In the dead of night, 72-year-old Miriam Narracott is found wandering on Exmoor, holding a knife and covered in blood. Inside the family farmhouse lies the body of her adult son, Gabe. CSI Ally Dymond is on compassionate leave, but when approached by the new DI, recently arrived from London and eager to have Ally's keen eye and local knowledge on the case, she finds herself being drawn back in.

With their only suspect Miriam unwilling – or unable – to talk, the team must dig into the family’s history to uncover a motive. Instead they find evidence that Gabe was involved with a criminal network, suggesting a completely different chain of events. But if Miriam isn’t the killer – then who is?