In the aftermath of a tragedy, the world needs an explanation.

In Edinburgh, after the Three Rivers College shooting, some things are clear. They know who.

They know when. No one can say why. For three women the lack of answers is unbearable:

DI Helen Birch, the detective charged with solving the case.

Ishbel, the mother of the first victim, struggling to cope with her grief.

And Moira, mother of the killer, who needs to understand what happened to her son. But as people search for someone to blame, the truth seems to vanish. . .