When new information comes to light about a possible suspect in a year-long unsolved murder investigation, DCI Joanne Davidson appears keen to finally bring Gail Vella's killer to justice. However, after her conduct arouses suspicions, it's not long before AC-12 are on the case. But as AC-12's investigation uncovers deeper and darker information, will DCI Joanne Davidson turn out to be their most dangerous 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.Hastings acknowledges that the disciplinary action against him put Kate and Steve in a difficult position, and a forensic report from Farida's house results in a shock for AC-12.
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.
