The following version of the novel was used to create this study guide: Nesbø, Jo. Killing Moon. Knopf, May 30, 2023. Kindle.

In the thriller Killing Moon by Jo Nesbø, the 13th installment in the Harry Hole series, Harry is desperate to save a friend from a Mexican crime family to whom the friend owes money. Harry accepts a job as a private investigator. The real estate mogul, Markus Røed, who is paying Harry to investigate the murders of Susanne Anderson and Bertine Bertilsen, is one of the main suspects in the crimes. As Harry works through the twists and turns in the case, he discovers Røed’s hidden sexual identity, a killer bent on revenge, and a unique parasite that has been bioengineered to make the victims compliant with their killer.

Both Susanne and Bertine, young women who go missing a week apart, were at a party thrown by Røed. Both were also his girlfriends. When Susanne’s body was found 17 days after she disappeared, it was missing her brain and one eye. Bertine’s body was found shortly thereafter. Her body was missing her head, making detectives believe that they were dealing with a serial killer. Katrine Bratt, a detective with the Oslo Police, requested Harry be called in to help on the case because he was a specialist in serial killer. However, her superior officers would not consider using his services.

When Susanne’s body was found, Røed and his lawyer believed that they needed to actively prove Røed’s innocence. So, they tracked down Harry in America and hired him to investigate on Røed’s behalf. Harry agreed because he learned his friend, Lucille Owens, was going to be killed by members of a Mexican mafia family if she did not repay them the $960,000 that she owed them. What Harry would make investigating the case would pay Lucille’s debt and ensure that Lucille and Harry would be allowed to live.

Meanwhile, the reader is introduced to Prim, the murderer. Prim was committed to getting revenge on his stepfather, Røed, who sexually abused Prim as a child. Prim also believed that Røed was responsible for setting the house fire that killed his mother and left Prim an orphan. Prim has bioengineered a parasite that will make his victims compliant. The parasites keep the victims from feeling fear and also make them sexually attracted to the host, who is, in this case, Prim.

Prim mixed parasites with a sample of green cocaine that was skimmed from a batch confiscated by the police and sent to the Forensic Medical Institute, where Prim works, for analysis. Prim attended the same party as Bertine and Susanne. He intended the cocaine for Røed, but Røed insisted that the women get to try it first. As Røed was preparing to snort the final line of cocaine, he sneezed, scattering the drug.

Prim had not intended to kill Susanne or Bertine, but he had to because he had not managed to infect Røed yet. Prim needed to ingest the parasites that had settled in the women’s eyes and brains to keep the parasites in his own system reproducing. Later, Prim also had to infect and kill Helene, Røed’s wife, for a new source of parasites. Prim finally managed to infect Røed with parasites when Røed came to the institute to identify Helene’s body. After the parasites had time to infect Røed’s brain, Prim approached his stepfather. Røed was sexually attracted to his stepson by the parasites and agreed to meet when Prim claimed he wanted a rendezvous.

Prim arranged for Røed to meet him in the remains of the house where Prim’s mother died in the fire. He claimed he wanted the encounter to occur in his old bedroom so that it would be as true to the past as possible. Prim had Røed remove his clothes. Then, Prim secured him to the bed. Prim secretly recorded as he ordered Røed to tell him what he wanted to do to him and what he had done to Prim when he was a child.

After Prim finished his recording, he told Røed he would be sending the video to all of the contacts listed on Røed’s phone. Prim poured gasoline throughout the remains of the house and set it on fire, leaving his stepfather to burn to death.

Harry and the group of detectives he had put together were distracted from the real killer by misleading clues Prim was able to plant on Susanne and Bertine’s bodies. They first arrested Røed, but it was determined that he was not guilty when a flake of skin belonging to cocaine dealer Kevin Selmer was found between Bertine’s teeth. Kevin was arrested, but he died in his cell of an extreme case of parasites before he could be questioned.

While the police believed Kevin Selmer was the killer, the murders still bothered Harry because they did not make sense. They ruled out Kevin as the killer because he had been at a showing of Romeo and Juliet on the night that Susanne disappeared. Harry and the others began looking again at people in the police department who had access to the shipment of green cocaine. He determined that Helge Forfang, a man who worked at the institute, was the one who had analyzed the cocaine when it arrived at that office.

Helge, known by his family as Prim, is apprehended and arrested on the roof of the Forensic Medical Institute where he and Alexandra, a co-worker were watching an eclipse nicknamed the Blood Moon. Helge was in love with Alexandra and proposed to her, but she refused his proposal. He believed she wanted to be with Harry, with whom she had past romantic encounters. So, he invited Harry to the roof to trade places with Alexandra. Prim intended to kill Alexandra and allow Harry to live.

Harry and Ståle Aune, a collaborator who was in the final stages of pancreatic cancer, tricked Prim when Aune pretended to be Harry. Prim injected Aune with a lethal dose of parasites. He realized too late that he had been tricked and was arrested.