Actress and director Sarah Polley is directing a documentary about her own family, chronicling stories leading to the revelation that she is a product of an extramarital affair. (Bret Hartman/FOR THE WASHINGTON POST)

The easy way to describe “Stories We Tell” is that it’s a documentary, by the Canadian actress and director Sarah Polley, about her mother, her family and a secret that had a seismic effect on all their lives.

A more nuanced way would be to say that “Stories We Tell,” which opens in Washington on May 17, is a delicate, funny, shocking, multilayered melange of nonfiction and drama, a brilliant collage that both addresses and exemplifies the slippery nature of memory and narrative itself.

“Stories We Tell” is simple: It’s about loss, grief and a long-buried truth. “Stories We Tell” is complicated (see “loss,” “grief” and “long-buried truth”). It’s disarmingly straightforward and craftily deceptive — all in the name of emotional honesty that is all too rare onscreen, not to mention in life. It’s a rigorously constructed, philosophically deep plunge into family dynamics and personal candor that owes most of its restrained beauty to the fact that it lays bare a story Polley has spent much of her adult life not wanting to tell.

“When it first happened . . . it was the last thing in the world I would have wanted to make a film about,” Polley said recently in a phone conversation from Los Angeles. “The impetus for making the film in the first place was how many stories were coming out of this story, and how many contradictions and complementary details were coming out. . . . I guess it made me think about how common an experience it is to have a family history that’s so subjective, and so different from the history of others in the same family, and how often members of the same family come up with different versions of the same events.”

What were those events? The facts are these: Polley, now 34, grew up in Toronto with her parents, Diane and Michael, and four siblings. Diane, an actress who appeared on local television and in theater, died when Sarah was 11. But she left behind some lingering questions that Sarah eventually explored and brings to the surface through interviews with family and friends, archival records and cinematic techniques that rival Errol Morris in their seamless re-creation of personal and social history. “Stories We Tell” is so carefully calibrated that it’s possible to finish watching the film and want to watch it again immediately to dissect the subtlety of its craft.

Or, maybe you’d just want to spend more time with Diane Polley, who emerges from “Stories We Tell” as a vibrant, palpable presence — a charismatic, fun-loving party animal and compulsive telephone-talker who exerts as seductive a pull onscreen as she did with the myriad people who loved and were confounded by her. Polley admits that the process of filming the movie “strangely made [her loss] a lot harder. There were so many times I felt like I should stop, mostly for mental health reasons.” As the film progresses, a painful discovery is revealed that sends “Stories We Tell” into ever more surprising trajectories. “Once I felt I’d processed [that discovery] and gone through the more shocking and painful parts of it, then I had to make this movie. It felt like rubbing salt in a wound over and over again.”

If “Stories We Tell” was difficult for Polley, the film itself bears no trace anxiety or resentment. Indeed, it’s a lyrically expressive, often exuberant evocation of 1970s life that often resembles a Canadian version of “The Brady Bunch,” given a mordant zing from the present-day interviews with Polley’s funny, self-aware brothers and sisters. The film is given its structure from a narration provided by her father, Michael, reading from his own memoir, which he can be seen taping in a recording studio, complaining sardonically at his daughter’s stern direction.

Even though “Stories We Tell” is nominally about her mother, Polley believes now that the reasons she made the film had more to do with her father. The most meaningful thing about her discovery and the way it reverberated, she says, wasn’t her mother or her secret, but “what felt most original and unique was my dad’s response, which was so unexpected and elegant, so full of compassion and humanity.”

The most moving passage of “Stories We Tell” comes toward the end, when Polley allows the camera to rest on the now-grown children Diane left behind, their faces offering wordless, wounded testaments to the lingering effects of her loss. In many ways, this film is a natural outgrowth of Polley’s directorial career, which began with the masterful “Away From Her” and continued with “Take This Waltz.” Both address issues of memory, marriage and selfhood similar to those that animate “Stories We Tell.” A few years ago, she felt compelled to write director Terry Gilliam an open letter recounting her experiences as a child actor on the set of “The Adventures of Baron Munchausen,” which left her feeling endangered and traumatized — another case of memory and story colliding. For her next directing project, Polley has optioned “Alias Grace,” the Margaret Atwood novel she quotes at the beginning of her new film: “When you’re in the middle of a story, it isn’t a story at all, but only a confusion.”

Giving voice to her version of her experiences, as well as those of everyone else involved, has a resonance that is almost political for Polley, who is well known in Canada for her activism in anti-poverty and progressive electoral politics.

“I do feel like it was really important that everybody get a chance to be heard,” she explains. “Nobody’s full version is in the film, but . . . we got every person’s perspective. That was also part of my decision not to do a voice-over and have it be my story. Because inevitably, that would overshadow every other version. It just seems to be a real injustice: getting to be architect of a film and deciding how it will be structured, and then being the person editing that film. To also then intervene with my own perspective, I felt like that would be kind of tyrannical.”

As for Diane, the mother she lost so young, the process of making “Stories We Tell” has had the contradictory effect of bringing her back and keeping her all the more elusive. “Even in the middle of all the noise and points of view of who she was, and getting to speak to all her close friends and family members, just immersing myself in those contradictory versions of her, I do have the sense that I know her better after making the film,” Polley says. “But,” she adds wistfully, “I will never know if that’s true.”

Stories We Tell

(108 minutes) rated PG-13 for sexuality, brief language and smoking, opens May 17 at Landmark’s E Street Theatre, AFI Silver Theatre and Cultural Center, and AMC Shirlington.