MANCHESTER BY THE SEA confirms what I saw in Casey Affleck the first time I saw him in a lead role onscreen: the man is a mesmerizing actor. In “Manchester by the Sea” he once again lures us into deep water as a man struggling to stay afloat amid the wreckage of a horrible personal tragedy. Oscar-nominated (“You Can Count On Me”) writer/director Kenneth Lonergan begins his movie with a whisper that becomes an earthquake which reverberates long after the film is over.

Affleck is Lee Chandler a janitor in Quincy, Mass who moves in a daze around the edges of other people’s lives, fixing their broken plumbing and not giving a scrap of himself away. He’s as immune to the hectoring of tenants as he is to the banal advances of women in bars. One night as he sits drinking alone, he spots a couple of guys across the bar whom he quietly approaches, asking if they know him. (One of them is played by terrific local actor Lewis D. Wheeler; later in the film we’ll recognize “Actors’ Shakespeare Project” ‘s Allyn Burrows, and Erica McDermott in bit parts.)

The encounter suddenly explodes in a fury of fists. His rage is a shock; Affleck never telegraphs a move, and it’s part of what makes him so absorbing to watch. The scene is shot wide, from a distance, asking us to witness a vulnerable man flailing against something we have yet to glimpse.

Lee is about to learn that his beloved brother Joe (the affable Kyle Chandler) has died, the inexorable result of congestive heart failure. What is unexpected is the responsibility suddenly thrust on Lee– the guardianship of Joe’s teenage son, Lee’s nephew Patrick (Lucas Hedges) whose alcoholic mother (an excellent Gretchen Mol) is not currently in the picture. Will this be a burden or an opportunity? Lonergan’s script keeps us in flux to the bitter end.  Lee is shocked to learn that his brother has left him in charge of the young man and this sudden challenge will require him to confront the pain of an earlier tragedy and his former family life.

The film drifts back and forth in time to the life Lee shared with his wife and kids in Manchester by the Sea, a northern town at the edge of the ocean with a name like a wistful fairy tale. Michelle Williams in a stunning, natural performance is Lee’s wife Randi, the exhausted, happy mother of three, catching her toddlers’ colds, and angrily chasing Lee’s noisy drunken pals out of the basement as they blow off steam late into the night.

Lonergan’s low key rendering of the familiar, messy rhythms of the couple’s marriage, and the ordinary, intimate tropes of their everyday working class life together, is masterful. The elusive Massachusetts accent so often a jarring false note in movies set in New England, here feels natural, like the fog which clings to every unglamorous detail.

The same can be said of the relationship charted between Lee and Patrick. Lonergan elicits something remarkably real in these performances and dialogue as the two navigate unknown emotional terrain with unexpected humor and pain. These are fundamentally good human beings doing their best in rough water, and Lonergan handles their evolving relationship with great tenderness but without a whiff of sentimentality.

The film ultimately builds to an excruciating climax in an encounter between Lee and his now ex-wife; as they confront a mountain of hurt, Affleck’s face crumples as Lee implodes from the weight of all he has been carrying, the massive fallout of guilt, pain, loneliness, anger, and the bitter acceptance of what can’t ever be changed.

The aftershocks ripple out and into the final scene, a bittersweet bookend of the very first: a boy and his uncle on a boat, pushing out into open ocean where sharks circle unseen– just below the promise of everyday life.

“Manchester By the Sea” is not to be missed. Look for Oscar nominations among cast and crew.