When three police officers are shot dead during the hijacking of a seized-drugs transport, AC-12 moves in to investigate possible police collusion.
Kate and Steve attempt to track down a lead; a fresh police leak enables another audacious raid; Steve's attempts to gather information put him in a dangerous situation.
Corbett's most daring plan yet requires McQueen to exploit links with corrupt police officers; AC-12 makes a shocking discovery.
Tensions rise in the OCG following the events at Eastfield, and with troubling questions hanging over the identity of "H," suspicions mount at AC-12.
As Hastings's secrets are laid bare, Kate and Steve are torn between loyalty and their duty to uncover the truth. Will the identity of "H" be revealed at last?
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.
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.
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.
Nick is brought in for questioning by AC-12, but while Nick denies any involvement in Steve's attack, his suspicions about Roz mount. AC-12 discover new anomalies in the forensic evidence and issue DCI Roz Huntley with a second Reg 15 notice.
DCI Roz Huntley struggles to allay her husband's suspicions. AC-12 find a new angle to pursue their case against her.
After Sergeant Danny Waldron and his armed response team shoot dead a hardened criminal, cracks begin to appear in their story and AC-12 is assigned to investigate.
DS Steve Arnott believes Danny Waldron compiled a list of his childhood abusers; while DI Cottan is commended for cracking the Waldron case, the spotlight falls on Arnott's questionable conduct in a previous enquiry.
Lindsay Denton sets out to clear her name, while Arnott and Hastings interrogate Bains on suspicion of murder.
Steve Arnott's colleagues scrutinize him.
Arnott is arrested on suspicion of murder, and DC Fleming's undercover operation may hold the key to his innocence and the identity of the mysterious perp.
Kate Fleming is planted undercover in Lindsay Denton's Missing Persons Unit.
Lindsay finds herself targeted in prison and tells Kate her theory of why she is being framed.
Lindsay is granted special dispensation to visit her dying mother.
Lindsay fights for her life as troubling details emerge.
AC12's investigation finally exposes a vicious criminal gang run from within the police.
Steve Arnott discovers his target is the city's top detective, Tony Gates.
Having been duped into covering up Jackie's crime, Gates is desperate to sever ties with her; as Arnott and Fleming close in on his secret, Gates is plunged into a more dangerous situation.
Desperate for proof of Gates's corruption, Fleming pushes her relationship with him to the limit; her behavior causes Morton to question if she might not be such a loyal colleague after all.
With Gates on the run, the anti-corruption team comes under pressure to close the case; Arnott is forced to make some tough decisions about his loyalties; Fleming and Hastings go all out to catch Gates.