In this era of big documentaries coping with such monumental issues as global warming, the war on terror, the nation’s electoral system, and suicide, the understated “As the Call, So the Echo” is a surprising and affecting documentatry on a far more intimate scale.

The film’s story is a straightforward one, about an American doctor taking a month-long vacation from his New Mexico practice to travel to Hue, Vietnam, in a bid to deliver a truckload of unused, state-of-the-art medical equipment to a hospital in need. It’s a simple, goodwill trip overseas; a chance for ear, nose, and throat surgeon Alex Moreano to, as he says, give something back. But as directed by Keir Moreano, Alex’s son, there is more at stake than the mere forfeiture of the comfort and security of home for three weeks.

Father-son relationships are complicated things and, as one might expect, Keir approaches his subject from a far different angle than other filmmakers would have. Moving along a trajectory that seems to leave too much unsaid — perhaps an unavoidable flaw in a son making a movie about a man he already knows so well — “As the Call, So the Echo”opens at an almost uncomfortable distance from the doctor, observing him as he performs an ordinary exam on one of his American patients.

It’s a casual, and frankly boring, procedure, but the perfect preface for the film’s larger themes of growth and rediscovery. Alex admits that in recent years he has felt his career sputter, lost amid the drudgery of day-to-day patients and routine exams.

In some ways, the trip overseas is his attempt to reconnect with a sense of purpose and fulfillment, to experience first-hand the difference he can still make. Later in the film, Alex offers a mysterious monologue (which Keir, sadly, never clarifies) in which he describes another, more emotional motivation for his journey. It’s one of many ambiguous sequences in “As the Call,” an ambiguity that seems noteworthy in itself. Alex, like so many doctors and professionals working in America, has felt the motivation evaporate from his life, and his decision to visit Vietnam seems less like a decision than an impulse. Keir calls attention to this, pointedly cutting together his father’s extended journey overseas, gradually moving further and further away from the isolation that has led to his disillusionment.

As Alex arrives in Hue, he is immediately received by a line of doctors and patients eagerly awaiting his arrival. We watch as he is surrounded by Vietnamese physicians, all fighting to overcome obstacles of language and technology, to learn from their American visitor as he makes diagnoses and performs surgery. Alex — sometimes engrossed, other times overwhelmed — quickly confronts patients suffering from a range of diseases, infections, and tumors more extreme than anything he’s seen before.

Keir, who removes himself from the proceedings and in the process denies us his insights on his father, keeps the story focused instead on Alex’s meetings with patients and doctors. One patient in particular becomes an obsession — a young mother suffering from a tumor so large it has expanded in her skull and wrapped itself around the back of her head. It is unlike anything Alex has ever seen; during surgery he’s unsure if it is malignant or benign, and he waits mid-operation for the results of a test that will lead him to either continue with the removal or sew her up as a hopeless case.

It isn’t until later, in the documentary’s brief, soft-spoken epilogue, that Keir is finally able to separate himself from his familiarity with his subject, sitting his father down in front of the camera and unearthing the heart of this story. It’s here that Alex reflects on the difficulty he’s encountered since returning home in sharing his experience with colleagues. He can’t quite put it all into words, and they can’t really relate. It is when he recalls the simple regards of a friend (“you did a good thing”) that Alex’s words fail him, his eyes well up, and the stoic doctor quietly sobs. It’s a revelatory moment of mixed meaning. Is he moved by how much he was able to help? Devastated by how little the doctors in that far-away place have to work with? Overwhelmed by the profound difference he could make with so little effort? Or perhaps a little of everything — a bewildering mix of pride, pity, and humility.

Given how many documentaries have invested themselves in exposing the unknown pains of the world, it’s difficult to know how to read this tearful denouement — the final note in a film less about pain than about how we can all do something to ease it. Here’s a movie that sees the slivers of hope that can still brighten the darkest of places, that hears the echoes that occasionally respond to the world’s calls for help.