Detective sergeant Alex Cupidi of the Kent Police has an insider's perspective, having recently spent her career finding bodies in cases gone cold. In Salt Lane, she joins forces with Constable Jill Ferriter, who takes the lead on solving the latest investigation: that of a pale body found floating in the dark waters of a drainage ditch. Alex isn't the only person bothered by the unearthing of the dead woman whose identity remains frustratingly unknown.

The case seems fraught with inconstancies. They think the victim is Julian Keen's mother and that she was the victim of some kind of assault. But while they wait for more detail, Alex and Jill traverse the ditches and waterways, convinced the woman could have been put somewhere else and floated into Salt Lane. Then another body is found at the local Horse Bones farm, drowned in a chamber filled with manure in what is possibly the worst forensic scene ever. The victim appears to have been a Muslim migrant. Injuries suggest that he was hiding from someone or something. Is the case about drugs and people-smuggling? Both bodies were disposed on farmland, about five miles apart.

As Alex attempts to tie the cases together, Shaw delivers a series of socially relevant themes culminating in a chase-out across the farms of rural Kent, where seasonal fruit pickers and illegal immigrant workers struggle to survive. On the ruins of Speringbrook Farm, a hijack situation goes horribly wrong, and the farm's owner, Stanley Eason, is nailed as a prime suspect. Meanwhile, Alex's boss, DCI McAdam faces a misconduct hearing: "they think I invented it all to justify what happened." The case takes an unexpected turn when Alex discovers a faded color photograph of two grubby boys sitting in front of a caravan. Using her quick instincts, Alex attempts to dig up the boys' long-forgotten history and tie them to the murder of a transient named Hillary Keen.

In this terrific companion piece to The Birdwatchers, Shaw rolls out a sophisticated narrative with a number of gang-masters, a series of befittingly nefarious suspects who may cost Alex her life. There's a complexity to Connie Reed, who jumps into a sewer first to save Ferriter, then to save Alex. She's brave and eccentric, entirely content to exist in her own terms. Alex and Jill aren't sure what to make of young frightened Najibas, who is at first reluctant to tell the detectives the truth about her shadowy colleagues, "the ghosts" who work in restaurants and brothels, cleaning offices or making beds. Even here in the countryside, Najibas's friends labor in the fields and in the warehouses, processing the food while toiling in the semi-darkness of summer mornings.

Alex plunges into these critical battles: a group of Roma families, a netherworld of immigrant desperation, and the two little boys, "two smiling urchins in a sunlit field." On the personal slide, Alex believes she had done the right thing relocating to Kent, though there's another secret motivation driving her to provide closure to her life in London. Alex tries not to let her frustrations show when her mother unceremoniously arrives. She's floundering with Zoe as badly as her mother did with her. Once again, Alex blames her mother for everything that went wrong. She spends much of the story frustrated, wondering when she stopped being able to talk to Zoe. When Zoe goes on the run, Alex knows that she has once again "shot her mouth off without thinking."

The windswept, sky-filled flatlands of Kent are the perfect backdrop to Shaw's action-filled narrative. The lights of the huge industrial bulk of the Dungeness nuclear power station blare out in the darkness, lighting up the southern sweep of this vast flat stony landscape. Beyond the kind of slow-burning read we have come to expect of British police procedurals, Salt Lane breaks the mold, providing an intelligent commentary on the desperation of asylum seekers and the high cost of finding justice.