Anyone who watched the beheading videos released by Islamic State group showing the deaths of hostages such as British aid worker David Haines will never forget seeing them.
Even those viewers with only the slightest amount of empathy must have felt horror and total emptiness when watching life disappear at the hands of an IS executioner.
These propaganda films were made to terrify, to sicken. And they succeeded.
Bethany was aged 16 when he was kidnapped - 17 when he was killed. Five years on, she described to me how she locked herself in a bathroom to watch the footage.
“I thought I would be really upset, but it was just numbness,” she said. “I think it was shock. So, for a while I didn’t feel anything.”
In a similar situation to hers, many of us might have been content to leave some of the burning questions unanswered – to find a way to simply ignore them. Not Bethany.
As I sit with her crossing from Iraq into Syria it is difficult to underestimate the magnitude of the journey that lies ahead.
She has brought with her files of hand-written research about IS and its western hostages including maps and colour-coded charts offering theories about where her father’s body might be.
Her work is impressive - even to my colleagues who have spent the last few years investigating IS.
After two days in Syria, Bethany is able to ask questions of a commander from the Syrian Democratic Forces.
He cannot tell her where David’s remains are, but promises that wherever they are found, a street will be renamed in his honour.
The commander takes us into Raqqa - the former de facto capital of the Caliphate – to see the spot where Mohammed Emwazi was killed.
Bethany says she is struck be “a sense of finality” after seeing the damage done by the drone strike.
Then at a camp for mothers and children who had lived under IS, one woman offers an apology on behalf of IS.
I’m struck by Bethany’s willingness to accept it – to forgive. But later, she is confronted by another former IS follower who dismisses the actions of the terror group.
Seeing the fury of this apologist close-up is an unnerving experience for Bethany.
But it is part of an experience that appears to leave her relieved and empowered.
Bethany smiles as she crosses the border back to the relative safety of Iraq.
She wants to return to Syria soon and hopes her next trip will be to bring her father home.