The marked car raced along Marylebone Street on blues and twos, narrowly missing an open-top bus that pulled out in front of them as they passed Madame Tussauds. Kelly listened to the response officers in the front discussing that day’s game at Old Trafford over the wail of the siren.

‘How Rooney could have missed that, I don’t know. If I was paying someone three hundred grand a week I’d bloody make sure they could kick straight.’

‘Can’t perform under pressure, that’s the problem.’

The lights changed to red at Euston Square. The driver pressed his horn, switching the sirens to a high-pitched warble, and the cars in front began to peel apart, allowing them through. They turned right into Bloomsbury and Kelly turned up her radio, waiting for the update they were all desperate for. It came as they neared the West End. Kelly closed her eyes and let her head fall briefly against her seat.

It was over. For Katie Walker, at least.

Kelly leaned forward between the two front seats. ‘You may as well slow down now.’

The driver had already heard the update and was switching off the sirens, dropping down to a more appropriate speed, now that there was nothing to gain from making on immediate. No one to save.

When they reached Leicester Square he dropped her off outside the Hippodrome and she ran towards the Underground station, flashing her warrant card to a bored-looking woman standing at the ticket barriers. She had come in via a different entrance than she had intended, and she looked around, trying to get her bearings.

There.

The door to the maintenance cupboard was scuffed at the bottom, where people had pushed it open with their feet, and a poster urging passengers to report any suspicious packages curled up at the corners. A sign told members of the public access was forbidden.

Kelly knocked twice on the door, then went inside. Even though she knew what she’d find inside, her heart was still racing.

The maintenance room was dark and windowless, with a desk and a metal chair on one side, and a pile of signs stacked against the opposite wall. A yellow bucket on wheels stood in one corner, filled with greasy grey water. Beside it, a young girl sat on a plastic crate, cradling a cup of tea. Even without the confident pout evident in the photo on the website, Katie was instantly recognisable. Her mass of highlighted hair fell around the shoulders of her coat; its padded white segments making her look bigger than Kelly knew she was.

Leaning against a wall behind Katie was a broad-shouldered man with dark hair. He stepped forward and held out his hand to Kelly.

‘John Chandler, covert officer with British Transport Police.’

‘Kelly Swift.’ She crouched down. ‘Hi, Katie, I’m Kelly, one of the detectives involved in this case. Are you okay?’

‘I think so. I’m worried about Mum.’

‘Officers are on their way there now.’ She put out a hand and squeezed Katie’s arm. ‘You did really well.’ DC Chandler’s radio message confirming that Katie was safe had been swiftly followed by confirmation of what Kelly had suspected: Zoe was being held by Melissa West, owner of several cafés in London, including Espress Oh!

‘It was horrible.’ Katie looked up at John. ‘I didn’t know whether to believe you or not. When you whispered in my ear, I wanted to run. I thought, “What if he isn’t an undercover cop at all? What if that’s just his cover story?” But I knew I had to trust you. I was scared Melissa would realise what was going on, and hurt Mum.’

‘You did brilliantly,’ John said. ‘An Oscar-winning performance.’

Katie attempted a smile, but Kelly could see she was still shaking.

‘I didn’t have to do much acting. Even though you’d told me what was going to happen, the minute you pulled me in here, I decided everything you’d said to me was a lie. I thought that was it. Game over.’

‘I’m sorry we had to put you through that,’ Kelly said. ‘We knew the CCTV had been hacked, but we didn’t know to what extent – we didn’t know exactly how much could be seen. When we saw your profile on the website we knew we had to get you safely off the Underground and away from anyone who might want to hurt you, without letting Melissa know we were on to her.’

‘How much longer do we have to wait in here? I want to see Mum.’

‘I’m sorry, we needed confirmation from the control room they’d switched over the CCTV feed.’

Craig had responded swiftly to Kelly’s concerns that Melissa might be able to see Katie and DC Chandler leaving the maintenance room, thereby blowing their cover. He had switched the live feed with recorded footage from the same time the previous day, when the footfall at Leicester Square would be roughly the same, and the risk of Melissa noticing the jump would be small. Kelly hoped he had been right. ‘It’s all fine now, we can leave and she won’t be able to see us.’

As she opened the door, Kelly’s radio crackled into life.

‘We need an ambulance to Anerley Road,’ came the disembodied voice. ‘It’s urgent.’

Katie’s eyes widened.

‘Tell them to make on silent, and hold off when they get to the address.’

‘It’s just a precaution,’ Kelly said quickly, as the younger girl’s eyes filled with tears. She turned the volume on her radio down until it was virtually inaudible. ‘Your mum’s fine.’

‘How do you know?’

Kelly opened her mouth to give more platitudes, then closed it again. The truth was, she didn’t even know if Zoe Walker was still alive.