What happens when one senseless mistake threatens to ruin someone’s entire life?

That’s the question posed in the Irish film “What Richard Did,” which had its world premiere at the Toronto International Film Festival last Sunday and will have a public screening this coming Saturday.

Directed by Dublin’s Lenny Abrahamson, the film stars Jack Reynor as 18-year-old Richard Karlsen, a star rugby player with a bright future who spends his last summer before college having as much fun as possible.

But everything changes when a drunken encounter at a party turns into a tragic event with life-long repercussions.

The film is based loosely on the book “Bad Day in Blackrock” by Kevin Power, and Abrahamson says he was drawn to the story because he was interested in exploring the idea of how a senseless act of violence could have tremendous consequences and cost someone everything they had worked for.

“It was just this idea of a very privileged but also pretty good kid who believed in the ethos of his school and was the sort of boy that you’d really like if you met and to think about that character coming undone; it was a very sort of powerful story.”

For Irish actor Reynor, who appeared in the ensemble thriller “Dollhouse” and recently signed with talent agency WME, it’s a story he says he could relate to because he shares a similar upbringing with his character.

“It’s funny how often I would see guys I went to school with and guys I knew just from being friends break down when something happened in their life that just didn’t go their way. It shattered everything they believed about themselves,” says Reynor.

Abrahamson says it made Reynor the perfect person for the role.

“He understands the world the character comes from because he comes from a similar world. In his own way, he has some really important traits in common with the character . . . Jack has the charisma that an actor needs and he’s also a great actor, so I don’t think we could’ve made the film without him.”

While some of the references in the film cater specifically to an Irish audience, both Reynor and Abrahamson believe the film has a more widespread appeal.

“No matter where you are, no matter what society you come from, I think it’s something that speaks to people on a very primal and personal level,” says Reynor.

“We all have to come to terms with failure at some stage in our life,” adds Abrahamson. “Most of us don’t have to do it that quickly, that early and to that extreme extent but we have to come to terms with not always being the people we fantasized about being when we were growing up.”

One of the most powerful scenes in the movie occurs when Jack tells his father Peter (played by Irish actor Lars Mikkelsen) what happened on that fateful night.

The struggle between doing the right thing morally and protecting your family is something that hit home for Mikkelsen, who is a father himself.

“If that happened I’d be in doubt of what to do, even though in being a real good person himself, it would be really hard to choose . . . We are at certain times immoral, if we’re put to the wall,” says Mikkelsen.

Abrahamson, whose next project will be a comedy called “Frank” starring Michael Fassbender and Domhnall Gleeson, says he wanted the film to focus on not so much the consequences of Richard’s actions, but how he lives with himself while trying to decide whether to confess.

“Possibly the most disturbing aspect of the film I think is . . . people do terrible things and they can compartmentalize it, they can convince themselves it would be to nobody’s benefit that they ruin their own lives just for the sake of telling the truth,” says Abrahamson.

“I think maybe people have quite a naïve idea of how they would behave in extreme situations and we really wanted to hunt down the truth of that in this character.”

A public screening of “What Richard Did” will take place Saturday, Sept. 15 at 9:30 am at the Scotiabank Theatre.