“Have you solved the code yet, boy?” Jack shouted down at Robert in the tank.

“Not yet,” Robert called back, “but I’m working on it.”

“You! Help him!” Jack thrust Lily down a metal staircase from the platform, holding up the lantern so she could see her way down to Robert.

“Hurry,” he told her. “Time and tide wait for no man!”

Lily staggered into the tank and splashed through the slurry, pulling Robert into an embrace, kissing his cheek in relief.

“Thank goodness you’re fine,” she whispered. “Hold steady, help’s on the way.”

She wondered where Tolly and Malkin had got to. And the police? Surely Anna had told them where to look? She should’ve waited for them to deal with Jack, she knew that now. She hoped she would live long enough not to regret her constant impulsiveness.

Robert shivered in relief in Lily’s arms and hugged her back as hard as he could. He had chalk in his hand and his clothes were damp with the humid stink of the tunnels.

“What have you been doing?” she asked.

“Puzzles.”

He let go of her and Lily rubbed her arm where Finlo had been grasping it. She stared about the tank, trying to figure out what he meant. The front of the safe was a circular dial filled with numbers. Beside it was etched a diagram of sorts. Lily peered closer at it and strange markings and inscriptions leaped out at her from the dark…

It was a pictogram of a diamond divided into a quadrant of separate chambers, like a heart, or the quarter hours of a clock. Around this were images of a lunar eclipse, and beneath each of those was a different code word.

“They’re numbers,” Robert whispered. “Because that’s what’s on the safe dial. You must have to read them clockwise, in the order of the eclipse.”

Lily nodded. He had to be right… But what numbers? A date perhaps? Captain Springer had told them the Blood Moon Diamond was discovered during a lunar eclipse. Could it be that date? What had it been again? As she racked her brains, the water sloshed about her calves. With cold, creeping shock she realized it was rising. Jack was right – they had to solve this fast!

She gazed at the diagram. It was more like four right-angled triangles than a diamond. Maybe Artemisia had used the same code as on the Moonlocket?

“It’s like the locket puzzle,” she guessed. “Each word has a translation triangle.”

“I started down that route,” Robert told her. “But it doesn’t work.”

“What are you saying?” Jack shouted down at them from the platform.

“Leave them be, Da,” Finlo said beside him. “They have to think.”

“No,” Jack said. “No secrets, remember, Fin, dammit!”

“It’s a similar code to the locket.” Lily’s words echoed round the tank, sounding more forceful than she felt inside. “It must mean the date of a lunar eclipse.”

“Did I ask you what it means?” Jack paced up and down the platform. “I don’t care what it means! I just need you to find the numbers and dial them into the safe before high tide. So get on with it!”

The water was at Lily’s shins now, soaking through the fringe of her already wet dress. She stared at Robert in alarm. His eyes were wide with alarm.

“We should leave, before we’re trapped,” Lily called out to Jack. “The tide’ll cut off your exit through the tunnels.”

Finlo turned to Jack. “She’s right, you know. If we stay too long we’ll drown.”

“No one’s going anywhere!” Jack shouted. “Not until I get my diamond!”

“Our diamond,” Finlo retorted. “And I don’t understand why we can’t come back another time?”

“I told you why.” Jack bristled. “I want to stand on Tower Bridge today, with that diamond in my hand, and watch the Queen of England go by in her parade. I want everyone to know that I outwitted her. And the Crown Prosecution Service, the prison guards, the police inspectors, the bobbies; everyone. I shall stand there as they pass and my picture will make the front pages, alongside the Queen. It’ll be the greatest trick ever pulled, the ultimate revenge – reappearing in public at the Jubilee with the very thing I stole fifteen years ago! Then afterwards, when I’m sure everyone has seen me with my diamond, I shall disappear in a flash of smoke before their very eyes, and get the hell out of this forsaken country for ever!”

“You don’t have to do those tricks, Da,” Finlo said. “They’re not important, they don’t matter. This isn’t a game. You’re not the world’s greatest showman any more. You’re a wanted criminal. What matters is getting the diamond out safely. We should go back into hiding and come for it another day, when things have quietened down.”

Jack shook his head. “Don’t you understand, you idiot? It’ll be too late by then. The police are onto us – with the information they’ve got from Selena and these two here, how long d’you think they’ll take to figure out where the diamond is? Then the only thing stopping them recovering it will be this code.” He pointed at the marks on the wall. “These two obviously like cracking mysteries or they wouldn’t have kept coming after me. It won’t take them five minutes to work out the cypher.”

While he’d been ranting, the water had risen above Lily and Robert’s knees. A dead mouse washed against the wall in the far corner, battering against a mess of leaves and twigs.

“We haven’t got long,” Robert whispered to Lily.

“Solve the code quicker or drown,” Jack shouted from above. “The choice is yours.”

