It was 54 years of wedded what?

A mystery in the form of a home movie, “51 Birch Street” tells the story, or tries to, of the 54-year marriage of Mike and Mina Block of Port Washington, N.Y., and what it seemed to be, and what it was, and what came after. (That it can only try and not wholly succeed is sort of the point.) Premiering tonight at 7 on Cinemax, it offers no especially strange closeted skeletons -- this is not “Capturing the Friedmans,” another, weirder documentary of Long Island life -- but only the ever semi-solvable mystery of other people, heightened by the apparent closeness of family. It’s not a whodunit so much as a whodunwhat, or whowasthat.

“Nothing ever is direct,” says Mina at the top of the film -- conveniently stating its theme -- as she’s being deposed on her life by son Doug Block, a documentary filmmaker and wedding videographer. (It’s a job he makes part of the fabric of “51 Birch Street.”) “It’s circuitous. It goes around in unpredictable ways.”

Yes, one thinks, but what else is new? But then Mina dies unexpectedly; Mike, at 83, just as unexpectedly marries his old secretary, Kitty, and prepares to move to Florida; and things start to get interesting. As the director’s childhood home is emptied of his family history, boxes turn up containing 35 years worth of Mina’s diaries and poems. And the relatively happy family Doug Block thought was his -- notwithstanding a lifelong disconnection from his father, who is described as having been “rigid,” “lost in his work,” “silent” -- shifts shape into something quite different, a place of roiling feelings, extramarital affairs and affections, of frustrated hopes and deep dissatisfaction.

Granted, such complaints are as common as crabgrass in the groves of suburbia. But director Block, making the diaries a kind of character and marshaling family photos that catch the spark of his parents’ younger selves, makes their particular life compelling on a level deeper than the mere facts of the case -- it’s a movie as much about time as anything else, and what it does to us. Mina wrote compulsively, we are told, to make some kind of sense of a life in which she often felt constrained: “a frozen-faced suburbanite ... programmed and locked in.” And though he never says so himself, we can see that Doug is also trying to create a narrative that will explain how his family is different from other unhappy families, to make sense not only of his parents, but to make sense of his own reluctance to make sense of them.

Because our own lives are so hard to grasp, so fluid and messy, we want our parents’ to be simple and consistent -- even when we discover them to be imperfect, we need them at least to be imperfect in a reliable way. Every step away from that surety makes the world less secure, and this is what gives “51 Birch Street” an aura of suspense and danger. The more we learn the less we seem to know, and as the film progresses the taciturn Mike becomes a richer and increasingly sympathetic -- even a heroic -- character.

That it is not the most ostentatiously artful piece of work suits the subject and works in the film’s favor. Modest images begin to speak volumes, so that the final shot of helium balloons floating above an anniversary party becomes unexpectedly moving, seeming to stand for the Blocks themselves, and for the rest of us, tied to Earth, blowing in the wind, sometimes bumping up against one another.

robert.lloyd@latimes.com

*

‘51 Birch Street’

Where: Cinemax

When: 7 to 8:30 tonight

Rating: TV-PG (may be unsuitable for young children)