Two young girls are killed on railway by a passing train. It does not seem to fit the profile of a normal suicide. So if this is a double homicide who would do this and why? Martin Beck and his team start following the step backwards in order to figure out what has happened. With good and intensive police work they find man side trails from the main path.

Hundred years ago, three students at the Hellestads Boarding School were brutally slaughtered, the murderer drowned himself in a lake nearby and his body was never found. The story has become a legend for generations of students as well as a yearly festivity. Sara, a student, is writting an essay based on the legend and uncovers new facts from the event that will cast dark shadows on the family name of one of the school's main benificiaries. On the night of the hundreth anniversary, the festivities go awry, students disappear and something dark and unknown is moving through the schools corridors...