The truth about mothers and fathers

How well do you know your mother and father? How well do you want to know them? The gulf between those two questions keeps even adult children locked in a naive suspension of disbelief. We think our parents are always as good as they're urging us to be, and surely they're duller, too -- if not, aren't we just like them?

Doug Block's profoundly touching "51 Birch Street" smacks into these issues like a bird hitting a window. It's another one of those ultra-personal documentary home movie projects, like "My Architect" or "My Father the Genius," in which filmmakers turn the camera on their parents hoping to find answers that, in most cases, are about the directors' own discontents.

In Block's case, the answers had nothing to do with him -- you can almost hear his polite shriek of boomer dismay -- and everything to do with the many secret lives a suburban marriage can contain. The Port Washington, N.Y. , couple he thought were boisterous mom and colorless dad turn out to have more shades and sadnesses and resentments than he ever knew. Or maybe he just wasn't looking hard enough? That's the thing about this genre: There's plenty of guilt to go around.

"51 Birch Street" begins so blandly you may feel you've stumbled into someone's family videos. The filmmaker's aging mother, Mina Block, fills the screen like a caustic, life-affirming force, while the father, Mike, recedes into the background. The 50-something director mentions that he always had a special bond with his mother, and the opening scenes feel like a toddler looking up at a beloved parent.

After 54 years of marriage and a brief illness, though, Mina died. Three months later, 83-year-old Mike married a woman he had known years before. To say his son and two daughters were blindsided is an understatement: In the wedding video, after the happy couple dance to "Only You," Doug gives a fumbling toast in which he notes "Mom would have approved."

Confronted with this jovial "new and improved dad," the son starts to dig. He interviews his siblings, a therapist, a family friend, a rabbi. He paws through old photos and finally comes across his mother's diaries, three decades ' worth of roiling, accusatory angst, and the scales fall from his eyes. "What must most marriages be like if this is considered one of the good ones?" Block asks us on the soundtrack, and he's not being rhetorical.

The film grows in power as it goes, finding ever more universal levels of feeling. The specifics of Mike and Mina's emotional lives don't need detailing here; they're no different from the infidelities and compromises you'd find behind any front door. What galvanizes "51 Birch Street" is that Doug and his sisters never knew, and that that not-knowing cheated them out of truly understanding the people who gave them life.

So whose fault is that? The children who habitually looked the other way? The proud, intellectual mother going stir crazy in suburbia but careful to keep it from the kids? The father who waited until his eighth decade to bloom? Do we underestimate our parents because we can't imagine them as flawed human beings, or because we're scared to? The lasting strength of this hugely compassionate movie is that it asks these questions, then sends us home to ask them again.

Ty Burr can be reached at tburr@globe.com. For more on movies, go to boston.com/ae/movies/blog.