The Watchers were gathered beneath a canvas tent on the roof of St Bride’s church. The rain was pelting down, finding every hole in the old fabric and falling on them in fat, cold drops. And there the Watchers slept fitfully, taking what rest they could. They would need everything within them if they were to rescue Ghost and Moon and then see Revolution Day through to the end.

When Ben woke his right hand was throbbing faintly. He sensed that it had something to do with the never-ending storm. If only he knew what.

Since Mr. Sweet had taken control, Ben had decided that it was vital for the Watchers of London to know where they could go if they were lost or alone and needed to regroup. Monday, the British Museum; Tuesday, the Temple; Wednesday, Westminster Abbey; Thursday, Tallow Chandlers’ Hall; Friday, Fenchurch Street Station; Saturday, St Bride’s; Sunday, Christ Church Spitalfields. Ben had made every Watcher learn the list by heart. It had served them well, so far.

Carter was there, with Nathaniel and Lucy, and maybe fifty or so other Watchers from across the city. But they could also see that their numbers were dwindling day by day. That was yet another reason why it was so vitally important to rescue Moon and Ghost. They would need everyone they could get for tomorrow, Revolution Day.

The canvas lifted and their eyes lit up to see Valentine.

“You got out,” said Nathaniel. “Well done, mate.”

“What about the escapees?” asked Ben, clasping his hand.

“Never let it be said that I let a chap down in their hour of need,” said Valentine. “Managed to get them all into the hands of some thoroughly decent Watchers running a safe house on Pye Corner. Close run thing though, don’t you know.”

Ben clapped Valentine on the shoulder.

“Must say,” Valentine continued, “emotions were running pretty high. Heard some things about what Sweet’s been doing that would make your hair curl. Seems he’s even been executing every red-headed boy that he comes across just in case the blighter is you, Benjamin.”

Ben shuddered.

Valentine went on. “You can be sure that every man jack of our escapees will be fighting alongside us when the uprising kicks off.”

“They won’t be the only ones.” A deep voice reached them from outside the tent. Ben’s heart missed a beat.

It can’t be…? “Pa!” shouted Ben, slipping out from under the canvas and running to him.

“It’s Saturday,” said Jonas. “So, St Bride’s.”

Nathaniel joined them too and both boys spoke over each other, not giving Jonas time to reply or even draw breath between their questions.

“We saw your flare.”

“How’s Molly?”

“What happened to the Liberator?”

“Are you hurt?”

“I’ve missed you.”

“We need you.”

Jonas enjoyed their embraces and attention. “I’m as right as rain,” he said, running his hand through his sopping hair. “But I’ll catch my death if we stand out in this – can’t we at least get under cover?”

The Kingdom boys happily yielded and brought their father back under the shelter of their makeshift tent.

“Right,” said Jonas. “First things first, Molly’s fine.”

Lucy smiled.

“She’s being looked after by the troops – they’ve sort of adopted her as a mascot – and there’s this brigadier, Carnehan his name is, commanding office of the regiment of Coldstream Guards. Really decent sort. He’s taken Molly under his wing.”

“And the Liberator? We saw the explosion.”

“She won’t fly again,” said Jonas sadly. “Hardly a matchstick remains, but she did the job. As far as I know, we didn’t lose anyone on her last flight, everyone got off in time. A little crisp around the edges maybe, but alive.”

“What of London beyond the Wall?” asked Carter.

Jonas breathed heavily through his nose. “People are in shock. The army is waiting to attack, but with the Queen still captive Sweet has got their hands tied. That’s the official story anyway. However…” Here Jonas grinned and Ben recognized his own expression in his father’s lopsided smile. “I’ve got some good news. Carnehan has brought in the sappers.”

Ben had heard of them; they were military engineers and explosives experts.

“Right now,” said Jonas, “the sappers are under the death zone preparing to blow open the tunnels again. The army can’t advance across the Dead Man’s Land without being seen, but when the revolution starts they are going to open up a dozen points of entry—”

“And sneak in under the Legion’s noses!” Ben finished the sentence.

