AC-12 investigate DCI Jo Davidson's apparent mishandling of the murder of an investigative journalist.

AC-12 suspect a cover-up as an officer on DCI Davidson's team appears to have sabotaged the inquiry into Gail Vella's murder.

A new witness comes forward claiming to have crucial evidence in Gail Vella's murder. Kate is torn between her loyalty to Jo and to AC-12.

DCI Davidson faces increasing pressure from all sides. AC-12's 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.

With time running out, AC-12 attempt to unmask 'H',the fourth individual commanding the network of corrupt officers.