“While there are filmmakers who look for stories,” he told the audience Wednesday night, “this was a story that found a filmmaker.”

Gwyneth Paltrow agreed to narrate the film and Lyle Lovett recorded a song.

As a little boy, Welles Crowther followed his father’s lead as a member of the local Empire Hook & Ladder Co., No. 1 in Upper Nyack. By age 7, he was helping clean the fire trucks. By 16, he became a junior member and gained full firefighter status at 18.

After attending Boston College, Welles Crowther went to work for Sandler O’Neill, an investment bank, and was working on the 104th floor of the South Tower on Sept. 11, 2001, when United Airlines Flight 175 hit the building.

After leaving his mother, Alison, a voice mail message telling her that he was O.K., he was never heard from again. Ms. Crowther said she followed her “mother’s instinct” after the attacks, searching for information about her son and his final moments. She combed through news coverage even after her son’s remains were recovered from ground zero six months after the attacks.

Two months after that recovery, on Memorial Day 2002, she read a lengthy New York Times article on the chaos inside the towers before they collapsed, which included eyewitnesses describing an unnamed rescuer: a coolheaded office worker who appeared in the Sky Lobby on the South Tower’s 78th floor.

“A mysterious man appeared,” who managed to locate the only passable stairwell and began marshaling down groups of injured and dazed people, according to the article, which also gave a telling detail about the rescuer that floored Ms. Crowther. He wore a red bandanna over his face to keep out smoke and debris.