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?

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.