Framed as an incredibly sad story of a loner whose life has been decimated by a single late-night moment of bad judgment, writer/director Kenneth Lonergan’s third consecutive masterpiece surprised the hell out of me. Given its hugely buzzed-about debut at last January’s Sundance Film Festival, it was a fairly safe bet that I’d respond well to Manchester by the Sea, the latest drama from the noted playwright, which he set and shot in the Cape Ann region of northern Massachusetts. My expectations were only heightened by my love for his previous pictures, 2000’s brother/sister drama You Can Count on Me and 2011’s Margaret, a messy film made even messier when it was released into theaters in a badly compromised version (Lonergan’s longer cut is well worth tracking down on video). But I was wholly unprepared for just how much a movie so raw and unflinching in its observations about death and its effect on the living could make me laugh.

A good deal of the humor comes from the lived-in, naturalistic interactions between Lee Chandler (Falmouth-born, Cambridge-raised Casey Affleck) and his 16-year-old nephew, Patrick (Lucas Hedges, impressing after small roles in Wes Anderson’s Moonrise Kingdom and The Grand Budapest Hotel). We first meet them as they ride in the rear of the fishing boat owned by Lee’s older brother and Patrick’s dad, Joe (The Wolf of Wall Street’s Kyle Chandler). Ah, but this lighthearted scene, which finds Lee gently making fun of Patrick while they fish (to the obvious delight of Joe), takes places in happier times, during one of the periodic flashbacks that Lonergan uses to punctuate the present.

In that present, Lee has withdrawn from life. For reasons that won’t be revealed till later, he’s abandoned his titular hometown for a basement hovel an hour away in Quincy, where he holes up when he’s not drinking, fighting or toiling as a janitor. It’s a purgatorial existence as grim as the gray skies captured by cinematographer Jody Lee Lipes. Yet Lonergan and his talented editor Jennifer Lame (who’s cut the last few Noah Baumbach pictures with equally great precision and timing) mine quite a few laughs during the montage that establishes our hermitic antihero’s daily routine of shoveling snow from the ground and shit from plugged toilets, all while remaining emotionally constipated himself.

Not 15 minutes into the film’s 2-hour, 17-minute running time, though, a little bit of life creeps back into Lee as a fateful phone call causes him to spring into action. He may be an introverted shell of his former self, but Lee is fiercely loyal to his family, and he’d do anything to see his beloved brother Joe one last time before congestive heart failure claims him—even if that means setting foot back in Manchester-by-the-Sea, a close-knit town whose inhabitants (including Lonergan in a humorous cameo) are prone to calling Lee a “fucking asshole” to his face.

Alas, in keeping with the bad luck that keeps turning up for Lee like a bad penny, he arrives too late. And with Joe’s soused ex-wife (Gretchen Mol) long out of the picture, Lee has to break the news of the death to Patrick. What’s more, he discovers that Joe has named him the teenager’s sole guardian. While it’s easy to see how Joe might have thought this responsibility would give Lee something to live for, we come to realize how terrible a gamble it may be once Lonergan reveals the devastating event that tore Lee’s own marriage apart.

It’s heartbreaking to witness what Lee has been through, which Affleck masterfully conveys as he practically swallows the short, clipped sentences from Lonergan’s script. The talented thespian—an Oscar nominee for his supporting turn in 2007’s The Assassination of Jesse James by the Coward Robert Ford—may be best known as Ben’s younger brother, but this perception should change for good once audiences see him opposite Michelle Williams, who will likely shatter you in her small but integral role as Lee’s ex-wife, Randi. Don’t be surprised if you finally see this three-time Academy Award nominee walking onto the stage of the Dolby Theatre in Hollywood in February to collect her first Oscar, alongside Affleck and two-time writing nominee Lonergan.

Lonergan is extraordinarily attuned to the way people talk and has a knack for casting just the right performers to mine both pathos and humor from his peerless dialogue. But Manchester by the Sea, one of the richest dramas of this year or any other, is also about what the characters can’t say to one another—and how the silences separate them. Lonergan trusts his audiences to do some of the talking, and they’re sure to discuss his films long after the lights have come up.

Manchester by the Sea ****

Starring Casey Affleck, Kyle Chandler, Lucas Hedges, C.J. Wilson, Heather Burns, Tate Donovan, Josh Hamilton, Kara Hayward, Anna Baryshnikov, Tom Kemp, Ben O’Brien, Matthew Broderick, Gretchen Mol and Michelle Williams. Written and directed by Kenneth Lonergan. At Boston Common, Coolidge Corner, Kendall Square and in the suburbs.