Fran Kranz was driving in Los Angeles three years ago when, suddenly overwhelmed, he pulled over. The actor had been listening to a news broadcast about the school shooting in Parkland, Fla., and needed a moment to collect himself as he processed the anguish of a parent interviewed on the radio.

Maybe it was that Kranz had recently become a father himself, he wonders aloud in an interview with The Washington Post, or he just experienced a human reaction to hearing someone grapple with the trauma of senseless violence. Numbness can at times feel like a natural response to such news in a country where school shooting drills have become part of curriculums. That trauma has shaped our national psyche. Kranz grew up with it; he was a teenager at the time of the Columbine shooting in Colorado.

But after Parkland, he couldn’t stop thinking about what the parents of school shooting victims endure, how they are left to wonder what it could possibly mean for their children to have died in this manner. He began to research their stories and, while doing so, also came across accounts from the families of people who have committed similar crimes. He began to see those parents as victims, too.

“There was an opportunity to give equal value to each of their stories,” Kranz says. So he took it.

“Mass,” Kranz’s directorial debut, can be difficult to distill. The film is at its core a contemplation of restorative justice, set six years after a school shooting and centered on a meeting between the parents of a victim (Jason Isaacs and Martha Plimpton) and those of the perpetrator who then killed himself that same day (Reed Birney and Ann Dowd). The tension makes for a heavy but captivating watch, the actors navigating in real time a conversation that goes from strained to charged, then back again.

Kranz’s storytelling approach required a delicate hand. What does it mean to assign equal weight to stories of grief that exist in opposition to one another? How do you mourn someone responsible for the destruction of human life? How much responsibility should a parent take on for the actions of their child? “Mass” doesn’t provide any firm answers, but it allows its characters to demand them.

“There’s a way we live with grief and try to keep the past in a way that helps us survive, but that might not necessarily be the most healthy thing. It might truly be eating away at us,” Kranz says. His film dives headfirst into “how we live with grief and take care of the ones we’ve lost,” as well as how we overcome “the fear of sharing it with the people you feel might not deserve to be a part of it.”

Dowd knew she wanted to sign onto “Mass” immediately after reading the script but worried whether she would be able to live with the level of heartbreak her character, Linda, experiences. She wondered whether she could capture the journey of a mother who truly believed she and her husband did all they could to care for their son, Hayden, who had increasingly externalized his violent thoughts. But the couple could never have fathomed that he would act upon them by committing such a devastating crime.

Eventually, Dowd says, she accepted “the honor, if you will, of entering the life of someone who not only has the grief of losing her son, but also has the profound guilt of her son killing others. ‘As a mother, what did I miss? How did I miss it? I loved my boy, and I know I have no right to do so at this point,’ would seem to be the worry that she has. Can you even imagine being that person?”

The premise of “Mass,” which Kranz wrote after also researching South Africa’s post-apartheid Truth and Reconciliation Commission, asks the four parents to envision life from each other’s perspectives. Aside from the bookending scenes, the bulk of the film takes place in the empty meeting room of an Episcopal church, where the couples sit across from one another. They’ve each brought family photos with them and ask about the other couple’s surviving children. Their initial interactions feel desperate and forced, as though the brief niceties will dictate how their relationship exists from that point forward.

Of course, the mood evolves. Each character arrives with expectations that unravel as the story goes on, each actor responding to their co-stars with a certain level of improvisation. The victim’s mother, Gail (Plimpton), enters the room with seething resentment cloaking her desire to finally forgive. Her husband, Jay (Isaacs), attempts a more methodical approach that counters his wife’s surfacing emotions.

But even Jay eventually strays from his original demeanor. When the shooter’s father, Richard (Birney), suggests that he understands the fear and agony the victims experienced as Hayden threatened their lives, Jay raises his voice into a defiant yell: “You don’t know,” he directs at Richard. “I know.”

“It felt like an honest sentiment for a father to express,” Kranz says. “I wouldn’t want to share my child with someone that I hated. My child’s memory. And that’s the difficulty of the journey Jason’s character is on. I don’t think he realizes how imprisoning it is, and how held back he is by his anger.”

Isaacs describes the emotional rhythm of the film as “beautifully calibrated, like a great symphony.”

“Mass” is political by virtue of its subject matter. But it avoids delving into any discussions of legislation beyond a moment in which Jay touches on the moral obligation he feels to speak publicly about what happened to his son, Evan, and another in which Gail says she wishes something had changed after the shooting so Evan’s death could carry that added layer of meaning. Kranz’s screenplay is more concerned with accountability as it relates to the characters blaming themselves, and one another.

To an extent, Isaacs adds, the exact incident that occurred in the film six years ago is irrelevant.

“It could be a car accident. It could be a political vote. It could be refusing to wear a mask and infecting someone in the family,” he says. “It’s about people who are divided. Four people walk into a room. We all have a pretty fixed idea of what’s gone wrong and who’s to blame. … They have a plan for what’s going to happen. And then, like in the best dramatic human encounters, it falls apart.”

At times, “Mass” is as interested in exploring marriage as it does parenting; though they never explicitly say it, Linda and Richard’s behavior suggests they are no longer together. The film considers how trauma drives people apart, all while moving toward the goal post of it binding the four parents together. A philosophical component that does relate to politics, per Isaacs, is how they’re “trying to find a way to free themselves from the crippling, paralyzing trenches they’ve dug for themselves.”

We all operate from these trenches, to some degree — perhaps especially on a societal level. The film takes place in a polarized nation that Isaacs describes as having served for four years as the “epicenter of blame.” Political and cultural divisions date back further. Somewhere along the line, according to Kranz, we “normalized hating people.” He hopes to help bridge the gaps.

“I worry about a country divided between us and them,” he says. “I worry about the country and the world my daughter is going to grow up into. If we can’t find a way to repair these relationships, I worry about our future. So I think there’s nothing more extraordinary and urgent and heroic than what these characters do that day. … They’re desperately trying to work through that to heal and survive.”

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