American veterans of the Vietnam War are often described as 'haunted'.
In the USA, it's a metaphor, shorthand for psychological trauma or PTSD.
But what happens when the veterans return to Vietnam - and the ghosts become real?
Nearly four decades after what's known in Vietnam as the 'American War', more than 300,000 Vietnamese people are still classified as Missing in Action.
Their families continue to search for remains.
They believe their loved ones have become 'wandering souls' - doomed to eke out an impoverished existence on the edge of the everyday world.
These ghosts are often angry.
They're imprisoned by bad memories - the traumatic and unjust nature of their final moments.
Many Vietnamese believe these unhappy spirits cause bad luck and frightening dreams...
or worse, can possess the living and make them experience the physical agony of a battlefield death.
During the War, the American military exploited this belief - playing the desperate moans of a wandering soul from helicopters to scare the opposition.
The 'wailing ghost' psy-ops tape was so frightening soldiers were instructed not to play it over friendly South Vietnamese forces - in case it encouraged desertion.
But nearly forty years on, some American veterans are moved by the beliefs of their old enemies.
They feel a connection with the Vietnamese ghosts...
it could so easily have been them.
And perhaps the identification goes deeper - between the ignored and angry wandering souls...
and the survivors of a war America would rather forget.
Cathy FitzGerald follows a group of American Vet.s as they search for a mass grave of soldiers they once killed.
Producer: Cathy FitzGerald
A Rockethouse Production for BBC Radio 4