After the Floating Market, events ran helter-skelter. Evadne rushed up and extracted Kingsley from a crowd that had lost interest quickly, especially after the body of the dead Spawn had been whisked away by its anonymous purchaser. Together they vaulted across a floating bridge and scrambled up a ladder into a vast, echoing tunnel junction. The vaulted ceiling was lost in shadows overhead.

Evadne had a satchel over her shoulder and an object in her hands the size of a cigar box. ‘That way!’ She pointed at the fourth tunnel on the right.

Kingsley sighed. It was the only one with water coming from it. He pushed back his hair with one hand and glanced behind him at the floating bridge. ‘Evadne,’ he breathed. ‘I suppose we should be getting used to it, but I think someone’s following us.’

Evadne swivelled and touched her spectacles. ‘It’s that man who was waiting outside the entrance to my refuge.’

‘Then I’d definitely say some accosting is in order this time.’

‘We can’t. We’d lose the signal.’ Evadne held up her box. ‘Hurry.’

She darted into the watery tunnel. Before following, Kingsley looked back. The man was slogging through a knee-deep drain, his coat bedraggled, his hat stuck on his head as if glued. He looked lost, but cast about with the sort of determination that Kingsley didn’t like to see in someone who could be a pursuer.

Then he straightened, waved and called out, his voice echoing from the walls. It was enough to jolt Kingsley into moving, and he set off after Evadne with another worry added to his ever-expanding bag of troubles.

The tangled route the Spawn took was a nightmare. They would have lost it a hundred times if it weren’t for Evadne’s clever box. It was elegantly made of dark wood and brass, with two handles on each end and a featureless top. Evadne kept her hands on the handles and was rewarded with a vibration if she turned away from the direction of the myrmidon that was shadowing the Spawn.

When they finally dragged themselves through the manhole they were confronted by the night-time Thames and a view that Kingsley had last seen in a Canaletto. The imperial bulk of the Naval College stretched along the bank, the twin domes, the Queen’s House behind, with the Royal Observatory in the distance.

‘Greenwich.’ Evadne said. ‘This must be the Isle of Dogs.’

‘What time is it?’

Evadne took out her watch. ‘Just after midnight.’

‘It’s later than I – Look.’

The myrmidon was waiting for them, running circles in front of a small domed building. It rose on its hind legs, scrabbling at the air and generally doing all it could to attract their attention without actually building a bonfire.

‘The foot tunnel,’ Evadne said.

The ratty construct almost rolled over with delight when they approached and was ecstatic when it led them along the gloomy, echoing sub-river tunnel, its claws ticking along. When they emerged on the Greenwich side of the river the rain was heavy – which suited Kingsley. Despite its saturating him immediately, he was grateful that ordinary folk would be kept at home.

The myrmidon guided them north-west, paralleling King William Walk and keeping to the shadows as much as possible. Once it reached the openness of the park it stopped every ten yards or so, its ratty countenance peering back at them to make sure they hadn’t become lost.

That was the moment when Evadne pointed at a furtive band crossing the park. ‘Now, that’s curious.’ She threw herself on the wet ground, careful not to land on her sabre, and adjusted her spectacles. ‘Very curious.’

Kingsley joined her, thinking that the mud might very well be an improvement on his popinjay garments. Yellow trousers. What had Evadne been thinking?

‘I’m glad it’s curious,’ he said, squinting through the rain and dark, ‘but a little more detail would be helpful. What can you see?’

‘The Demimonde is abroad.’ She dragged her satchel up so she could prop her elbows on it. ‘Something is afoot.’

‘Gangs roaming about Greenwich in the dead of night? I’d say so. Shouldn’t we inform the police?’

‘I doubt that the police would be able to help here.’ Evadne touched her spectacles and peered through the darkness. ‘They’re Neanderthals.’ She turned to him. ‘They hate us, you know.’

Kingsley stifled a growl. ‘Those brutish murderers? Here? For any particular reason?’

‘I can’t think of anything, apart from our ancestors’ hunting them nearly to extinction.’

‘Mm. I can see how that could lead to deep-seated, brooding enmity.’

‘Indeed. And when you blend that with supreme mechanical artificing, humanity would have been in grave danger if not for their small numbers and their inability to work together very well.’

‘You’ll be telling me that you have dinosaurs in the Demimonde, next.’ He waited. ‘You don’t, do you?’

‘I don’t know everything about the Demimonde, but I wouldn’t wager anything on it.’

‘I think I need to know more about these Neanderthals,’ Kingsley muttered.

‘True, but not now.’ Evadne lifted herself. ‘My, it’s like Oxford Street out here.’

‘Someone else?’

‘A man, by himself. I think it’s the one who was following us after the Floating Market.’

‘Who could he be?’

‘Someone of little concern, I hope. Did you see where the Neanderthals went?’

‘You’re not thinking of going ahead? Not with those Neanderthals hereabouts?’

‘I’m not leaving.’

