A tracking shot shows the house full of community members, some preparing food, setting the table, talking in small groups.

With the long beards and plain clothes, in the shadowy interior spaces, they look like figures in a Vermeer or some other Dutch master, a connection Weir emphasizes with his use of light and shadow. The first words in English are a brief exchange about the late farmer’s virtues and foibles, ending with an earthy quip about testicles — the first of a number of indications that we aren’t in a world of stereotypically puritanical fastidiousness.

The quip comes from Daniel (Alexander Godunov), who makes a point of seeking out the women, who are sitting apart from the men, to offer his condolences to the grieving widow, Rachel (McGillis). Later Daniel comes by the train station as Rachel and her young son Samuel (Lukas Haas) travel to Philadelphia en route to visit Rachel’s sister in Baltimore — and from the note of surprise with which white-bearded old Eli, Rachel’s father (Jan Rubeš), greets Daniel at the station, we gather that Daniel is being quite forward in making known his interest in Rachel.

“First time in the big city?” Daniel grins broadly at Samuel. “You’ll see so many things.” They are familiar things from our world, but by seeing them through Samuel’s eyes, with lingering low-angle shots and Maurice Jarre’s hypnotic, Vangelis-like synthesizer score, we witness them anew: a brightly colored hot-air balloon, a water fountain at the Philadelphia train station, and an immense, evocative bronze statue — Walker Hancock’s Pennsylvania Railroad World War II Memorial, depicting Michael the Archangel raising up a dead soldier.

Samuel is also witnessed by others at the train station: a blond girl about his own age gives him the stinkeye. And Samuel smilingly approaches a familiar-looking black-clad, bearded figure in a brimmed hat — but his hopeful smile vanishes as the Hasidic Jewish man looks down at him, not quite frowning at the little Christian boy, and Samuel uncertainly grasps that this man is not of his tribe after all.

While “Witness” doesn’t deny a dark side to the Amish community or rays of light in the outside world — the world of the “English,” in Amish parlance — it’s far from a balanced portrait of either. The Amish world is bathed in a golden glow, while the ugliness and griminess of the outside world is seen in an unforgivingly harsh light (literally in both cases, not just metaphorically).

That bronze memorial in the train station that transfixes Samuel is sublime, but it’s also a monument to man’s inhumanity to man. This inhumanity is far more dramatically and momentously attested by the crisis that sets the plot in motion: a horrifying murder in the men’s room that Samuel witnesses from one of the stalls.

No matter how many acts of movie violence we’ve seen, this one is especially appalling, because in that instant Samuel loses his innocence. The sequence is shot in thriller mode — there is a moment involving the boy’s hat (which his mother bade him to wear!) that almost recalls Indiana Jones — with Samuel barely evading detection through luck and presence of mind. But it’s more than a thriller moment: It’s where the movie’s two worlds, until now a study in contrasts, become locked in a potentially deadly struggle.

The murder brings the unwanted presence of Detective John Book (Ford) into Rachel and Samuel’s lives. John is the best we see of the “English” world, but his existence is far from ideal. He carries a gun and a badge, and with Rachel and Samuel unwillingly in tow, he cruises mean streets amid rough people at all hours of the night. Other than a divorced sister and her two sons, he has no family; if he were killed in the line of duty, there would be no house full of people gathered around his sister and his nephews, making food and offering words of comfort.

John is a good cop, but by his own account he’s the exception, not the rule. The restroom murder victim, it turns out, was also a cop, and when John learns that at least one of the killers (Danny Glover) — identified by Samuel in a wordless scene that’s among the film’s best — is a cop as well, John realizes that both he and the boy are in mortal danger. It’s a danger he doesn’t quite evade; while he manages to get Rachel and Samuel back home to safety, the Amish are forced to take in a gravely injured John as well, for his own protection and for theirs.

As he recuperates, John gradually becomes acclimated to the Amish way of life. To avoid drawing attention to himself, he must dress “plain,” and he learns what it means to wake up at 4 in the morning to milk cows, and the firm hand that work requires. He puts his carpentry skills to work, making a marble-maze toy for Samuel and rebuilding the birdhouse he damaged when he arrived among the Amish. Above all, in the great barn-raising scene, working side by side with the rest of the community and even sharing a glass of lemonade with Daniel, John earns the respect of his Amish hosts — though suspicions swirl around his relationship with Rachel.

Ironically, it is here, among her own people, that Rachel is most drawn to the English world and the things shunned by her people as “proud” and “worldly.” Perhaps the film’s most winsome token of our world is Sam Cooke’s lilting “Wonderful World” (“Don’t know much about history …”) played on John’s car radio in Eli’s barn, with John leading a laughing Rachel in the one and only dance of her life.

Not coincidentally, this is also the moment in which the dark side of the Amish world comes out: A scandalized and frightened Eli remonstrates with his daughter, warning her that her wayward actions could result in her being shunned by the community, so that even her own father would not be able to sit at table with her. To Eli’s charge that she is acting like a child, Rachel retorts that she’ll be the judge of that. “No!” he cries. “They will be the judge of that! And so will I … if you shame me.”

“You shame yourself,” Rachel replies defiantly, and the indictment is not of her father alone, but of her community as well.

The tension between John and Rachel reaches its highest point the evening after the barn-raising. John is never more Amish than on that day, and both he and Rachel feel it. On this day, it is not impossible to imagine John choosing a new life for himself, and this unspoken thought leads to a moment that tests how transgressive each of them is willing to be. There is a threshold Rachel is not willing to cross, but she has become “English” enough, for the moment, to literally leave the door open for John to cross it if he wishes to, and to take her where she can’t quite choose to go herself. For a long moment, neither is willing to move toward the other or to walk away.

The next day, visiting Saltzburg, John’s English-ness reasserts itself when he learns that the corrupt cops searching for him have murdered his partner. Seething with impotent fury, John sees a knot of thuggish young tourists lightheartedly taunting and harassing Daniel, and something in him snaps. “It’s not our way,” Eli protests, but John answers that it’s his.

What follows is an ostensibly crowd-pleasing bit of formulaic Hollywood comeuppance, but there are two problems. John’s reaction is glaringly disproportionate to the provocation; the thugs who knock off John’s hat and paint dabs of soft-serve ice cream on Daniel’s impassive face are contemptible cowards, but not remotely deserving of the bloody beating John dishes out. And John’s violence has the worst possible consequence: It attracts police attention, and John’s enemies catch his scent.

The action climax revisits the contrast between the professional violence of John’s world and the principled nonviolence of the Amish. To its credit, the film avoids definitively endorsing one or the other, instead allowing each to be true to itself. It’s a thrilling sequence, with John outnumbered and handicapped without his gun, but having the home court advantage, knowing the terrain. He tries to send Samuel to safety, but the boy, knowing John is unarmed, returns to try to help him. Will he try to retrieve John’s gun for him? Or will he try another tactic? “There is never just one way,” Eli told Samuel when contemplating John’s gun, and Samuel shows that he learned the lesson well.

In this sequence, as at the outset before the funeral, the Amish seem to rise up from the earth, responding to a community member in need, as they did for the barn raising. The title “Witness” has many meanings, and one of them is that the Amish themselves bear witness to a way of life radically different from our experience, including the possibility of true community.

We cannot belong to Amish world, any more than John can; the very act of watching the Amish onscreen (or actors portraying them) places us as emphatically outside that world as John’s use of violence. On the other hand, watching the Amish — hopefully in a spirit closer to John’s openness than the tourists’ gawking — may also heighten our awareness of our own world, and all that is wrong with it, and perhaps move us to think about things we could do differently. We won’t, most of us. But we could.