“We still can’t win unless Ben destroys the Crown of Corruption,” Carter reminded them.

“That’s why we have to rescue Ghost and Moon,” said Ben. “The rebels are going to need Watchers to guide them or the revolution will be a shambles.” He clenched his teeth. “We’ve got to get them out of Sweet’s circus.”

“I can help with that too,” said Jonas, pulling out the poster that Munro had given them. “Ben, you once told me about a hunchbacked boy you met in the Under—”

“Munro.”

“That’s the lad,” said Jonas. “He’s left the Legion and he’s safe with Molly and Carnehan. He’s been able to give us lots of information about what’s happening inside Sweet’s headquarters at the Tower of London – numbers of guards, shift patterns, those sorts of useful details. He also brought us this.” Jonas spread the poster wide.

“One o’clock today,” said Valentine, reading down.

“At the Lyceum,” said Nathaniel.

“Right,” said Ben. “Then let’s get moving.”

Hans Schulman, the Watcher spy, was tied to a chair in Sweet’s private chamber. Mr. Sweet examined him with the cold precision of a surgeon. Hans could not bear to meet the fierce gaze which bore into him through the silver sockets of Sweet’s mask.

“Do you think you are cleverer then me, boy?” asked Sweet.

Hans said nothing.

“Did you honestly believe that I wouldn’t be able to sniff out a traitor in my own ranks? You Watchers are so arrogant! You probably thought that you were a brilliant spy when you overheard me talking about the crossbones key. It never occurred to you that I meant you to hear me. You fell straight into my trap.” Sweet laughed. “And now you are going to betray Ben Kingdom.”

“Never,” said Hans. “I’ll die first.”

“You don’t have any choice in the matter, boy,” said Sweet. “I wear the Crown of Corruption. I can make you do anything.”

Before Hans could react, Sweet grabbed the boy’s face in both hands. Sweet brought his own face in close, the beak of his mask scratching Hans’s skin, his eyes piercing into the German lad’s mind, stripping away all mental defences.

“I. Am. Your. Master,” hissed Sweet.

Hans tried to screw his eyes shut to avoid Sweet’s hypnotic stare, but as the words bored into his skull he found that he was unable to resist.

“I. AM. YOUR. MASTER.”

The words exploded inside Hans’s head, pushing every other thought aside, until only Sweet’s voice remained.

Hans spasmed, like a medium at the seance table overwhelmed by a spirit that is not their own.

“You are my master,” he repeated.

“You will obey me.”

“I will obey you.”

“Take this to Ben Kingdom,” said Sweet, placing an iron key in the boy’s hand. “Give him this message and then return…”

Hans had no choice but to listen. Mr. Sweet’s lie ingrained itself in the wet clay of his mind so that he would be able to repeat it word-perfectly when the time was right.

“Now go!” Sweet ordered.

“Yes, master.”

The Watcher’s own spy was now working against them. Hans Schulman was Sweet’s puppet and he jerked away on his errand like a good wooden boy. Sweet licked his cracked lips in delicious anticipation.

The voices inside Sweet’s head kept telling him that Ben Kingdom would defeat him, but with the German boy under his control, Sweet had sealed Kingdom’s fate once and for all.

It was almost too simple. Propelled by his schoolboy sense of patriotism, Ben Kingdom would never allow Queen Victoria to rot in a cell if he had been handed the key. So it was a given that Kingdom would come running, place the key in the lock, turn it and…

Sweet slammed his hands together gleefully.

The Watchers, in their earnest naivety, always failed to see the depth of cunning that resided in the souls of the Legion. The Under had many secrets, most of which could inflict pain.

The dungeon doors had two keys. One which opened the lock. And another which released a vicelike trap – steel spikes which would spring out from their hiding place in the wall and put an end to any rescue attempt. For ever.

Guess which key he had sent to Ben Kingdom?