Robert couldn’t sleep. He had too much to consider. It was like arriving at a fork in the path, and suddenly seeing ahead of you an alternative…a future where things could go differently. Another life with another family – one who’d never even existed before in his mind.

He watched Selena and Caddy dozing on the bare mattress. It was strange, he’d not only found a mother, but a sister too – two good people, who were both as alien to him as complete strangers, because his ma had chosen to leave him behind… She’d said it was to keep him safe from the Doors, but could he ever understand that? It hadn’t even worked in the end… Could he possibly forgive her? Perhaps, in time…

Lily was somewhere out there. She was sort of a sister to him too, though he’d known her only eight months. He sat back down on the mattress, and listened to the night sounds of the city. A drunk was singing somewhere in the street below, and there was a screech of fighting cats, or could it be a fox? With a sudden pang he thought of Malkin – he hoped his mechanimal friend was taking care of Lily, wherever she was.

Robert stifled a yawn. He’d better stay alert for Jack’s return. He focused on the sound of his ma’s soft breathing, and Caddy’s occasional murmurs in her sleep. It was funny how she was his sister and yet her voice, her accent, sounded nothing like his. Her face seemed so calm and tranquil. Despite everything, he was beginning to feel a great deal of warmth towards her, and, in some ways, for Selena too.

The fear of what Jack might have in store for all of them – especially Selena and Caddy – filled him with anxiety. He only hoped that, whatever it was, it wouldn’t split his new-found family apart.

He needed a plan to get them out and away, but he was so tired, so lost. His usual sharpness had leached out of him, and his thoughts were a mushy haze. Soon, though he tried to fight it, sleep crept up and pulled him through her dark doorway…

“Time to get up.” Rough hands shook him awake.

“What’s going on?” Robert asked, as he sat up and rubbed his eyes. This night wasn’t over yet. The room was still black. Beside him Caddy and Selena lay asleep. Finlo stood over them, holding a lantern. Jack wasn’t with him.

“You’re coming with me, you scrawny runt.” He reached out to grab Robert.

Robert balled a fist and hit at his uncle’s face, but all that did was knock his bowler hat askew. Finlo laughed.

Robert trembled and looked down at his sister and Ma. Selena hugged Caddy, holding her close. She snorted and threw a hand over her face, twitching at the light, but she didn’t wake up. The pair of them were cuddled together, spooning like slumbering angels. He was leaving them in the middle of the night, just as his ma left him all those years ago. If he went with Finlo, he might never return. “I don’t want to go,” he said, his voice quaking.

Finlo nodded at the sleepers. “You’ll do as you’re told. If you want them to wake and see another day.”

Robert felt sick. “Where are you taking me?”

“Never you mind,” Finlo spat back. “Put your jacket on quietly. Or it’ll be worse for you later. And for them.”

Carefully, Robert reached for his coat and cap on the chair beside the mattress. He struggled into the coat and pushed the flat cap back into shape before wedging it jauntily on his head. He wished he felt that way inside. He hoped Selena and Caddy would be okay without him. What if he never saw them again? What if he never got the chance to say all he needed to say? He bent down to kiss them both goodbye but Finlo grasped his sleeve and jerked him away.

“None of that,” he growled. “I warned you, didn’t I? Now, come on, no time to dawdle, Jack’s waiting!”

Finlo dragged Robert along murky streets filled with grey smog and a watery drizzle, which floated up from the cobbles and made Robert’s eyes water and his throat itch.

In the distance, behind London’s shadowy domes and turrets, a blanket of black clouds was rolling in across the night sky. The air was filled with a muggy, tingling tension that seemed to promise the coming of a great storm.

At one point, as they passed beneath the glistening spotlight of a gas street lamp, Robert thought he saw the tall helmets of two policeman coming towards them along the pavement in the fog. Finlo shielded the lantern and clasped a hand over Robert’s mouth, steering him down a side alley.

“Why do you do as he says? Jack, I mean,” Robert gasped, when Finlo finally removed his hand. “You don’t need to, you know.”

Finlo gritted his teeth. “I didn’t ask your opinion.”

“I just think—”

“Keep quiet, nephew, and do as I say,” Finlo snapped, “or your ma and your sister’ll never get out. You follow?”

Robert nodded, but didn’t reply.

“Not so chatty now, are you? Speak up if you understand.”

“Yes.”

“Yes, Sir.”

“Yes, Sir!”

“Good.”

It started to rain heavily. The water splatted off them in drops. They had reached an area that looked vaguely familiar. Robert could hear a soft rustling from the drain covers they passed, as if water was running under the streets.

They turned another corner into a curving road lined with elms, and there, lurking beneath the tallest one, was a tramp in an old overcoat. His hands were thrust in his pockets and his collar was turned up against the foul weather. As they approached, he turned his head into the lantern light and Robert saw that it was Jack.

“What took you so long?” Jack asked Finlo.

“Problems.” Finlo took off the bowler and scratched his head. “We ran into some peelers when I went to collect this one. But we got by unnoticed.”

“You ought to be more careful. There’s been none round here.” Jack darted a look across the street. Robert followed his gaze and realized they were standing opposite the Doors’ old rental house – number forty-five, Queen’s Crescent.

