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.