10-year-old Helén Nilsson goes missing from the small Swedish town of Hörby over Easter break 1989. It is a chilly evening and she is going to meet two of her classmates, a few minutes' walk from her home. But the young friends never find each other. Six days after her disappearance, Helén's mangled body is found, raped and strangled. Detective Pelle Åkesson can see that the investigation is starting to drift dangerously off track under Chief Krister Berg's leadership.

With Helén's case on the backburner, Pelle is worried that the investigation has stalled. Then Sofia Jansson is found dead, her body covered meticulously with cut branches. Volunteer groups start to patrol the streets. One of the volunteers is Bengt Karlsson. Pelle and his team interrogate suspects and follow up leads, but power struggles and political positioning take their toll on the investigation.

Pelle, Monica, and Erik have solved several murders, but Helén Nilsson's unsolved case still plagues them. Helén's mother has decided not to put a headstone on Helén's grave until her killer has been apprehended. In an attempt to move the investigation forward, the police start to look into the murder of Jannica Ekblad, a prostitute who may have come into contact with the same man who killed Helén. A new is suspect named, a man named Grisen.

Pelle returns from sick leave and starts to systematically go through the Helén Nilsson case in secret. Pelle has found a person in the investigation material that he wants to look at closer. Leif Storm lived next door to Helén when she was murdered and was seen at the discount store an hour before Helén went missing.

Pelle has gone against Berg's orders and secretly sent DNA taken from Helén's body to a lab in Birmingham, England. After waiting in tense anticipation, the results arrive - the killer's genetic profile. Now they need a DNA match. Every name that goes on the list to get tested has to be approved. Krister Berg makes things difficult for them, but an old colleague helps Pelle get the necessary permits to continue.

Pelle can prove with DNA that Ulf Olsson raped Helén, but not that he murdered her. The police tear apart the cabin where Ulf Olsson lived when Helén went missing. When they pull up the floorboards, they find traces of blood. It turns out that the blood in the cabin isn't just Helén's, but Jannica's as well. However, Pelle needs additional physical evidence, or a confession from Ulf, before the prosecutor will press charges.