“Are you ready? Okay, let’s roll.”

The final words of Todd Beamer, one of the four men believed to have overpowered hijackers on United Airlines Flight 93, match the sustained tone of National Geographic’s latest six-part memorialization of the attacks on Sept. 11, 2001.

9/11: One Day in America, an all-new, comprehensive retelling of the infamous day that launched a forever war and shocked the world, can only be described as heartbreakingly thorough. With never-before-seen footage and all-new interviews, the documentary series from the production team of Dan Lindsay and T.J. Martin (“Undefeated” and “LA 92”) is a devastating yet mesmerizing portrait of America on perhaps the nation’s worst day in generations.

While the documentary begins like many accounts that preceded it — imagery of New York City firefighters and businessmen who worked at the World Trade Center — little time elapses before realizing this series is different.

Audio and video footage spans past and present narratives while highlighting vantage points that range from the casual New Yorker standing horrified outside of a taxi to the frantic personnel inside the actual towers.

Calls previously released from American Flight 11 are included, as is an account of one man working in the WTC whose sister and niece were onboard the plane that hit the North Tower. There’s the fire chief who lost his brother after giving him one final order, a mother who hoped her kind-hearted son could be a killer when he needed to be, a security officer who sang people to safety, a man buried alive and left behind, a fighter pilot faced with a decision to kill innocent Americans — and herself with them — before another plane reached its target.

Still, perhaps the most beautiful aspect of this oft-agonizing and meticulous series lies not within the footage of witnesses, but in the moments of hope and resiliency that underscore those fateful events and the days that followed.

Viewers will watch horrified one moment as clouds of ash, smoke and debris swallow screams and people alike, only to be rejuvenated the next by a woman, who should have died, sitting in a hospital bed happily showcasing a gift from her fiancé, or even a former Marine-turned-firefighter recounting the heroism of a colleague who saved his secretary.

Every viewer may know how the story ends, but this immersive retelling leaves no doubt as to why that day 20 years ago should remain prominent in our memories.

“This was my world, never to be the same again,” one interviewee says.

That same sentiment will no doubt resonate with viewers.

National Geographic’s 9/11: One Day in America is currently available to stream on Hulu, and will air in three-episode segments on the National Geographic channel on Sept. 10 and 11.

Observation Post is the Military Times one-stop shop for all things off-duty. Stories may reflect author observations.

Rachel is a Marine Corps veteran and a master's candidate at New York University's Business & Economic Reporting program.

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