DCI Roz Huntley is under intense pressure from her superiors to apprehend a serial murderer after months of fruitless investigation. When another young woman is abducted, Roz is on the scene to track down and charge a 24-year-old man. But doubts around the young man's guilt lead the chief forensic investigator to ask AC-12 to investigate. Is Roz ignoring forensic evidence that might prove the young man's innocence? AC-12's Supt Hastings places DS Kate Fleming undercover inside Roz's team to dig deeper into the case. With DS Steve Arnott piling on pressure from the outside, Roz is forced to act decisively. A mother of two and wife to Nick, Roz will do anything to stop her life from unravelling.

A dismembered body is discovered, and worries grow when DCI Roz Huntley fails to report for duty. AC-12 crank up their investigation into Operation Trapdoor, concerned that Michael Farmer will go to jail for crimes he didn't commit. With suspicions that Roz may have withheld crucial evidence, a new DCI is drafted in to run Operation Trapdoor. Kate makes progress in her undercover role, but AC-12's investigation falters when they realise that their main informant, forensic investigator Tim Ifield, has also gone missing.

As AC-12 wrestle with the new forensic evidence casting doubt over Tim Ifield, Kate is approached by another woman claiming to have been assaulted by Michael Farmer. While Steve fumes over this contradictory testimony, Kate uses her newfound respect to inveigle herself with the Trapdoor team. Meanwhile, Steve challenges Roz's husband Nick to vouch for his wife's whereabouts on the night Tim Ifield went missing. With Nick proving evasive, Steve senses a whole new angle on the investigation.

Roz's husband Nick is brought in for questioning by AC-12. But while Nick denies any involvement in Steve's attack, his suspicions about Roz mount. Meanwhile, AC-12 discover new anomalies in the forensic evidence and issue DCI Roz Huntley with a second Reg 15 notice. AC-12's case appears watertight until Roz starts dismantling their evidence with inside information of her own.

DCI Roz Huntley struggles to allay her husband's suspicions. AC-12 find a new angle to pursue their case against her.

While Nick Huntley faces lengthy questioning, AC-12 remain convinced of Roz's involvement.