Sarah Polley’s third film—following the dramas “Away from Her” and “Take This Waltz”—is a very personal documentary constructed in a way that raises questions of narrative truth while telling a poignant family saga. Cinematically “Stories We Tell” is like a painting in which the subject is portrayed simultaneously from different angles. And by the end the film itself comes to be seen as just one more perspective—Polley’s—as individual and incomplete as all the others.

The film starts out as an effort by Polley to paint a portrait of her mother Diane, who died when the director was eleven in 1990. At first it appears a fairly conventional piece, made up of what appear to be home movies and excerpts from interviews with Polley’s father Michael and her siblings John, Mark, Susy and Joanna, as well as a few other relatives and friends, whom she invites to talk about Diane, a vivacious stage actress, from the beginning, as she puts it. They respond, some more reluctantly than others, but there’s a peculiarity from nearly the start, since Michael’s remarks are divided between some that he delivers extemporaneously seated at a kitchen table while others are recorded in a studio as he reads from a prepared text and is occasionally prodded by his daughter to provide another take on a line or two.

But things get stranger still when Polley interviews actor Geoff Bowles, with whom Diane co-starred in a play, and asks him about persistent rumors that he, not Michael, was her biological father. He denies it, but Polley takes up the subject again when she talks with Harry Gulkin, a producer who knew both Diane and Geoff during the play’s run. That leads to revelations that come as a bit of a shock both to Polley and her relatives but also to the audience, indicating that Diane’s life was more complicated than even those closest to her knew.

“Stories We Tell” ignores the usual trappings of a conventional documentary. It doesn’t report much about Diane’s childhood, and doesn’t bother detailing anything about her first marriage until very late in the running-time. Even then it doesn’t explain much about how the family came together in the aftermath of her divorce. Nor does it fit easily into the personal essay form, because while in a sense Polley goes in search of her roots, in a curious way she remains incidental to the fragmented biography of her mother she manages to create. And even its very documentary character is called into question when footage of Polley shooting what has been passing as quasi-archival material occurs, showing that these are actually newly-filmed sequences featuring actors and rendered (by cinematographer Iris Ng) to look “authentic.” And, of course, though Michael Polley is certainly who he claims to be, the fact that he, like his wife, was an actor—as well as his scrupulous preparation of his narrative about Diane—make it clear that his contribution isn’t exactly spontaneous.

As the title indicates, though “Stories We Tell” is about Diane Polley, it’s also about Sarah, and Michael, and Harry Gulkin, and everyone else who appears in the course of it. In the end it’s like a hall of mirrors, in which the images reflect on one another in distorted, incomplete form. And its real subject isn’t a single person or even a cast of related people; it’s the nature of storytelling itself. The result is an absorbing exercise not only in documentary excavation but in narrative construction.