He'd sat down to watch a documentary about the Second World War and suddenly found himself looking into the eyes of a man he'd faced 74 years previously - a man responsible for countless barbaric deaths.
The name too - Heinrich Neuschwanger - had remained burned into John Harding's brain ever since that day in 1946 when the pair had been handcuffed together in the back of a truck.
The 93-year-old from Cardiff had only been a teenager when he was assigned to escort the German - one of the Nazi regime's most feared officers - to be tried for war crimes.
Unaware at the time of the atrocities Neuschwanger had ordered, the former Llanrumney store worker said he couldn't believe he'd rubbed shoulders with the embodiment of pure evil and had never told a soul about the encounter - until now.
"I was watching a programme called Nazi Hunters when it hit me - 'Good God, I was there'," he said.
"I'd been a private in the Third Monmouthshire Regiment and had been assigned to escort various German officers to court in Wuppertal, Germany.
"I'd been stationed over there a couple of months when my path crossed with Neuschwanger, and I never ever forgot that name."
John added that the feared Nazi lieutenant - notorious for stamping on his prisoners before having them shot and nicknamed 'Stuka' after the German dive-bombers - had told him in broken English that he was a doctor.
"That's not all he was though, all of which I found out about after he'd had his hearings."
The documentary, shown on the Military Channel, told of how Neuschwanger - of whom very little footage or photos exist - oversaw the grisly deaths of four female special ops agents who'd been captured whilst undercover.
He had them injected with carbolic acid, thereby causing agonising multi-organ failure - he also beat one POW so badly his bones showed through his skin.
After the war ended he was eventually captured by a top secret SAS unit formed to hunt down Nazis-in-hiding.
Neuschwanger was discovered living in the Russian-controlled zone of Germany, beyond official British jurisdiction, but was lured across the border after the crack Allied team duped him into taking part in a bogus 'get rich quick' scam on the black market.
"I wish I'd known just who he was at the time - I'd have given him a lot more stick," said John.
"As it goes, he did ask me if I could pull the truck over at one point so he could go to the toilet.
"But, as I told the blokes I was with, there was no way I was going to stand handcuffed next to him while he did a number two at the side of the road.
"So he had to wait until we got to the court room at Wuppertal."
And it where there Neuschwanger was found guilty and handed a death sentence - eventually being hanged on August 3, 1946.
"After his hearing we took him and a couple of other Nazis to prison and he sat there in silence the whole journey.
"I was with two brothers by the name of Clark from Ebbw Vale and they had a length of rope they'd fashioned into a noose, and one of them dangled it in front of the Germans.
"One Nazi got up and went to hit him, but the other brother drove his rifle butt into his shoulder and he went down like a sack of spuds.
"Didn't stop them turning and giving us the Nazi salute when we dropped them off at the jail, mind."
John said that it felt good to finally talk about what he'd done during the war.
"I came home after two years and never really discussed what had happened with my family or friends," he added.
"But seeing that documentary made me realise that I'd been there at a pivotal moment in the conflict.
"Looking back, I'm proud of the part I played."
