It is only the beginning of May but in Oslo a brutal heat wave has coincided with an alarming increase in violent crime. In the latest instance, police investigator Hanne Wilhelmsen is sent to a macabre crime scene on the outskirts of town. An abandoned shed is covered in blood. On one wall an eight-digit number is written in blood. There is no body--nor any sign of a victim. Is it a kid's prank or foul play? Is it even human blood? As more bloody numbers are found in isolated locations throughout Oslo, Hanne's colleague Håkon Sand makes a startling discovery: the digits correspond to the filing numbers of foreign immigrants. All are female, all are missing. Is there a serial killer on the loose in Oslo? Meanwhile, as the trail heats up, the victim of a horrific unsolved rape case and her father have each decided to take justice into their own hands. Hanne and Håkon soon discover that they aren't the only ones on the hunt for the killer.