DS Annie Redford is called to the shore of Loch Ness to clear up the site of a gory prank, supposedly the washed-up remains of the Loch Ness Monster. Soon afterwards, reports come in of a body found at the foot of a cliff. When the pathologist determines that the man was murdered, DCI Lauren Quigley is tasked with leading the investigation.

When her daughter's involvement in the Loch Ness Monster prank is revealed, Annie is disappointed to be off the case. Quigley focuses her questioning on Leighton Thomas, who was convicted of a double murder as a teenager. Forensic psychologist Blake Albrighton believes he knows the key to finding the killer.

The police appeal for the safe return of Jonjo, with DCI Smilie insistent that the young man's disappearance is not linked to the murder. But when Albrighton informs the press that there may be a serial killer at large, the town erupts into chaos. Meanwhile, Evie grows suspicious of Dr. Marr.

As Annie becomes more entrenched in the investigation, her relationship with her family begins to suffer. The police question school principal Craig Petrie as the main suspect in Jonjo's murder, but when doubt arises about Leighton's alibi, Annie is forced to investigate.

A frustrated Quigley fires Albrighton, but he warns her that the killer isn't finished. Evie continues her search for answers about Dr. Marr's involvement with the McGrellish family, and while following new evidence, Annie finds herself in a race to track down a suspect before more people die.

Quigley and Annie watch as the body found on Alan's radar is pulled from the water. Annie visits Mhari to learn what she knew about Dessie's plans, but Mhari refuses to believe her son could be a killer. Despite being threatened with arrest, Albrighton continues to obsess over the evidence and risks his life to find the truth.