Carly’s chest tightened with fear. Frantically she swept her arm back and forth, all the while retreating. She couldn’t leave any footprints, she just couldn’t.

She could hear the sound of boots now, squelching through the mud. He was almost upon her. Certain she’d be seen she held her breath as she sidled backwards again.

Behind her, pressing against her socked feet, a wall. She’d reached the building. Keeping low, she scrambled inside.

Safe for now, but for how long?

Arms wrapped around her, her sisters’ small bodies trembling with fear, as Carly stood, her eyes straining to adjust to their surroundings. With the small amount of moonlight trickling through the patchy roof, she could just make out a rusting sign: DANGER. RISK OF CONTAMINATION.

Darkness loomed ahead but she couldn’t risk staying in the doorway. In the absence of footsteps she knew the men would soon turn their attention to the buildings and this was the closest one.

Suddenly lightning flashed again, briefly illuminating the twins. Before Leah could scream Carly dropped her shoes and pressed her hand over Leah’s mouth.

‘It’s okay. Count and see how far away the thunder is,’ she whispered. It’s what her mum would have said.

‘One. Two. Three.’

Thunder boomed and as it faded another noise took its place.

Whistling.

And the calmness, the ordinariness of that sound was the most frightening thing Carly had ever heard. Swiftly she located the twins’ hands and pulled them forward to the next room.

Hide.

They had to hide. In the eerie light she hurriedly took in their surroundings. On the wall were two metal doors. One said SHOES the other CLOTHES. Above them somebody had spray-painted, Abandon hope all ye who enter here. Carly shuddered as she realized they were in the Gas Decontamination Chamber she’d learned about. There was nothing else in the room. Nothing to take shelter behind.

‘Quick,’ Carly whispered, moving on.

The next room was full of showers. She remembered seeing this in Mr Webster’s photos but then they had curtains over the cubicles. Now they were empty, pipes ripped from walls. Mounds of debris clogging the drains. Carly backed up against a wall, felt slime coat the backs of her legs.

‘Here!’ came a shout. ‘I’ve found some shoes.’

Carly cursed herself as she ushered the girls through a doorway into another empty room. Where could they hide? Her panic rose as they crossed a corridor. Here the roof was whole. The blackness was absolute. Carly released the girls and fumbled against the walls until she found a doorway.

‘Quickly.’

They fell into the room, a slash of sky visible above their heads. On first glance in the dim moonlight this room appeared empty too but then Carly spotted a flat metal trolley on its side. She darted forward but as she crouched behind it she realized there wasn’t space for them all. Her eyes slid around the room, resting on rows of lockers on the walls – some were gaping black holes where their doors had been wrenched off, but some were intact.