During the early days of Russia’s 2022 invasion of Ukraine, extraordinary photos and videos were emerging from the besieged port city of Mariupol.

Those visuals, from The Associated Press, included scenes that have become defining images of the war: A maternity hospital, bombed. Dead children and grieving parents. Mass graves.

As it happened, we were already collaborating with the AP on what would become our award-winning work to document potential war crimes, and we began a conversation about whether the story they’d been capturing in Mariupol could become a documentary.

That’s how we would come to know Ukrainian AP video journalist Mstyslav Chernov. He and his colleagues, photographer Evgeniy Maloletka and field producer Vasilisa Stepanenko, were the last international journalists reporting from Mariupol as Russian troops attacked.

“At some point I realized that every single moment matters,” Mstyslav would later tell me. “That I have to just record everything.”

That’s exactly what he did, filing news dispatches whenever he could find a signal, and eventually escaping Mariupol with 30 hours of footage. From my very first meeting with Mstyslav, soon after his escape, it was clear that he was thinking about what he had documented with a larger story — and history itself — in mind.

As he told me earlier this year, “I know that we form our understanding of the current events of the world around us by watching news and consuming news. But we as a generation form our understanding of our past with documentary films … Film is a medium which carries meaning across time, for generations to come.”

We soon began working with Derl McCrudden, vice president of news and head of global news production at the AP, Mstyslav, and FRONTLINE producer and editor Michelle Mizner, poring over Mstyslav’s footage and making the feature film that you will see on PBS tonight. Called 20 Days in Mariupol, it won the audience award for World Cinema Documentary when it premiered at the Sundance Film Festival earlier this year, and it is an unflinching account of journalists risking their lives to share the truth of the conflict with the world.

As you watch the documentary tonight, you’ll see that, again and again, the people Mstyslav encounters implore him to keep filming — from a doctor performing an operation on a gravely wounded four-year-old and saying, “Show this Putin bastard the eyes of this child and all these doctors who are crying,” to a policeman outside a destroyed maternity hospital begging the world for help.

Mstyslav, Evgeniy and Vasilisa have watched this film with people who escaped Mariupol. It’s been incredibly meaningful for these people to know that what happened to their city was documented, did not go unnoticed, and is in fact being seen.

The Guardian calls 20 Days in Mariupol “a brave, visceral, merciless masterpiece,” The New York Times says it is “essential. A relentless and truly important documentary,” and The Wall Street Journal calls it “grueling but vital.”

You can stream 20 Days in Mariupol starting tonight at 7/6c at pbs.org/frontline and in the PBS App, or watch at 10/9c on PBS stations (check local listings) and on our YouTube channel.

Thank you for watching — and in doing so, joining Mstyslav and his colleagues in bearing witness.