They heard the mother before they saw her. Two streets away, through the open windows of an unmarked police motor, and they could hear the screams. On what felt like the hottest day of a baking summer, the sound was chilling and incongruous. To Detective Inspector Sarah Quinn's way of thinking, the flawless turquoise sky called out for ice cream, paddling pools, children's squeals. Not this cry against nature.

Maybe it was the cooling sweat trickling down her spine, but Sarah shivered again as she reran mentally what little they'd picked up on police radio. Scanning the surroundings, her shrewd grey eyes took in a stream of probably inconsequential images on the way to the bigger picture: a whippet trailing after a fat man with a ponytail, an old woman dragging on a cigarette as she tugged along a tartan shopping trolley, a chav yelling into a mobile, while pushing a double buggy one-handed.

It was force of habit for Sarah, the constant observing, recording, filing for possible future use. Anyway, detail via the radio had barely provided crumbs let alone food for thought, it had been sparse to the point of useless; the original triple-nine made from a call box, anonymous and garbled: a missing pushchair, White's newsagent just off Small Heath high street and a young mother who, quote, needed a good slapping. Sarah arched an eyebrow: the tip-off was right on that score – the hysteria was evident and she hadn't even seen the woman.

She wasn't alone. Passers-by turned cocked heads trying to pinpoint where the screams came from. Sarah glanced at her sergeant, part of her willing him to drive faster, the rest scared they may already be too late. Either way, picking up speed wasn't an option on inner city streets heavy with traffic and heaving with people. It was uniform's shout by rights. Sarah and Hunt just happened to be in the vicinity when the call came in. She'd suggested taking a quick look, was already beginning to think better of it; quick was likely the last thing it would be. Thursday evenings were French conversation class – she was lucky if she got to one in three. C'est la vie.

'Next left, John.' Her clipped tone verged on curt; tight vocals the only sign of stress. It certainly wasn't a case of least said soonest mended. If the screams were anything to go by, the damage had already been done. The feeling was so unlike Sarah; she didn't do instinct, sixth sense, whatever. So why couldn't she shake off the uneasiness?

John Hunt's muttered, ma'am, articulated more than her senior rank. Born and bred in the second city, the detective sergeant needed directions like Delia Smith needed cookery lessons. Registering his pale face and pinched features, Sarah realized this call-out could be a too-close-to-home nightmare for Hunt. He and his wife had lost their first baby in a cot death just before Christmas.

Focus woman, she told herself, curling twitching fingers to stop them tapping; it wasn't even clear yet that a child was missing, let alone permanently. Even before Hunt cut the engine, Sarah was out of the Vauxhall, long legs striding towards a growing crowd of gawpers. Through a sea of jeans and shorts, saris and salwar, she caught a flash of yellow. The warrant card and her air of seemingly effortless authority were enough for most spectators to step aside and for the first time Sarah saw the woman.

Down on bony knees in the middle of the dusty pavement, she was oblivious to the curious stares and occasional asides, to the fact her bright yellow sun dress had ridden up, exposing hollowed white thighs. Head cradled in her hands, she rocked and wailed as tears dripped from her chin, spotting a paving slab already mottled with discarded gum.

A man crouched at her side, his hand hovering but not quite making contact with her bare shoulder. The concerned glances he cast at the open door of the newsagent's, as much as the biro tucked behind his ear, suggested he was the eponymous owner.

Sarah stopped just short, rapidly deciding on the best approach. The woman's misery was almost palpable but it wasn't getting anyone anywhere.

'Took your time didn't you?' The biro man had a voice. And a point. Sarah was well aware the traffic had held them back. Five to four now – the call had come in eleven minutes ago. It was a long time in policing. It could be a lifetime if they didn't act fast.

'Mr White, is it?' Over his shoulder she glimpsed Hunt in conversation with a couple of uniformed officers. He'd be briefing them; they'd start questioning people on the street, all potential witnesses.

'That's right.' White staggered to his feet, the brusque tone slightly mollified. Sarah heard relief in it as well. Like most people he was happy to let the cops deal with it, whatever shit it was. 'Can't you do something for her?' He jabbed a thumb at the hunched figure as if there was any doubt, as if Small Heath was a hotbed of women on the verge of a meltdown.

'Let's get inside. I can't do anything here.' The noise was grating and the woman needed to get out of the heat, drink some water. They'd have to call an ambulance soon if she didn't get a grip.

'She won't budge. I've tried.' White's bald pate was surrounded by what looked like a fallen halo of wispy cotton wool. He pulled dung-coloured slacks over a middle-aged paunch. 'She's terrified to move in case the baby comes back.'

Comes back? Suppressing a growing sense of urgency, Sarah moved in closer. 'Calm down, love. Take some deep breaths.'

Maybe the girl hadn't heard. The rocking and wailing continued. Her sallow complexion was marred by twin trails of shiny mascara-blackened tears. Staring straight ahead now, she seemed oblivious to the crowd, to Sarah, to anything but her own misery. She was obviously in shock, but had she witnessed something vital? Or someone?

'We're wasting precious time, love.' Sarah's patience was at a premium, too. It took an effort not to show it in her voice. For a few seconds, the woman appeared to focus on Sarah's face and the screaming stopped. In the brief period of eye contact, Sarah was surprised to see how young she was: sixteen? Seventeen? Not much more than a child herself. She managed a tight smile of reassurance, encouragement, but the girl started screaming again.

Conscious of Hunt at her side, Sarah snapped a command. 'For Christ's sake, John, stop her making that sodding noise.' The words came out louder than intended and sounded harsh even to her ears. Hunt failed to hide his reaction. She didn't care if he was appalled; if a baby was missing hurt feelings were neither here nor there. Without a word, he went to the girl, squatted beside her and laid a strong arm around her scrawny shoulder. His six-two well-padded frame made her appear even smaller, even frailer. 'Come on, sweetheart. Let's get you inside. See what we can do to help.'

Hunt was good at the touchy-feely stuff; unthreatening despite his bulk; a passing resemblance to David Tennant didn't hurt. It was enough to silence the girl, she cut Sarah a glance that was difficult to read, then allowed him to half carry, half steer her into the newsagent's. White must've retrieved the girl's bag from wherever it had fallen. He clutched it self-consciously as he showed them through to a cluttered storeroom then slipped it on the floor near the only chair, an upright that had seen better days. Hunt gently lowered the girl onto the seat where she hung her head, curtains of dull mousy hair concealing her...