Dahina Sy-Le Guennan was one of Fourniret's earliest victims. He won her trust, abducted her and then raped her. She was only 14, but clearly remembers him explaining, "he was a virgin when he got married, and that on his wedding day he found out his wife was not a virgin. And that since then he had been chasing the myth of virginity." Dahina survived the ordeal, and would later testify against him. The obsession with virginity that she speaks of would prove the defining characteristic of Fourniret's career.

It was also what led him to Monique Olivier. "The first time they met is at an early trial. They make a sort of criminal pact. He will give her the revenge she seeks on the men who hurt her. In exchange she will give him a number of virgins." Once out of jail, Fourniret moves in with Olivier. Remarkably, the police make no effort to track him. Within 6 months he has committed his first murder.

Fourniret and Olivier were happy to talk the police through their techniques. They would identify a victim and follow them in the car, before manipulating them. In once case, "Olivier was 8 months pregnant, they said she needed a doctor." Then Olivier threatened the victim with a gun, while Fourniret tied her hands. "He then told Olivier to check that the girls are still virgins. She feels to see if they are still virgins."

"You'll notice that whenever Monique Olivier was involved there were no survivors. All those girls were murdered." Stephane Bourgoin is a psychologist who has analysed the psyche of several serial killers, "there is a perverse love between Monique Olivier and Michel Fourniret that can only be consummated in the presence of a third person. It is a threesome." It is this terrifying reliance on each other that led to the murders.

But outwardly the pair appeared so normal, Gerard Cade lived next door to the Fournirets, he shows us a video of his son's communion party, "We invited him because he used to give my son some pocket money every now and then. Just a month before, he had murdered a girl. We had let a monster into our house. That doesn't bear thinking about."

Fourniret has always tried to present himself as a sophisticated man. "But there is nothing behind it. Nothing. Not a shadow of remorse, not a trace of conscience." His wife's complex obsession with power bounced off this emptiness. Ultimately, she would prove his downfall. It was Olivier who turned Fourniret in to the police.

This tragic, twisted case has seen families devastated by grief and anger and surviving victims scarred for life. Finally, Fourniret and Olivier have been convicted, sentenced to life imprisonment. But in a terrible last twist, they still hold the cards. Only they know where the remaining victims are buried, and they refuse to tell.