When a nurse is found dead in the woods, the case bears similarities to a solved historic case from twelve years ago, the 'heart knot' murders; fearing there's a copycat killer, DI Ffion Lloyd is called in to lead the investigation.

A manhunt grips the town as a suspect flees, causing Ffion to raise questions about the past; new evidence linked to her sister Lisa emerges and a bitter clash ensues.

The team builds a case against their prime suspect, but an unexpected development unfolds when a gunman enters the frame; a new link to the past and the revelation of a secret threaten to throw everything into chaos.

Following the school shooting, the gunman is charged with Abbi's murder and the investigation intensifies when it's revealed he is connected to the heart knot murders; a personal confession comes to light.

The past continues to stalk the present after news from Paul leads Ffion to uncover a secret about Rick; when a new suspect emerges, Ffion questions if Abbi's killer is still at large.

Helen is nowhere to be found and fear sets in; believing that his wife may have been killed, Rick decides to follow his intuition, with devastating consequences.