“You know what I think happens?” Sebastian Junger says in his new documentary, “The Last Patrol” (Monday night on HBO). “I think men protect women, and women protect children.” Expanding on his idea, he says of the 2012 theater shooting in Aurora, Colo. — where several men died shielding their girlfriends — “not one woman stepped in front of a man, because that’s not their job.”

“The Last Patrol” covers a lot of ground in its 88 minutes. It’s an American road story, shades of Kerouac and James Fenimore Cooper, that looks like a war movie or a Depression-era adventure when its four long-distance hikers ford streams or take cover to avoid railroad cops. It’s a Robert Frank-style collection of vignettes of everyday Americans along the Amtrak corridor north of Washington. It prominently features a lovable dog named Daisy.

Mostly, though, it’s a film about manhood and male camaraderie: the latest of Mr. Junger’s meditations on men and combat, with a subtheme of how fathers can unknowingly push their sons toward war. These subjects are treated with intelligence and finesse, but despite this — or perhaps because of it — what may stand out for many viewers are the moments when the maleness gets a little stark, as in the Aurora discussion. It’s not the only time in the documentary when women are referred to as “they.”

The idea for “The Last Patrol” came about when Mr. Junger and his filmmaking partner Tim Hetherington were on a train and Mr. Junger, looking out the window, imagined walking the tracks. Mr. Hetherington, who would die while photographing combat in Libya in 2011, happened to be filming as they spoke, and that moment provides an eerie opening scene. “We’ve got to find this footage sometime in the future,” Mr. Hetherington says. “After the world has changed.”