4 actors, 1 room unspool a shooting’s aftermath in ‘Mass’

Jason Isaacs was at a Starbucks when he sat down to read the script for “Mass “for the first time. It was a decision he’d come to regret, if only because at some point he started “heaving with sobs” and couldn’t stop. Strangers in the shop looked at him like they were debating whether to call a professional in to help.

But he was too wrapped up in the story to really take notice. When he came to the end, he put the script down and took a breath.

“I felt like I’d been through something enormous,” Isaacs said. “I remember thinking this is insane. But it’s also the reason I became an actor.”

“Mass,” which is now playing in theaters, has a simple set up: Four adults gather in a non-descript meeting room at a church six years after a shooting at a high school. One set of parents had a son who was killed. The others are the parents of the perpetrator. And for almost 100 minutes, they talk.

The Starbucks breakdown was just the beginning of what would prove to be a life-changing experience for Isaacs and his co-stars Martha Plimpton, Ann Dowd and Reed Birney. The resulting film is no less significant: It is one of the most wrenching of the year. And it comes from a first-time writer and director, actor Fran Kranz.

Kranz wrote the script after the Parkland High School shooting. He was a new father at the time and trying to make sense of things — trauma, violence, grief, tragedy, forgiveness — and reading about the South African Truth and Reconciliation Commission before putting words to the page. And it will come as no surprise that it started as a play, before Kranz and his producer realized they’d have a better shot at getting it made as a movie.