Frances Cairnes begins her search for the person who killed her six-year-old son in a hit and run. DI Strangeways arrives on the Isle of Wight, hoping for a fresh start after the death of his colleague.

Tracking down the man that she believes is responsible for the death of her son, Frances sets out to inveigle her way into his home as she plots her revenge. Struggling to deal with his PTSD, DI Strangeways’ takes a closer look at the investigation into Frances’ son’s death.

Staying on as a house guest, Frances must now work out how she is going to see her murderous plan through. Strangeways's suspicions over his predecessor's handling of Frances's case begin to grow.

As Frances becomes more embedded in the home, she makes progress with her plan: she's found her weapon. Things heat up for DI Strangeways as he becomes ever more convinced of the possibility of foul play.

DI Strangeways, blindsided by a surprising discovery, scrambles to find answers.