He led them up the other stairs and they found themselves in a maze of dusty rooms that had been built into the tower. They didn’t fit perfectly so that some of the big, fancy windows had been cut in half by new walls and floors. Blue kept catching glimpses of London – rooftops, chimneys, the world outside. It reminded him of just how shut in they were. After the wide-open spaces of the galleries downstairs it felt claustrophobic up here; the low ceilings seemed to be pressing down on them and everywhere was clutter: shelves loaded with carefully labelled fossils, cabinets and drawers stuffed with rocks and crystals, boxes filled with junk. You could hardly move.

And there were a thousand places to hide.

Maxie was sticking so close to Blue that they were cramping each other. It irritated him and he was biting his tongue, anxious not to snap at her. He looked at the floor. Now there were footprints in the dust. Someone had been here recently. But there was a horrible stillness, and the air felt hot.

The smell was making him want to puke. His eyes were watering and he was breathing through his mouth in quick pants, like a dog.

They heard something moving again, deeper in the tangle of rooms. Maxie tightened in even closer to Blue and he felt trapped, wanted to shout out and smash the place up. He almost said something to her and just stopped himself in time. Moved quickly away from her. Looked out through a window that faced the museum roof. It stretched away, bigger than a football pitch, like the plateau on the top of a mountain range. A world above the world. There were all sorts of walkways and ladders out there. Easy for Paul to get around and into other bits of the building.

Blue’s feet crunched on something and he glanced down to see the skeleton of a pigeon with some scraps of skin still attached.

Maxie’s voice broke the stillness.

‘Holy crap … Look.’

He turned round. She was pointing with her eyes to a pile of stuff on the floor. It looked like a nest of sorts. A bed. Made of feathers and old clothes and various other scrappy bits and pieces.

‘We’ve found him,’ Maxie whispered. ‘This is where he’s been hiding.’

‘Is he here now?’

‘I don’t …’

Maxie suddenly jumped back as there was a blur of movement and something came at her with a whirring, rattling noise. She yelped and Blue froze in shock.

And then he saw that it was only a pigeon. It must have got in somewhere and was flapping in panic. It came at Maxie again and she instinctively swiped at it with her sword. There was a squawk and a shower of feathers and the pigeon spiralled into the wall.

Blue giggled with relief.