Acclaimed filmmaker and actress Sarah Polley is now ready to reveal what her new documentary, "Stories We Tell," is about. And it’s rather a shocker.

The documentary has been the topic of much speculation in recent months because Polley has refused to discuss it. The film’s co-producer, the National Film Board, has said only that it’s about family secrets.

Now we know that the family secret is Polley’s own.

And the revelation is that Polley’s biological father is not the dad who raised her. Instead, he’s a man her mother had an extramarital affair with back in the 1970s.

While such a secret is not so very unusual, what is unusual is that Polley has decided to take the story to film, to explore why secrets are kept and what occurs when they are finally out in the open.

In a guest blog post on the NFB website, dated Wednesday evening, Polley explained that she had long suspected her mother Diane, who died of cancer in 1990 when Polley was 11, may have had an affair that led to her conception.

But the writer-director didn't know for sure until 2007, when a DNA test revealed that a man she had met a year earlier -- “almost by accident,” she says -- was her biological father.

Polley says she kept the secret to herself for a year and only revealed it to Michael Polley, the dad she grew up with, after learning a journalist had gotten wind of it and might reveal it first.

"My father's response to this staggering piece of news was extraordinary," Polley writes in the online post.

"He has always been a man who responds to things in unusual ways, for better or for worse. He was shocked, but not angry. His chief concern, almost immediately, was that my siblings and I not put any blame on my mother for her straying outside of their marriage."

Her father then began to write about Polley’s mother, to help explain their marriage and her mother’s affair.

“My biological father, at my behest, had also begun writing the story of his relationship with my mother. He is a fine storyteller too and one of the most interesting people I’ve ever met,” Polley writes.

“Each of us had a deep and growing need to tell the story, different parts of it, in different ways, with emphasis on different details, in a way that reflected our own experience and what was most important to us as we are now.”

When her siblings revealed their own different recollections of the events, Polley realized that she had a topic for a film: How emotions can colour memory and change different people’s remembrances of the same events.

"Making this film was the hardest thing I've ever done," she writes. "It took five years and tormented me.”

The film debuted Wednesday at the Venice Film Festival, along with Polley’s guest blog on the NFB website.

Polley, who has also directed the acclaimed "Away From Her" and "Take This Waltz," is hoping not to do any interviews about this latest film until it's released theatrically. Until then, she says, she wants the film to "speak for itself.”

"Stories We Tell" will have its North American debut at the Toronto International Film Festival, which begins Sept. 6.