“Love is Strange” is now playing at Sundance Cinemas. R, 1:38, three and a half stars out of four.

Ira Sachs’ “Love is Strange” begins with a shot of hairy legs, intertwined in a bed. They belong to a longtime gay couple, Ben (John Lithgow) and George (Alfred Molina), who sleep together in a bed that’s much too small for them. It’s a shot of casual, taken-for-granted domestic bliss. In public, the couple are more discreet, walking a few feet apart on the street. The world may be changing, with gay marriage now legal in their native New York City, but they’ve lived long enough to know it doesn’t change that fast.

Ben and George decide to get married, making official a love affair that has gone on for nearly 40 years. Unfortunately, George teaches music at a Catholic school, and while the priest in charge could turn a blind eye to Ben and George living together, he can’t to marriage. George is fired.

This sets in motion a series of unexpected late-life crises in Ben and George’s life. Sachs’ film is quiet and understated, never going for the obvious emotional payoff, but is in its own way tremendously moving. What Ben and George go through feels so commonplace, a reminder that even when we think our lives have settled into place, tragedy can always lurk just beneath. The mechanisms of the world that turn for us can suddenly start turning against us, and those support systems of friends and family that we often take for granted get put to the test.

Ben is a dilettante painter with no income, so after George is laid off, the couple can’t afford their New York apartment. They sell, and while George looks for work, the couple stay with others. George lives with the two gay cops downstairs (they turn him on to “Game of Thrones” in a very funny scene) while Ben moves in with his favorite nephew (Darren Burrows) his wife (Marisa Tomei) and their teenage son Charlie Tahan. Both living situations are much less than ideal, and in a few deft strokes, Sachs captures that feeling of loneliness and displacement, of having to live in someone else’s life. Ben and George made a commitment to be together, but that commitment is now keeping them apart.

The performances large and small are all wonderful — Molina as the neat, patient George and Lithgow as the more free-spirited, even a little naughty Ben. In their scenes together, you sense a shared lifetime of experiences, not just in the dialogue but in the way they gaze at each other. I’m not sure they could even articulate why their relationship has lasted 40 years, but it has, and they’re grateful.

Sachs films New York in scenes of extraordinary visual poetry, set to Chopin piano ballads. Part of the reason for these interludes is dramatic — making us understand why Ben and George don’t just leave New York and move somewhere cheaper — and some of it is more esoteric. “Love is Strange” captures scenes, such as a day Ben spends painting the city from his rooftop, of ordinary, perfect moments, ones that turned out to be the end of something but you didn’t realize it at the time. And the last shot, which I won’t reveal, is so beautiful that you’ll leave the theater with your chest full.