On 5 November they abducted 21-year-old Denise Brown as she was waiting for a bus on Stirling Highway. Denise was a funloving girl who worked as a part-time computer operator in Perth and spent a lot of her spare time at dances and nightclubs. She shared a flat in Nedlands with her boyfriend and another couple. Denise spent her last night at the Coolbellup Hotel with a girlfriend. She accepted a lift from the Birnies outside the Stoned Crow Wine House in Fremantle. A close friend said later: "She was someone who would do anything to help anyone. She trusted too many people. Perhaps that is why she didn't think twice about taking a lift."
At knife point Denise was taken to the house in Willagee, chained to the bed and raped. The following afternoon she was taken to the Wanneroo pine plantation. Along the way they nearly picked up another victim. After the Birnies' capture, a 19-year-old student told police how she was offered a lift by two people who she later recognised as Catherine and David Birnie from photos in the newspapers.
After finishing university for the day, she was walking along Pinjar Road, Wanneroo, when a car pulled up beside her. There were two people in the front and another slumped in the back seat. Later she realised that the person in the back was probably Denise Brown.
She went on: "I felt uneasy. I didn't recognise the car. There was a man driving and a woman in the front seat of the car. The man kept looking down, not looking at me and the woman was drinking a can of UDL rum and coke. I thought the fact that she was drinking at that time of day was strange. He didn't look at me the whole time. It was the woman who did all the talking. She asked me if I wanted a lift anywhere. I said, "No, I only live up the road".
"They continued to sit there and I looked into the back seat where I saw a small person with short brown hair lying across the seat. I thought it must have been their son or daughter asleep in the back. The person was in a sleeping position and from the haircut, looked like a boy but for some reason I got the feeling it was a girl. I told them again I didn't want a lift because walking was good exercise. The man looked up for the first time and gazed at me before looking away again. By this time more cars had appeared and I started to walk away but they continued to sit in the car. Finally the car started and they did another U-turn and drove up Pinjar Road towards the pine plantation. It wasn't until I saw a really good photo of Catherine Birnie that I realised who they were. Somebody must have been looking after me that day. I don't know what would have happened to me if I had got into that car."
Safely in the seclusion of the forest, David Birnie raped Denise Brown in the car while the couple waited for darkness. They then dragged the woman from the car and Birnie assaulted her again. In the light of Catherine's torch, Birnie plunged a knife into Denise's neck while he was raping her.
Denise didn't die straight away. Catherine Birnie, still holding the torch, found a bigger knife and urged her lover to stab her again. He didn't need much prompting. He wielded the knife until Denise lay silent at his feet. Convinced that the girl was dead, they dug a shallow grave and lay her body in it.
As they were covering Denise Brown with sand, she sat up in the grave. Birnie grabbed an axe and struck her full force on the skull with it. When the girl sat up again, he turned the axe head around and cracked the girl's skull open. They then finished covering her with sand.