Jack put a hand on Robert’s shoulder. “You’re going to get us in, boy. If you do as I say, and all goes well, then you’ll see your ma and Caddy again. If not, well, it’ll be the worse for you…”

Robert gulped back a wave of panic.

Jack looked around, then nodded to Finlo. “Coast is clear.”

They dashed across the road and sneaked up the steps to the porch of the house, to shelter from the brewing storm.

Jack pointed out a small square window above the front door. “Loosen that transom,” he whispered.

Finlo put down his lantern, took a crowbar from his coat, and jemmied open the tiny window. The dark space behind it was far too narrow for a man to squeeze through…but not, Robert realized, a skinny boy. Jack clenched his arm hard.

“You’re to climb through that gap, son, and open the locks on the door, and be quick and quiet about it. Or else.” He kneeled down. “Get onto my shoulders.”

Robert did as he was told, and Jack stood up and thrust him upwards, as if they were in some sort of acrobatic show.

Robert found himself soaring towards the dark window; he reached out and grabbed hold of the frame. The buttons on his coat scraped against the edges as Jack and Finlo shoved him through from behind.

Then he fell, tumbling into the hall. Luckily the doormat and some piles of newspapers broke his fall and muffled his landing. He worried that they might wake the landlady. Then he remembered the old woman had told him she slept like the dead. Lucky really, or Jack might have seen that she meant it for real. For a moment he considered refusing him entry, but the thought of his grandfather’s rage and what he might do to Selena and Caddy made him change his mind.

He stood and took a deep breath. Then he undid the locks, one by one, and opened the door.

Outside he could make out the shapes of Jack and Finlo in the glow of their lantern’s light.

“You did it!” Jack rubbed a callused hand through Robert’s hair as they crept silently inside.

Robert felt a moment of strange pride, before remembering the many reasons to hate his grandfather.

“Let’s go,” said Finlo, closing the door softly. “We have to find the diamond and get out before daylight or else the police’ll work out where we are for sure.” He held up the lantern and they stumbled down the stairs, descending into the basement.

They found themselves in a long, narrow room with an earthen floor, which stretched out under the street. In its centre was a large manhole cover.

Finlo took out his crowbar once more and forced it open. Air wafted from the hole, humid and claggy, and filled with a fecund stink. There it was: the Fleet Sewer. It ran all the way down to the Thames, and somewhere in its straggling maze of deep, dark tunnels it held the Blood Moon Diamond in its clutches.

Robert stared down into the hole. Iron rungs descended into a darkness deeper than he’d ever known.

“Get down there and wait at the bottom,” Jack told him. “We’re coming after you.”

“Please…” Robert whispered but he tailed off. He coughed and felt a gritty dry emptiness in his mouth. He wanted to refuse. Wanted to tell them that he couldn’t swim. He tried to push a reply from his throat, but when he thought of Selena and Caddy locked up, the words stuck there, wedged behind the great lumps of fear filling his chest, as scratchy and solid as tree bark.

“Don’t think about being brave,” Jack told him. “And don’t think you can protect Selena from my retribution. Remember what I told you, boy, she’s not worth saving. She doesn’t want you. Doesn’t care! Family means nothing to her.”

Jack thrust the lantern into Robert’s hand and pushed him into the hole.

Robert stumbled down the rusted ladder. As he reached the last rung, his feet dangled in noxious water. It slopped, cold and clingy, about the tops of his socks and he wished he was wearing stout boots rather than the thin leather shoes he had on. The turn-ups of his trousers soaked through and slapped heavily against his ankles. He was in a narrow arched passage of filthy yellow and red brick. The stench of sewage surrounded him, seeping into his every pore.

He held the lantern high above his head and glanced upwards.

Jack and Finlo were descending. Jack had the Moonlocket around his neck, with the strange map Robert’s da had engraved on it long ago at the request of Artemisia Door. It was the only guide they would have down in these underground ways beneath London. Could the jewel on its back really mark the location of the Blood Moon Diamond? To think a part of the locket had been hidden in his house those many years…

At the top of the ladder, Finlo paused to pull the manhole cover across behind him.

“Come on!” Jack whispered angrily at him. “Hurry it up!”

Finlo’s posture tightened for a moment, then he carried on.

Robert shoved his hands deep into his pockets, hoping to find something, anything, to help him thwart Jack’s plan. Beneath his fingers was a thing the size of a bean, hard and crumbly. He took it out. It was the nub of chalk Jack had given him to complete the code.

Quietly, while Finlo and Jack weren’t looking, he marked the wall beside him. At least if the police, or Lily for that matter, had worked out what the locket map meant, they might see this mark and be able to follow him. Though heaven knows how long that would take.

Robert glanced up one last time. Finlo was nearly at the base of the ladder. He’d left the cover open a chink – probably in case they had to return that way. Holding onto his hat, he jumped down the last few rungs into the water.

Jack had been reading the locket in his palm. Now he nodded, indicating the tunnel to the right and strode on ahead. Robert traipsed behind him and Finlo took up the rear, urging him onwards into the dark sewer, following the path of the Fleet.