We connect to that day in our own way, and the storyteller in the broad-brimmed ranger’s hat is no different. He was born into the military, his father an Army lifer who served in World War II’s European theater, his mother a daughter of the French underground. They were married at the Cathedral of Notre-Dame in Paris, and went on a short honeymoon in an Army jeep.

Their son Rob spent the better part of two decades flying Army Hueys and Black Hawks and training other soldiers how to fly helicopters. He left the service in early 2001, and was at home on Cape Cod, Mass., that Tuesday, watching the news. He thought of those he had trained, and felt guilt for not being among them for the deployments sure to come.

Mr. Franz focused on a real estate career, volunteered with the local veterans’ committee and worked briefly as a police officer. Then, in late 2011, he spotted a listing on a government website for a seasonal job as an interpretive park ranger at the Flight 93 Memorial. He quickly applied, he recalls, sensing a chance to “complete the circle.”

Soon he was driving about 600 miles west to Shanksville every April, and staying until October. He proved to be such a powerful storyteller, his presentation informed by his knowledge of aeronautics, that he was recently offered permanent employment, which he accepted.

“He told this story unlike anyone I had heard,” says Stephen Clark, the superintendent for the national parks in western Pennsylvania. “And, of course, being a veteran makes it all the more special.”

Sometimes Mr. Franz stands at the memorial plaza, answering questions about the time of the crash and the location of the bathrooms. He commiserates as people recount their own connections to the day, and keeps his counsel as conspiracy theorists question whether such a crash even occurred. “If somebody’s made up their mind, there’s nothing I can do,” he says.