Paul Bernardo and Karla Homolka met in October 1987 and instantly became obsessed with each other. At six years older than Karla, Paul had significantly more sexual experience, both consensual and non-consensual. Paul was the Scarborough Rapist, attacking at least 19 women between 1987 and 1990. Karla maintained that she never thought Paul could be the rapist terrorizing the area.

Paul proposed to Karla in December 1990, and they were married in Niagara-on-the-Lake on June 29, 1991.

As the Scarborough Rapist, Paul terrorized his victims: stealing IDs, noting addresses and threatening to kill her family. He stabbed some of his victims, and his attacks were so violent, he broke another victim’s arm.

Karla worked at a veterinary clinic and had access to both Valium and halothane (a general anaesthetic), which she stole to use in crimes against her sister that she and Paul would commit.

On December 23, 1990, Homolka mixed her sister a drink of eggnog and rum to get her intoxicated, making it easer for Karla and Paul to implement their overall plan.

Homolka prepared a dish of spaghetti for her 15-year-old sister, lacing it with Valium she had stolen from the veterinary clinic. She was excited to give Paul his Christmas present: her 15-year old virgin sister. She swiftly succumbed to unconsciousness.

Karla poured the halcion that she stole from the veterinary clinic on a kitchen towel, and she and Paul took turns holding it over her sister's mouth to keep her unconscious. They also take turns raping her, all the while filming the entire assault.

Three weeks following Tammy Homolka's passing, Karla and Bernardo created a video known as “The Fireside Chat” within the Homolka residence, which would later serve as evidence in court. The video commenced in the basement recreation room but eventually transitioned into Tammy's bedroom. While in the recreation room, Karla confessed to Bernardo that she had taken pleasure in Bernardo's assault on Tammy. Additionally, she expressed a desire to leave a rose at Tammy's gravesite during the recording. As they moved into Tammy's bedroom, Karla further immersed herself in the role, donning Tammy's attire and imitating her sister. Disturbingly, they engaged in sexual activity on Tammy's bed.

In the early hours of June 15, 1991, Bernardo took an unplanned detour through Burlington, situated halfway between Toronto and St. Catharines. It was during this detour that he came across his victium standing outside her residence. The 14-year-old had missed her curfew after attending a friend's wake and found herself locked out of her house. Bernardo decided to leave his car and approach Mahaffy, initially claiming that he wanted to break into a neighbor's home. Unperturbed, she inquired about cigarettes, and he asserted he had some in his vehicle. Bernardo then led her to his car, blindfolded her, and forcibly placed her inside. He subsequently drove her to Port Dalhousie, where he informed Homolka that they had a victim.

Bernardo and Homolka videotaped themselves torturing and sexually abusing their victim while they listened to Bob Marley and David Bowie.

The following day, Bernardo claimed, Homolka fed their victim a lethal dose of Halcion; Homolka claimed that Bernardo strangled her. They put her body in their basement, and the day after the Homolka family had a Father's Day dinner at Bernardo's and Homolka's house. After the Homolkas and their remaining daughter Lori left, Bernardo and Homolka decided that the best way to dispose of the evidence would be to dismember the victim and encase each part of her remains in concrete. Bernardo bought a dozen bags of cement at a hardware store the following day; he kept the receipts, which were damaging at his trial. Bernardo used his grandfather's circular saw to dismember the body. Bernardo and Homolka made a number of trips to dump the cement blocks in Lake Gibson, At least one of the blocks weighed 200 pounds and was beyond their ability to sink. It lay near the shore, where it was found by a father and son on a fishing trip.

Paul started to subject Karla to brutal assaults, and during one such occurence, Paul would repeatedly strike Karla with a metal flashlight and stab her with a screwdriver. The force of the blows was so intense that it caused her brain to collide with the front of her skull, resulting in a distinct “raccoon-eyes” appearance.

On April 16, 1992, Bernardo and Homolka drove through St. Catharines to look for potential victims. They spotted a 15-year-old walking home and pulled into a nearby parking lot. Homolka got out of the car carrying a map, pretending to need assistance. The girl wanted to help, and looked at the map with Karla. Bernardo then attacked from behind and forced her into the front seat of the car at knifepoint. Homolka subdued the victim by pulling her hair from the front seat.

Following a horrifying ordeal of 24 hours involving rape and abuse at the hands of both perpetrators, the victim met her tragic end on June 15, 1991. According to Homolka's account, Bernardo resorted to strangling her with an electrical cord for a second time when the initial attempt had rendered her unconscious for a brief period. It is worth noting that Homolka, who worked as a veterinary technician, possessed access to sedative drugs, which were employed to subdue the victim — a technique mirroring the one used in the previous rape and unintended killing of Homolka's sister six months earlier.