From the beginning, Sarah Polley opens the hood of her documentary “Stories We Tell” and shows the machinery whirring inside. The film begins with her and her father entering a recording studio, and a montage of Polley and her crew setting up cameras and lighting for the film’s interview subjects.

It’s an introduction that’s tantamount to a magician’s “nothing-up-my-sleeve” insistence, because Polley has something far more interesting and unusual in mind that the typical nuts-and-bolts documentary. “Stories We Tell” is a powerful and very personal story from Polley, but she’s also constantly mindful that it’s a “story,” a series of facts and opinions shaped into a narrative, and as such isn’t entirely to be trusted.

And this is, in a very literal sense, Polley’s story. The film digs deep into the lives of her parents, Mick and Diane Polley, who met on the Toronto theatrical scene. Mick cut a dashing figure onstage, while Diane was beautiful and vivacious, the sort of woman who “make the record skip” when she walked into a room in more ways than one.

As Mick settled into kind of a middle-aged fog, Diane grew frustrated, and took an acting job in Montreal in 1978 that took her away from home for a couple of months. What happens next, the viewer should discover from the film, as Polley digs through layers after layers of reminiscences and rumors, interviewing her four siblings and friends of the family.

As we’re following the sudden revelations of the story — and there are some doozies, expertly revealed by Polley as she shifts from perspective from perspective — we’re also becoming aware of how each account differs. Some people have the facts a little off, others just view the same facts differently than others. One man, who seems to be just a supporting character in the drama, insists that his account and only  his is the truth, and all the others are just noise.

The closest the film has to an authority is Mick, who reads from his writing about the family history in plummy, theatrical tones throughout the film. But even he doesn’t have the full picture of his own family.

The perspective that is largely missing, ironically, is Polley’s — although we see B-roll footage of her about to be interviewed, it never appears in the film. Instead, she seems to be getting at the truth of her life the way a submarine uses sonar, pinging off the other people in her life until she finds a spot that overlaps. It’s a fascinating process, and Polley (an actress turned acclaimed director of the dramas “Away From Her” and “Take This Waltz”) blends the different voices expertly until a bigger picture emerges.

The film also weaves in Super 8 home movie footage of Diane and the family, although it turns out that the way Polley is telling the story contains as many secrets as the story itself. Put it this way; critics who like their documentaries the way Joe Friday likes his witnesses — “Just the facts, ma’am” — are going to have a big problem with this movie.

For me, it’s a brilliant film that manages to be both one family’s shared history and all families’ shared histories. Would “Stories We Tell” have been as compelling if Polley didn’t add all the post-modern devices to it? On the level of pure storytelling, probably. But the way she tells the story takes it beyond “What happened?” and forces us to examine the too-tidy narratives of the stories we tell ourselves.