When Steve Arnott is transferred to a police anti-corruption unit, he finds his target is the city's top detective, Tony Gates. Can Gates really be as good as he appears? Arnott must engage in a cat-and-mouse struggle to uncover Gates' secret.

Having been duped into covering up Jackie's crime, Gates is desperate to sever ties with her. But, as Arnott and Fleming close in on his secret, Gates is plunged into an even more dangerous situation than he first thought.

Arnott is convinced Gates played a part in Jackie's disappearance. With Fleming's help, the anti-corruption team tries to trap Gates into revealing his involvement in covering up Jackie's crimes.

Desperate for proof of Gates's corruption, Fleming pushes her relationship with him to the limit. But her behaviour causes Morton to question if she might not be such a loyal colleague after all.

With Gates on the run, the anti-corruption team come under pressure to close the case once and for all. Arnott is forced to make some tough decisions about where his loyalties lie. Fleming and Hastings go all out to catch Gates, leading to a shattering conclusion.

A brutal ambush on a police convoy leaves three officers dead and a protected witness fighting for his life. AC-12 turn their spotlight on Lindsay Denton, the officer who led the convoy but escaped unscathed.

Following the tragedy at the hospital, AC12's Kate Fleming is planted undercover in Lindsay Denton's Missing Persons Unit. But Lindsay in turn gathers compromising evidence against the anti-corruption team.

Lindsay is denied bail and finds herself targeted in prison. Growing increasingly desperate, she tells Kate about a relationship with an executive police officer and her theory of why she is being framed.

Following the violent attack she suffered in prison, Lindsay is granted special dispensation to visit her dying mother. Meanwhile, deputy chief constable Dryden shrugs off allegations about his relationship with Lindsay and possible links to the ambush.

Lindsay fights for her life as troubling details emerge of a vicious conspiracy involving corrupt police officers. With growing evidence of links between DCC Dryden and the ambush, Hastings is under pressure to arrest his senior officer.

AC12's investigation finally exposes a vicious criminal gang run from within the police. But while Steve now appears convinced of Lindsay's innocence, Kate remains determined to prove she had a role in the ambush.

Sergeant Danny Waldron and his armed response team shoot dead a hardened criminal.

After the shooting at the drugs house, DC Fleming increases the pressure on Danny Waldron.

DS Steve Arnott believes there is a dark secret in Danny Waldron's past.

Fresh testimony launches AC-12 back on the trail of a corrupt officer.

Loyalties are divided when DS Steve Arnott comes under scrutiny from his colleagues.

DS Steve Arnott is arrested on suspicion of murder but continues to protest his innocence.

DCI Roz Huntley is under intense pressure from her superiors to apprehend a serial murderer after months of fruitless investigation.

AC-12 crank up their investigation into Operation Trapdoor. Kate makes progress in her undercover role.

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.

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.

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.

Following the deadly hijack of a police convoy, AC-12 target a ruthless organised crime group known to have links with corrupt officers.

Kate and Steve attempt to track down a lead. A fresh police leak enables another audacious raid, while 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.

Tensions rise in the OCG following the events at Eastfield. With troubling questions hanging over the identity of ‘H', suspicions mount at AC-12.

As the hunt for 'H' intensifies, Steve and Kate wonder if Hastings is guarding a dark secret.

In a nail-biting 90-minute finale, Hastings's secrets are laid bare. Will 'H' be revealed at last?

DCI Jo Davidson receives new information about the whereabouts of a suspect in the unsolved murder of investigative journalist Gail Vella. En route to raid the property, she spots an armed robbery in progress and stops the convoy. Suspicious of her motives, AC-12 launch an investigation. Still dealing with the fallout from their explosive investigation into undercover officer John Corbett, AC-12 soon realise they are facing their most enigmatic adversary yet.

Having opened an official inquiry into DCI Jo Davidson and Operation Lighthouse, AC-12 begin to suspect a cover up. Steve and Chloe dig into Gail Vella’s controversial reporting for clues to her murder, and find links to previous AC-12 cases. Their investigation takes a shocking turn when they begin to scrutinise Jo's personal life.

On the strength of evidence from a new witness, DCI Jo Davidson brings Terry Boyle back in for questioning about Gail Vella's murder. Kate is torn between loyalty to Jo and wanting to assist AC-12's investigation, and tensions increase when Kate encounters a face from the past with links to organised crime.

DCI Jo Davidson faces increasing pressure from all sides as AC-12 make major breakthroughs in the investigation, but an attempt to interview a key witness goes horribly wrong.

AC-12 link Gail Vella’s murder with a historic case of police corruption, and Kate hatches a plan to tell once and for all if Jo is bent. They are closer than ever to cracking the case, but when Hastings’ authority is undermined it leaves his team in a potentially dangerous situation.

As AC-12 struggle to deal with the repercussions of tragic events, Hastings makes one final bid to uncover institutionalised corruption before his time runs out.

With time running out, AC-12 attempt to unmask 'H', the Fourth Man (or Woman) commanding the network of corrupt officers behind the murder of Gail Vella. But sinister and powerful forces appear intent on orchestrating a cover-up.