Even in the dark Kingsley could see her determination. She was a crusader, however much she might decline the title. ‘Child abductors, you say?’

‘It’s worse than that, Kingsley, but I’m not sure if you’re up for it.’

‘Worse than abduction? What could be worse?’

‘They’re magicians, and they’ve lived for a long time. A very long time, through their magic.’

‘So you say.’

She flared. ‘It’s not me who says it. The whole Demimonde knows about the Immortals, and fears them.’

‘So they’re nasty.’

She looked at him and then she dropped her gaze to her hands. She twisted the ring on her little finger. ‘The Immortals are rumoured to be able to manipulate objects and minds, and even time itself. They are totally without conscience. They use whoever and whatever for their own ends.’

‘Including children.’

‘They use young children to help them stay alive. They wear them out, you see, and then transfer their essence to fresh new flesh after it has been prepared with secret magic. Again and again, over centuries,’

‘I understand.’

‘I doubt it.’

Kingsley was tired. He pointed. ‘I think they went up there. Near that shed.’

Evadne rose, hefting the satchel over her shoulder. ‘After this, we’ll find your foster father. I promise.’

When they reached the shed – which was a rather more substantial brick building than Kingsley had thought – they found the door open.

Evadne nudged him. ‘You still have the Incapacitator?’

‘The steel wool weapon? Is that what you call it?’

‘What’s the good of making unique weapons if you can’t give them gaudy names?’ She shook her sleeve and produced a startling five-barrelled handgun. ‘I have the Crushing Reply.’ She reached behind her collar and plucked out a fine spray of steel and handed it to him. ‘And the Scorpion.’ She pushed aside her pleated skirt and reached into her boot. Something like an icicle crossed with a sickle emerged. ‘This is the Life Changer.’

‘Life Changer?’

‘They take one look at it and start seeking another occupation.’

‘Naturally.’

The tunnel led them downward. Kingsley held the Incapacitator in front of him in one hand and the pen light in the other. The silence had a weight of its own, pressing down like doom. The walls were tiled with black, shiny rectangles that caught the light and bent it in streaks along the arched way.

The tunnel ended in a five-sided door. It was guarded by two glowering Egyptian statues.

Evadne dropped to her knees and rummaged in her satchel. Kingsley was startled when she pulled out a tiny brass cylinder, hardly as big as his finger. ‘Is that explosive?’

She tucked the cylinder behind the base of the female statue. ‘It’s a phlogiston-based material of my own devising. Very powerful.’

‘That tiny thing?’

‘If I can secrete a few more of these, I don’t think the Immortals will be using this place again.’

‘You can’t,’ he said, aghast. ‘You’ll destroy the observatory.’

‘It’s worth it to put an end to these creatures.’

‘No it’s not. The observatory is our heritage.’

‘You’d put a pile of bricks against the chance of getting rid of the most hideous monsters in the Demimonde?’

‘It’s not just a pile of bricks. It’s enlightenment, it’s rationality, it’s the first step in humanity making sense of the universe in a way that matters.’ He ran his hand through his wet hair. ‘It’s Wren, it’s Hooke, it’s Flamsteed. It’s solving the longitude problem, it’s seeing the world clearly for a change.’

‘You’re serious,’ she said.

‘Of course.’

‘Then what is the alternative?’

‘Alternative?’

‘We’re here. If I’m right, the Immortals are behind that door. We have explosives, they have the Spawn. What are we going to do?’

‘I suppose a good ticking off is out of the question?’

‘Habits of millennia aren’t likely to change due to a scolding.’ Evadne’s face became serious. ‘Trust me. They need to be exterminated.’

Something ran deep in Evadne’s set against the Immortals. ‘What is it? Why are you so down on them?’

She twisted the ring on her finger. ‘It’s a secret.’

‘We all have secrets. Some are meant to be shared.’

‘I’m sorry.’ She shook her head. ‘I’m a solo performer, an independent operator, self-sufficient and complete.’

‘If that’s the case, what are you doing with me?’

‘I . . .’ She closed her mouth, then opened it again, perplexed. ‘It was a whim, at first, but I think I became carried away by circumstances.’

‘They can do that, circumstances.’

‘You have a secret, too,’ she said softly.

‘Don’t we all?’

‘Yes, but I happen to know yours. That makes us uneven.’

Kingsley went to answer, but he was interrupted by a roar that came right through the marble door in front of them. He dropped to all fours and a growl tore from his throat.

Evadne looked at him, unafraid and with some satisfaction. He climbed to his feet, abashed, and wiped his hands together. ‘Sorry,’ he muttered.

‘We all have secrets, but yours is one of the more interesting.’ She glanced at the door. ‘In the spirit of an open and honest working partnership, let’s do what we can in there – without destroying the observatory – and afterwards I’ll share my secret with you.’

She offered her hand. He took it. They shook, solemnly. Kingsley realised that for the first time, he’d met someone who he didn’t mind knowing his secret.