I do not know whether Kenneth Lonergan is an only child. I do know that the siblings he has written into his films are some of the most indelible relationships I have ever seen onscreen. His debut film, 2000’s “You Can Count on Me,” featured a beautifully complex relationship between adult siblings who lost their parents as children. It remains one of my favorite films ever. Now, in his critically acclaimed “Manchester by the Sea,” among the many deeply felt pairs onscreen none is more powerful than the brothers played by Casey Affleck and Kyle Chandler.

Affleck is heartbreakingly real and perfect as Lee, a man we meet eking out an existence working as a handyman in an apartment building. Fluidly integrated flashbacks show him happier, on a boat with his brother and nephew, but that is in the past. When we next see the man, his elder brother has just succumbed to a long-diagnosed heart condition.

The film’s many flashbacks are so powerful because they function just like memories. As Lee sits in a lawyer’s office hearing his brother’s will read, his mind goes to the devastating event that has made him the broken man we see today, and it is absolutely masterful. It is a sequence so perfectly delivered that, although painful to watch, it is also amazing to behold. And it gives us a few moments between the brothers that tell you everything about their bond, that show you why Lee’s love for his brother might give him back some measure of life when he must care for his nephew.

The many tragedies of “Manchester by the Sea” are almost macabre, but the script is so natural and well written that every minute of the story feels completely true. Michelle Williams is also unforgettable as Lee’s ex-wife; I found a single embrace between the two the most indelible moment in a film full of perfectly expressed moments.

A young actor named Lucas Hedges, who had roles in “Moonrise Kingdom” and “Grand Budapest Hotel,” defies expectations of what a young man who just lost his father might look like. His character, Patrick, is a popular guy with a garage band and two girlfriends, one of whom is played by “Moonrise Kingdom” actress Kara Hayward.

I cannot imagine that Casey Affleck won’t receive a Best Actor nomination for this role. He takes the unimaginable and shows us what it looks like, on his face. It’s unfortunate we have to wait so long between Kenneth Lonergan’s masterpieces, but when they come they certainly pack a punch.

If you’ve never seen “You Can Count on Me,” do yourself a favor and seek it out. And do not miss “Manchester by the Sea,” because it will be the movie everyone will be talking about, and you will never forget the (tear-soaked) experience.

“Manchester by the Sea” is now playing at the Crescent Theater and Carmike Wharf 15.