On a chilly evening over Easter break, 1989, ten-year-old Helén Nilsson goes missing from the small Swedish town of Hörby. Six days after her disappearance, Helén's mangled body is found.
Pelle, Monica and Erik have solved several murders, but Helén Nilsson's unsolved case still plagues the group. Helén's mother has decided not to put a headstone on Helén's grave until her killer has been apprehended.
Pelle disobeys his boss and sends DNA from Helen for analysis in England. This may be the last chance to prevent the investigation from being terminated.