Director Sarah Polley (left) and Michael Polley (right) in scene from STORIES WE TELL. (Courtesy of Roadside Attractions)

Sarah Polley begins her documentary, “Stories We Tell,” with an epigram from Canadian author Margaret Atwood. But this rich, sensitive, densely layered piece of poetic nonfiction also conjures a famous aphorism of William Faulkner: “The past is never dead. It’s not even past.”

Polley, an actress who has pursued an accomplished directing career with the fictional features “Away From Her” and “Take This Waltz,” brings the past to vivid, astonishing life in “Stories We Tell,” an excavation of her own family that hinges on a long-buried secret, how it’s reverberated with everyone it’s touched and the contentious ways they explain it. Sitting her four funny, smart, attractive siblings down for talking-head interviews, Polley makes a simple request: Tell the story from beginning to right now, in your own words.

They do, and thanks to their observant storytelling and Polley’s judicious editing, what unfolds is a riveting drama that grows even more so as it plays out. Held together by a memoir written and narrated by Polley’s father, Michael, “Stories We Tell” begins in the 1960s, when he and her mother, Diane, met and fell in love while doing a play. Looking back, he says, he realizes that Diane — an outgoing, charismatic actress who appeared frequently on Toronto television — probably fell in love with his dashing, intrepid character.

Using home movies and original material filmed on vintage-looking Super-8, Polley creates a vibrant tapestry of the facades and false assumptions that form the captivating subtext of “Stories We Tell,” which is nominally about the mysteries left in the wake of Diane’s death when Polley was 11. Indeed, those questions inspire a lurid sense of curiosity, a truth Polley cleverly winks at in her use of old-fashioned, melodramatic music played on a slightly tinny piano. Interviewing Diane’s family and friends, Polley begins to apprehend the inner life of a mother she could never know fully, and who — no surprise here — harbored needs and desires that few would have suspected of a beautiful, vivacious “good-time Charlie” (as one witness describes her).

But what makes “Stories We Tell” far more exceptional than a mere re-hash of reality-TV episodes is Polley’s rigorous adherence to her own principles: letting multiple truths have their day and never allowing the thrill of the hunt to overwhelm the grief that still exerts its unspoken, centripetal force.

With its ingenious structure, seamless visual conceits and mordant humor, “Stories We Tell” is a masterful film on technical and aesthetic values alone. But because of the wisdom and compassion of its maker, it rises to another level entirely. Its finest, most shattering moment isn’t a grand revelation, pivotal encounter or sly piece of visual legerdemain. Rather, it’s an achingly simple, wordless tableau, as the people who loved Diane reflect on the woman who has passed but will never be consigned entirely to the past.

PG-13. At area theaters. Contains thematic elements involving sexuality, brief strong profanity and smoking. 108 minutes.