Anxious knocks at her flat door brought 68-year-old Elizabeth Gordon to answer in a hurry.
It was a work colleague of her neighbour across the landing, Brenda, and they'd called by arrangement to do some work. Trouble was, Brenda wasn't answering.
Brenda's workmate knew she was friendly with Elizabeth Gordon and, if she'd had to go out, she would've left a message with her. No message.
Elizabeth kept a key to Brenda's flat though. Their tenement, 13 Allan Street in Aberdeen, was the kind of place full of good neighbours you could trust.
Besides, Elizabeth and Brenda were pals and often spent evenings chatting.
Asking the colleague to wait, Elizabeth slipped into Brenda's home.
In the flat, everything seemed normal and Brenda's three cats purred as they rubbed against Elizabeth's ankles.
Not wanting to frighten her pal, Elizabeth edged the bedroom door open gently. Right into a scene of horror.
Brenda was in bed all right. Not sleeping. Dead.
"I saw nothing but blood and hair," the deeply traumatised Elizabeth later recalled.
It didn't take long for the cops and forensic team to work out the details.
Sometime in the evening of July 13 or early morning of July 14, 1978, someone broke in through the window of a spare room at the back of the house.
Brenda had been battered on the skull and face with a heavy, blunt instrument like a poker or spanner.
She was fully clothed and lay across the bed as if she'd fallen there. She hadn't been sexually interfered with.
The cops' immediate thoughts turned to a burglary gone wrong when the thief was disturbed. Nothing had been stolen.
Then, as usual, they considered a relationship soured by jealousy or money.
Brenda had been divorced three years before but she was on good terms with her former husband, Dr Christopher Harrisson, a biochemist.
They checked but his movements that night were accounted for.
[...]
The people of Aberdeen were terrified. A killer was on the loose and they could be next. Then the cops dropped a bombshell.
Some evenings every week, Dr Brenda Page worked as an escort girl. Everyone assumed that meant high-class call girl.
Surely even expensive prostitutes came in contact with lethal punters?
The cops were forced to go public. Dr Brenda Page wasn't a prostitute, they said, but worked for a respectable company that provided good-looking, well educated females as dinner guests for businessmen away from home.
Tall, beautiful, with lush brown hair, well read and polite - Brenda certainly fitted that bill. What was more, she didn't keep her "hobby" a secret.
But then why would she? It was the late 1970s - no HIV, free contraception, sex clubs a-go-go, all-night clubs, women's liberation.
Forget the 1960s, the 1970s was the true time of sexual and social freedom.
On the night she was killed, Brenda had been working as an escort dining with two businessmen at the plush Treetops Hotel.
Very quickly, the police cleared the pair of businessmen of suspicion. They were drawing blanks at every turn.