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10-year-old Helén Nilsson disappears on Easter 1989 in the small community of Hörby. It's a chilly evening and Helén will meet two classmates at the discount store a few minutes' walk from home, but the young friends never meet. Six days after the disappearance, Helén's wounded body is found, raped and strangled. Detective Commissioner Pelle Åkesson wants to work with the investigation but is abruptly rejected by her boss. When Helen's mother shows up at the police station and asks Pelle to take on the case, he promises to find Helen's killer.

The Helen investigation is on fire and Pelle Åkesson is following the investigation's dwindling existence. Two women are found murdered and night walking groups begin patrolling the streets in Kristianstad. Pelle and his team interrogate suspects and follow up on clues, but power struggles and political games complicate the investigation. Pelle's new colleague Monica Olhed's interrogation methods are starting to be discussed at the police station and a night walker catches Pelle's attention and is taken in for questioning.

The unsolved murder of Helén Nilsson continues to torment Pelle. To move on, he begins to dig into the murder of the prostitute Jannica Ekblad. The investigation leads them to a new suspect, a man called the Pig. Pelle wants to investigate the murders in parallel, but this leads to a conflict with his boss which has major consequences.

Pelle is back from sick leave and given an office in the police station's basement. He secretly begins to go through the Helen case and the group that can solve the murder reappears, now the hunt begins in earnest.

Pelle secretly and against his boss's will sends DNA from Helen to a lab in Birmingham for analysis. Now Pelle is in a hurry. This is the last chance before the investigation will be closed forever. The prosecutor gives Pelle the go-ahead to explore a number of DNA suspects.

15 years have passed since Helen disappeared. Pelle decides to tear down the cottage Ulf Olsson lived in when he disappeared. It is newly renovated but when they break up the floorboards they find traces of blood. But further technical evidence is needed for the prosecutor to prosecute.