Interviewed for this documentary about veterans living with post-traumatic stress disorder, war photographer Lalage Snow talks about her series of triptych portraits of soldiers: three photos, taken before, during and after deployment in Afghanistan. You find yourself searching for the psychological scars written in their tired, thinner faces. In some of the “after” shots, it’s as if war has completely rewired them. Their features look subtly but noticeably different, muscles in their faces clenched, hardening their expressions – or somehow making them softer. It’s extraordinary work.

Snow says she always wanted to go back, photograph the soldiers a decade after the war, but hasn’t been able to get funding. Instead she talks here in Kate Blewett’s straightforward sympathetic documentary following three veterans living with PTSD (none of them featured in Snow’s work as far we know). One of them is Amber, who enlisted at age 16 after a childhood of domestic violence and sexual abuse. Her emotional rawness and vulnerability are very brave, but almost unbearable to watch in places. Former Royal Marine sniper Stuart is on his second attempt at rowing solo across the Atlantic. He wants to prove that PTSD doesn’t have to hold you back. It’s also a kind of therapy, he says, “To keep my demons at bay.” Blewett gently asks him how it changed him, being a sniper. He won’t be prodded. “It’s a job, you get paid to do it.” Which leaves you wondering if there is trauma he doesn’t want to go near.

We also meet a former US marine called Ryan, now serving with Vetpaw, a group of veterans protecting wildlife in Africa. People assume they are a bunch of gung-ho, trigger-happy bros, but Ryan says that they’ve all seen enough death to last a lifetime. What Vetpaw gives him back is the “brotherhood” he had in the marines. Back home in the US he was sick of the pity and questions. “I don’t want to reintegrate into society. Society sucks.”

• Disorder is released on 28 November on digital platforms.