The murder of 15-year-old Peter Watts, exactly 40 years ago, is a story with a beginning, a middle, but no end.
The teenager went missing from his home in Colwyn Bay in Wales on January 18th 1976 and was found dying in the Euston Underpass the next day.
There were no witnesses or forensic evidence and the details of how and why Peter came to be in London are still unknown. His brother Mark told me his death has been a mystery for 40 years and he was "very afraid" it would remain that way.
Just before he disappeared, Peter is thought to have written a note saying he was going to a friends to study for a mock exam he had the next day. His parents had taken Mark for a driving lesson.
Taking ten pounds, it's thought Peter left his home shortly before 4pm. A British Rail ticket clerk believed Peter gave him a £10 note for a train ticket to Chester but he could not be sure.
Despite his uncertainty the focus of the entire police investigation then concentrated on the possible journey with missing posters being handed to commuters who would have been on the same train. However, extensive inquiries did not reveal a single person who'd seen Peter.
Peter was discovered by a taxi driver ten hours after he was last seen by his family, lying barely alive in the Euston Underpass - 250 miles from home.
He had suffered an extensive fracture of the skull and other internal injuries which were consistent with a fall but there were no fragments of dirt from the road in the head wound and nothing to show he'd been in a fight or involved in an road accident.
Personal belongings, including his watch and glasses were missing.
The inquest jury returned a unanimous verdict of murder by a person or persons unknown. In his closing remarks the coroner said the case was odd, and notes written by the lead detective at the time show he was baffled.
Detective Inspector, Susan Stansfield, from the Metropolitan Police Homicide Special Casework Investigation team which specialises in cold cases met with Peter's brother to look through the file.
Could the missing piece of the puzzle be this green car?
Peter's parents always believed their son had been abducted but could never be totally sure. Mark, who is the only surviving member of the family, said his parents spent their life feeling like they had let Peter down in some way.
The case remains a mystery and after examining the file DI Stansfield says the only hope of ever solving it is if someone who knows what really happened to Peter is still alive and prepared to come forward.
For Mark, the urge to solve the mystery of his brother's death remains, but he is not hopeful.