Single mother and off-duty detective constable Dinah Kowalska pursues a murder suspect through the streets of Manchester, only for him to get killed by a bus. As a result, Vivienne Deering, her outspoken detective inspector superior, instructs her to withdraw her application for sergeant, the promotion going instead to the nervous Joy Freers. On a visit to the morgue, Kowalska sees the corpse of a girl with Down Syndrome, the second victim with that condition to be hauled out of a river in recent weeks, and suspects a serial killer is at large.

Kowalska is taken off the murder case as her involvement with Cathy Calvert has made her too subjective but Deering asks her to assist when a student is found dead of an overdose. Drugs expert Randolph Miller directs the team towards a house suspected of being a drugs factory. Former chemical factory employee Angelica Costeros is brought in but refuses to incriminate potential co-defendant Jensen Raphael until Kowalska and Spike Tanner unite to get her to talk.

The serial killer is showing no signs of slowing down, while local racial tensions are also running high when an Asian woman is killed in a suspected racist attack.

The Friday Street team uncover a scam encouraging people in debt to sell their kidneys, while the search for another girl abducted by the serial killer leads to a big breakthrough.

The police set up a trap for the prime suspect in the serial killer case, but their plans could be derailed entirely after a notorious child murderer's new identity is leaked on the internet.

Deering leads a manhunt through Manchester as the net closes in on the serial killer. Joy and Spike discover an unsettling slavery case after a brutal attack on a young homeless man.

The fatal shooting of a respected surgeon leads the team to an unlikely suspect. Meanwhile, Dinah and Deering make disturbing new discoveries in the serial killer case, fearing it might not be over.

In the final episode, Dinah clashes with Deering, setting off an irreversible chain of events, and an attempt to save the lives of a mother and baby goes disastrously wrong.