Lily felt sick with fear. She could barely think straight. She had to get a grip. The only way they were going to get out of this alive was if they did as Jack asked. She stared hard at the diagram and the code.

“Wait,” she said. “I think I’ve got it! What if it’s the same system but reversed… The coded words go down the middle – along the straight edges of the diamond – then you fill the triangle with the preceding letters to get the translation along the slanted edge.”

“How did you guess that?” Robert asked.

“It’s the easiest possible variation on the cypher,” she replied. “Now hurry, we have to work it out.”

The freezing water was lapping their thighs. Robert began to write on the wall, filling in the gaps as Lily suggested with the wet chalk. It left sludgy white lines. His fingers shook so much he could barely write. After he’d done the first triangle, he passed the chalk to Lily and she took over, scrawling the other coded words down the straight sides of the triangles – and then adding the letters alphabetically before and after them to fill out each space.

The water had risen to their hips now. Lily filled in the last few parts of the code, and then she was done.

The pictogram diamond was a mess, but she read the words out clockwise from around its edges.

“Twenty-one, June, Eighteen, Fifteen.”

“Ha!” cried Jack. “That’s it! Open the safe, quick!”

Lily waded through the water, which was now level with her waist.

Robert held his arms out for balance and read the numbers aloud to Lily as she dialled them into the safe.

“Twenty-one,” he said, “then June… June’s letters, not numbers.” Lily could hear the panic in his voice.

“It’ll be six,” she said, “June is the sixth month.” And she turned the safe’s dial to six.

“Then eighteen and fifteen,” Robert said.

He felt himself shift along the floor as the current gambolled around his feet. He braced himself to stay upright.

Lily spun in the last two last digits on the wheel and the safe gave a loud click!

She hoped upon hope that the diamond was inside. Then they could get out of here before they drowned.

The lever on the safe turned, and she tugged the door open.

The shelves were scattered with old rings and trinkets…and in amongst them was a diamond bigger than all the rest. A perfectly cut red stone at least five times the size of all the others: the Blood Moon Diamond. It was the most impressive gemstone Lily had ever seen, red as wine and shifting in colour. In the feeble glow of the lantern it twinkled bright as a star, reflecting the light back at her in fizzing red droplets that sparkled across her face.

What a wonder it was! And it would surely be even more radiant in daylight – if they were ever destined to see it in such conditions, for the water had almost reached their chests. Without further ado, she pulled the diamond from the safe and, holding it above her head, waded towards Jack, dragging Robert along behind her.

Jack held the lantern high and rushed down the metal steps of the platform to meet them. He snatched the diamond from her hand and was about to stuff it in his pocket when there came a clang from behind him.

Finlo rested his crowbar against the metal rails of the platform. “I think you’d better give me that, Da,” he said. “I know the kind of tricks you’re capable of pulling. I imagine you’ve no intention of splitting the diamond, and I’m afraid I can’t let you walk away with it.”

Jack grasped the stone tight in his fist. “I spent fifteen years inside waiting for this moment, boy, and I’m not going to let you take it from me. You’ll get what’s coming to you, if you do as I say.”

“That,” said Finlo, “is what I’m afraid of. But not this time.”

“What’re you going to do?” Jack asked. “Kill me? Your old dad?”

“I will if I have to. You can’t bully me any more, Jack.” Finlo took a step towards him. “I’m my own man now.”

“Are you?” Jack asked. “Or are you the same snivelling worm you’ve always been?”

“Don’t underestimate me.” Finlo’s face turned purple with fury, he raised the crowbar over his head.

Lily braced herself for the thud.

But Jack laughed and, with a wave of his hand, the diamond vanished and a gun appeared in his grasp. A tiny pepperbox pistol.

“You know the difference between you and me, Fin?” he asked.

Finlo shook his head.

“I know when to keep my powder dry.” Jack jerked back the trigger and a bullet exploded from the muzzle of his pistol. Finlo dropped the crowbar and it clattered to the floor of the platform. He clutched at his chest.

Lily blinked in shock as Finlo listed sideways and plunged over the rail with a splash. Blood seeped into the water. His body sunk for a second then drifted upwards in the rising tide. His bowler hat bobbed from his head and bumped against the base of the metal staircase.

Robert gasped and scrambled up the steps away from it.

Jack had killed his son. And for what? A stone. He was no father, no grandfather either – just an evil, selfish man.

“Time to go.” Jack raised the lantern and waved his pistol at them both. “I’ve a boat waiting in the Thames. I’m going to need someone to row me to Tower Bridge in time for the big finale. You two’ll do fine for that, I think!” He grinned, patting at a bulge in his pocket. And, with a start, Lily realized that he’d hidden the diamond there during his sleight of hand.

Back in the main tunnel the Fleet River had engulfed the path. Here the water was fast-flowing and had already grown knee-deep. Lily felt queasy. Jack stopped to consult the engraved map on the locket round his neck, and used his fingers to trace a path. When he’d got a sense of where they were, they headed off again.

