In 2007, Canadian actress-turned-director Sarah Polley got a tip that a local journalist had uncovered a family secret about the identity of her biological father -- one that she knew but hadn't yet revealed to the man who raised her, and who still believed she was his daughter by birth. She begged and ultimately convinced the reporter not to publish the story but realized then that the truth could not stay hidden for long.

“Stories We Tell” documents the unraveling of the Polley family history, though not the family itself: The director's father (the man who raised her) and siblings, who all offer candid interviews with varying levels of cooperation, are remarkably accepting and good-humored about Diane Polley's betrayal, which led to the birth of her youngest child. Diane, Sarah's mother, was a beautiful, aggressively charismatic actress who enjoyed minor success; she died of cancer when Sarah was 11. Sarah's arrival was a big surprise in the Polley family. She was believed to have been the result of a rare (and conveniently timed) physical interlude between Diane and Michael Polley, a retired actor and talented writer. He and Diane's marriage was in some ways happy and loving, but he had a habit of withdrawing, and he admits he didn't give Diane the attention and affection she yearned for. An affair was almost inevitable.

Much of “Stories We Tell” is “adapted” from Michael's engaging account of his marriage and fatherhood, which he wrote in short order (and in the third person, notably) shortly after learning he was not Sarah's biological father. From this text, Michael narrates the footage of the Polley family's early years; indeed, “Stories We Tell” opens with Sarah coaching her father on his voiceover performance.

I don't know if there is a variation of the "meta" descriptor that can accurately capture what Polley has done in “Stories We Tell.” It's a story about the telling of a story, and through that, a third story emerges. Polley doesn't just blend narrative filmmaking and nonfiction reporting; nor does she just blur the line between them; she creates a kind of chemical reaction that transforms them. It's possible that she's invented a new genre of filmmaking, and while she absolutely pulls it off, I hope her approach doesn't become popular among aspiring auteurs: I was half-expecting a “Don't try this at home” disclaimer to appear at the bottom of the screen.

Polley's approach is risky not only from a family dynamic perspective (she's abdicated any chance of hiding behind journalistic objectivity if her subjects aren't happy with the final result): She uses a technique involving copious archive footage that is downright controversial. But she does it (“it” shall be left undefined) so skillfully, and reveals it with just the right amount self-awareness and cheeky humor, that the audience needn't feel unforgivably betrayed when they learn they weren't always seeing what they thought they were.

Further, this method allows Diane to be a character in the story, instead of a ghost haunting the narrative. And it gives “Stories We Tell” a much-needed element of true mystery. The identity of Sarah's father isn't revealed until later in the film, but from the very start, it's pretty clear it's not Michael. We also learn early on that Sarah's bloodline has been a constant source of speculation in the Polley family. It is treated not as a melodrama but as a running joke, one that's funny because it's true.

Truth is the shape-shifting devil running through “Stories We Tell.” False leads and false witnesses stymie Polley's search for her biological father; two of her siblings have different memories of the same family anecdote. Some of the pivotal “action” scenes -- like a key meeting at a coffee shop -- are obvious re-enactments; others, like the scene where Sarah tells Michael he's not her biological father, may or not be. (At least, it wasn't clear to me.) Polley wants us to believe, and seems to believe herself, that her primary concern is to examine the manner in which people shape and share their memories; and that the family drama is just her chosen tool. But ultimately, we're not completely convinced, because with her presence in the film it's impossible to ignore how the movie is in the strictest sense the story of her life.

It's faint, and it doesn't distract from the enjoyment of the film, but Polley's demeanor carries a whiff of “who, me?” manufactured innocence. Maybe “innocence” is not the best word, since Polley is not guilty of anything but trying to tell her family's story in the most irresistible way, with respect to its many interpretations. But her determination to package the story as if it were not about her -- by withholding her own interpretations and keeping her observations to a minimum -- sometimes comes across as a bit disingenuous. Of course it's about her. Polley is the de facto focal point not only of “Stories We Tell,” but of her family in general -- she's the baby, extraordinarily bright and talented, and a control freak, though in a quiet, well-mannered, non-freakish sense.

We can't forget that she is a major star in Canada (and the film doesn't try to pretend otherwise.) While Polley has a reputation for rejecting the trappings of celebrity, she is nonetheless quite famous, even more in her native country than in the U.S., and she enjoys the power that celebrity affords -- on her own terms.

One of these terms is an unwavering loyalty to her home country, which she is eager to advertise. The director's first two features, “Away From Her” and “Take This Waltz,” were both set in Canada; “Take This Waltz” got its title from a song by Leonard Cohen, who is also Canadian. Her next film project is an adaptation of Canadian novelist Margaret Atwood's “Alias Grace.” A passage from Atwood's novel serves as the epigraph to “Stories We Tell.”

While the director may have been deceiving herself to some degree about what she really aimed to do, we know that Polley is ultimately an honest artist because she allows us -- even invites us -- to doubt her stated intentions, along with Michael, who directly challenges her. She also reveals a surprising and somewhat heated artistic power struggle with her biological father, one that she inevitably wins, but it doesn't flatter her.

Polley's reserve and self-possession are even more apparent when her scenes are contrasted with the footage of her vivacious, uninhibited mother, who is seen always searching out the camera, always eager to be the center of attention, always aware of what people see when they look at her. But there is a certain kind of narcissism that can exist without vanity. If the stories we tell are about ourselves, even in a roundabout way, it must mean that on some level we believe we are fascinating. Fortunately for us, in Polley's case, it's true.