It was lunchtime when Joyce went missing. She usually went home to see her mother who'd have a meal waiting on the kitchen table.
But, on 28th September 1939 she didn't come home.
Someone had taken the four year old, sexually abused and murdered her.
Who did it? Nobody knows for sure.
Joyce Cox's murder shocked a nation and it coincided with a cataclysmic world event - World War Two had just broken out.
Police at the time had a suspect in mind but they didn't have the evidence. DNA hadn't developed to the standards we have today.
Despite the passage of time, detectives from South Wales Police never closed the case. From time to time they've reviewed the evidence using advances in forensic science.
ITV Wales understands that only this week police have been able to say for certain that the prime suspect from the time is dead.
So why investigate? Well Joyce Cox' family want closure. Her cousins Terry and Peter have fought for justice. Terry told ITV Wales he will never be able to move on but he hopes his family can.
DCI Mark o'Shea heads the cold case unit at South Wales Police and is hoping someone who's held a secret for the last 78 years might come forward.
"Whoever the killer was would have probably have told somebody at some point. It would be a big secret to keep. If anybody has ever received any information about this murder please come forward, please let us know."