But soon they heard the echo of footsteps, and glimpsed a faint far-off light. Jack threw Lily and Robert against the edge of the passage and crowded in behind them, blowing out the flame in the lantern. “Keep quiet, both of you, or else,” he said, raising the gun and resting it on Lily’s shoulder.

Lily’s legs shook. She leaned forwards and narrowed her eyes, trying to see who was coming.

In the dimmest of dim lights, she could make out a silhouette holding a lantern. As it pushed through the waters, she saw a furry creature wound around its neck like a stole. Tolly and Malkin!

Lily’s heart ticked hard in her chest. Tolly couldn’t see them in the dark; he didn’t know Jack was armed.

Jack took aim and cocked the hammer, and with a lurch of horror Lily realized he actually intended to shoot again. “Tolly!” she cried. “Hide! Jack’s got a gun!”

“Don’t come any closer!” Robert added.

Tolly threw himself behind a narrow brick pier.

“Be quiet!” Jack shouted and pushed them both out of the way. Lily’s head bounced off the bricks. In a haze of pain, she glanced at Jack. He waved the gun about, searching the tunnel for Tolly’s light. There wasn’t much time…

She barrelled forwards, throwing her weight against him, and pushed.

Jack slipped on the slime-coated floor, and tumbled. He yelled and threw out his hands to break his fall, and then plunged into the stream of sewage. Ripping the gun free, he raised it and fired – but it fizzled, wet and useless.

“Blasted thing!” he cried, tossing it away.

Robert rushed forward in a flash, smacking one hand against Jack’s chest, and snatching the Moonlocket from around his neck with the other. While Robert had him distracted, Lily seized her chance and shot a hand into Jack’s pocket, plucking out the diamond. “OY!” Jack grabbed wildly at them both, trying to get his treasures back, but he didn’t know who to go for first.

Suddenly, with a lurch and a scream, he slipped. His feet teetering in the current, arms flailing about the air. His face darkened as he opened his mouth to curse them all, but then fell into the raging water instead.

Instantly he was wrenched beneath the surface and swept away.

They waited for him to come up, but he did not.

Lily hugged Robert and stood there, breathing heavily, shocked and relieved. Jack was gone, and they had the diamond and the Moonlocket – their map to freedom. But they weren’t safe yet.

Tolly bounded up the tunnel towards them, and embraced them both. “We heard a splash,” he said. “Thank tock it wasn’t you!”

Malkin, who was around Tolly’s neck, sat up and licked their faces. “Yeuch! You’re covered in sewage!”

“We could have done with your help half an hour ago,” Lily said. “Where were you?”

The fox looked sheepish. “We got lost in the dark,” he admitted.

“We couldn’t find anything,” Tolly added. “And then we heard a noise coming this way and thought it might be Anna arriving with the police at last. It’s a good thing you shouted a warning to us or I never would’ve realized you were with Jack. Was that him falling in the water?”

Lily nodded. “He never came up.”

“Well, good riddance,” Tolly said. “I hope he washes out to sea with the rest of the sewage. What happened to the other one – his son?”

“Jack killed him.” Robert put a shaking hand to his temple; his head was spinning fast, as if the sound of the gunshot was still echoing inside him.

It was like his da’s death all over again. Even though he hated Finlo and what he’d done, Robert did feel sorry for him. It seemed to him that Finlo had taken part in his father’s plan not only for the money but also to please him. But you couldn’t please a father like Jack; no one could. He was a cold-hearted killer who had valued a gem over his flesh and blood. How could Robert bear to be part of a family like that? Well, he didn’t need to be. Not when he had Lily and John, who he loved and who loved him. His home was with them, and if he got out of this alive, he’d make sure they knew it. He only hoped his ma and Caddy would be fine together now that Jack and Finlo were gone.

“What on earth…?” Tolly said suddenly, glancing over their shoulders, up the tunnel, the way they’d come. The engorged Fleet raged in a torrent over the weir. “I don’t think we can get back over that.”

“We need to find another exit.” Lily pointed at the locket in Robert’s hand. “The opening to the Thames is marked on the Moonlocket. It’s a storm drain under Blackfriars Bridge. Jack told us he’d moored a rowing boat there. If we follow the map, we should make it.”

“Thank goodness!” Malkin said. “And whatever happened to that clunking Blood Moon Diamond that everyone made such a fuss about?”

“I picked it from Jack’s pocket.” Lily opened her hand and it glistened.

“Played him at his own game, eh?” Malkin said. “Perhaps his book wasn’t so useless.”

Lily put the diamond away in her pocket. “When we get out of here,” she said, “I shall return it to its rightful owner. Until then, at least we’re alive and we have each other.”

“Only if we get moving though!” Tolly said.

Robert nodded. He’d been consulting the Moonlocket. “It’s this way, I think,” he said, pointing a finger off into the distance, and they set off at once, following the map.