TORONTO -- A horrific First World War battle and the valour of Canadian soldiers who fought there take centre stage as Paul Gross's long-awaited epic "Passchendaele" kicks off the Toronto International Film Festival on Thursday.

It's a prestigious opening slot for the ambitious tale of love, regret and heroism, with Gross -- the film's writer, co-producer, director and star -- confessing to some fruitless second-guessing as the result of his bold efforts is set to debut.

"You get so obsessed with details, and I'm fixated on this one bomb that I always thought was on the wrong side of the soundscape," says Gross.

"It's one explosion among thousands, and I have it in my head that if I don't get that moved over to the right side, this film will be a disaster."

Such obsession will have to be excused for someone who's been visualizing this battle since before he even knew he was a filmmaker.

Gross's fascination with the First World War began as a teen when he would pester his grandfather for real-life tales from the front.

These were doled out reluctantly and with more than a tinge of regret, Gross recalls. One day his grandfather confessed to killing a German teen with a bayonet through the forehead, for absolutely no reason that he could articulate.

It was a senseless act of violence that haunted his grandfather to his death, says Gross, noting that the man's final moments were marked by a stream of apologies as he was plagued by wartime hallucinations.

Gross's grandfather did not fight at Passchendaele, but his harrowing stories pushed his grandson to learn all he could about the First World War. Some 12 years ago, Gross began his own battle to document the Canadian experience at Passchendaele, near Ypres, Belgium.

"In many ways, Vimy Ridge is the more famous of the Canadian engagements in the First World War," says Gross, known for his starring roles on the TV series "Due South" and "Slings & Arrows" and for his 2000 curling comedy, "Men With Brooms."

"And it's not terribly well known, but we did win Passchendaele. ... Partly, I wanted to choose that battle because the win is so ambivalent. There's something very paradoxical about the First World War. It's kind of amorphous -- yes, it was a victory, but of what kind?"

The battle of Passchendaele is known for its heavy casualties and a punishing rain that turned the battlefield into a thick muddy swamp. On Oct. 26, 1917, the Canadian Corps began a series of attacks that, some two weeks later, succeeded in taking the village of Passchendaele.

But the cost of gaining eight kilometres was tremendous, with 16,000 Canadian casualties among some 310,000 Allied dead.

Gross's film focuses on the intimate details of war, exploring what it does to family and love. He plays Sgt. Michael Dunne, a Canadian soldier who falls in love with Sarah, a nurse played by Caroline Dhavernas, while recovering in a Calgary hospital. Dunne returns to the battlefield when Sarah's asthmatic brother David (played by Joe Dinicol of "Train 48") decides to join the fight, with Dunne feeling compelled to protect the boy from the horrors of war. They soon find themselves at the epic battle of Passchendaele.

Dhavernas says she was intrigued by Sarah's broken spirit and the relationship her character forges with an injured soldier.

"Throughout these atrocities and wars, what remains, for these two characters, is that they will make it because they are in love," says Dhavernas, also set to appear in the upcoming HBO series "Pacific," about the Second World War.

"They'll be able to save themselves because they have each other and because I think love does lift you higher like that when you're in times of trouble, and it's the only way sometimes to make it through."

The climactic battle scene was shot at an aboriginal reserve about 20 minutes outside of Calgary. Enormous rain towers transformed into the land into a swampy battlefield, flooded by tanks of water from the frigid Elbow River. The ordeal proved to be the most difficult of Gross's career.

"We were constantly reinventing and refining ways of thawing our hands really quickly or moving people into warmer tents," Gross says of the October shoot.

"The first night we shot in the rain we sent 20 people off to the Rockyview Hospital with early signs of early-stage hypothermia. So it was terrible, and knock on wood and thank God nobody got hurt and I'm not really sure how we managed that."

Pulling together money to fund the $20-million production was another challenge. A decade of canvassing from public and private investors finally garnered enough to launch the project, with Gross saying he was pleasantly surprised by how receptive individual donors were to supporting the film.

"For a film of that kind of ambition, it's still extremely low budget," Gross adds. "If this were a U.S. studio picture they wouldn't really contemplate it for anything less than $100 million."

The initial stake came from the Alberta government, which contributed $5.5 million from its centennial fund in 2005. More money came from Telefilm Canada and various media companies. Gross turned to the private sector for the rest.

"And there's a truism that is actually true -- rich people did not get rich by giving their money away," he says, laughing.

"So it took years of flying back and forth across the country, having dinners with billionaires, but in the end what was really encouraging and slightly surprising is there's an enormous amount of interest out there in the private sector for what we do in this business."

Given the size of the Canadian market, Gross says he knows he won't make that money back at the box office. Recouping those investments will have to come from wider distribution, which he will be pursuing at the Toronto festival.

But already, Gross has partnered with the Dominion Institute to create a study guide for schools. Eventually, "Passchendaele" will be made available to high schools across the country so that the story of Canadian valour lives on, he says.

"Passchendaele" opens in theatres Oct. 17.