The team uncover connections between racist ring-leaders, politicians and mass killings of immigrants, and are forced to go undercover in order to investigate.
A visitor to Plummer's relish factory turns up dead, and Barnaby is called upon to investigate. The investigation carried out by him and Scott brings a web of lies and secrets to the surface.
The fatal shooting of a respected surgeon leads the team to an unlikely suspect; Dinah and Deering make disturbing new discoveries in the serial killer case.
When Dale Barge is found dead in a playground, Nikki and Jack face the alarming prospect that his murder is linked to the disappearance of a little girl, Lizzie.
Social worker Louise fears the worst when Daniel Garvey, the estranged stepfather of teenager Kevin, whose case she managed, is found dead in his car.