The search for a serial killer in the area around Loch Ness becomes a matter of life and death for local detective Annie Redford, who is trying to cope with her first murder case. When the body of a local man is found at the foot of Carn Mhor Mountain and an isolated human heart on the loch shore, the town's normality is shattered and the nightmare begins.
A human heart is discovered on the shores of the Loch, and forensic psychologist Blake Albrighton is brought in to join the investigating team. He quickly forms a disturbing theory about the motive behind the seemingly inexplicable murders, but Chief Inspector Frank Smilie is quick to dismiss his ideas. However, if the new arrival is correct, the once peaceful Highland community could be set to endure far more deaths, as a serial killer prowls the area.
Smilie holds a press conference to appeal for information about Jonjo, as well as reassuring the public and preventing news of the human heart found by the lake from spreading further. However, his plans are disrupted by Blake, who reveals everything and shares his fears that the community is haunted by a serial killer. An unexpected discovery is made outside a town meeting.
Craig Petrie becomes a suspect in both murders, and Blake assists in questioning him. However, he quickly turns the tables on the police and casts doubt on Leighton's innocence. Annie investigates his claims further, and her loyalty to Leighton is shaken when she discovers his alibi is a lie. Smilie tries to comfort Jonjo's father, but is rejected.
Annie continues her determined investigation into the killer's identity, and reaches a breakthrough when she discovers the weapons that were used in the murders. Seeking help from Quigley, she is finally able to track down the culprit, and the two detectives put their lives on the line to put a stop to the slaughter. However, their relentless pursuit of justice leads them into a dangerous confrontation, as they stalk the armed murderer through the dark corridors of the area's hydro-electric dam.
A body is pulled from the Loch, and a photograph in his wallet provides a vital clue. Annie tries to gather further evidence by talking to a desperate Mhari, who refuses to believe the terrifying possibility she is presented with. Albrighton is threatened with arrest by Quigley, but nevertheless continues to pursue his own theory about the killings - even if it means putting his life on the line.
