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

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

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

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

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.

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.

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 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.

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.

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

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

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.

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.

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.

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.

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.

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

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 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.

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

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.

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