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.

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.