North London; in the depths of a sweltering summer, teacher Samuel Szajkowski walks into a school assembly and opens fire. Three pupils and a teacher are shot dead before Samuel turns the gun on himself.

As the only woman in her office at CID, Detective Inspector Lucia May is finding it difficult to be taken seriously by her colleagues. When she is assigned the school-murders case, she is expected to tie things up quickly and without a fuss. The incident is a tragedy that couldn't have been predicted and Szajkowski a psychopath beyond help. But as Lucia begins to piece together the testimonies of the teachers and children at the school, a much uglier and more complex picture of the months leading up to the incident begins to emerge - one which leaves many at the school culpable.

As the pressure to bury the case builds and the high jinks of her colleagues take a more sinister turn, Lucia begins to realise that she has more in common with Samuel Szajkowski than she could have imagined. And she is determined to tell the truth about what really happened, whatever the consequences...