Oscar clenched his fists as he stared after the two ghosts. He couldn’t believe they were leaving him like this, with so many questions. His eyes fell on the photograph of his father hanging on the wall.

It was his favorite picture. The last one taken of his dad before the accident. Oscar was in it, playing with a toy car. His dad was grinning down at him. He looked very proud. And then Oscar had a remarkable thought. Maybe his dad was a ghost too.

Could he find him? Speak to him? Oscar had imagined his dad’s voice in his head a million times—he’d love to hear it again, just once….

Oscar burst out of the front door as fast as his crutch could carry him.

The carriage was already turning around in the street. The two ghost detectives had climbed up into their seats.

“Stop!” Oscar shouted, leaping in front of the carriage. The terrifying skeleton horses loomed over him.

“Please don’t make a scene, sir,” Sally said.

“But I want to know if I can talk to my dad! He could be a ghost too.”

Sally seemed to look at him with a flicker of pity for a second. “No. Against the rules.”

“I’m not moving.”

“Suit yourself.”

The knight flicked the reins, and the skeleton horses walked forward. Both of them snorted at Oscar as they marched right through him, sending a wave of ice through his body. Their translucent bones gleamed silvery green. Bits of withered flesh still clung to them here and there like badly chewed chicken drumsticks.

Oscar tried grabbing the carriage to pull himself up, but his hand went straight through the wood, making it tingle.

The carriage kept rolling off. The detectives’ legs passed through his shoulders and head. Now his chest was poking through the bed of the carriage, the boards flowing around him as if Oscar was a rock standing in a fast-flowing stream. “Stop!” he pleaded. He could just make out his feet through the transparent planks. The sheer oddness reminded him of before, when he’d turned invisible and fallen from Mr. Jenkinson’s grip.

The carriage finished passing through him, and Oscar turned to watch his last chance to get some answers—to speak to his dad!—rolling faster and faster away down the street. The hope that had burned so brightly for a moment flickered out.

He was so desperate that he almost didn’t notice the change that had come over him.

Oscar looked down at his shimmering hands. They were see-through.

So were his legs. His arms. His feet.

For the second time that day, without quite knowing how he’d done it, Oscar Grimstone had become a ghost.

“Hold up!”

Oscar didn’t wait to wonder how it had happened. He was already running after the carriage, making no footprints in the mud. Around him, Oscar noticed that the houses and bushes and cars appeared dulled, like he was viewing them through sunglasses. But as everything darkened, the ghosts and their carriage sparkled brighter, like they were made of pure moonlight.

Oscar pumped his ghost legs. Then he was floating, his wheeling feet only faintly brushing the ground.

A huge grin spread on Oscar’s face as he moved faster toward the carriage.

His crutch had turned ghostly too, but now he didn’t need it because his legs were awesome! He dropped it in the road. The crutch snapped back to normal as soon it left his hand. It clattered on the asphalt.

“Stop!” Oscar shouted, but the skeleton horses accelerated.

That didn’t matter. He took three great, bounding strides and jumped straight into the back of the carriage.

He landed with a thump on the same boards that had flowed about him seconds before. The two detectives were staring at him openmouthed.

“You look like you’ve seen a ghost,” Oscar said.

“By Mortis’s beard!” Sally gasped.

Cedric opened up the lower half of his visor. “Stab me with an otter! That’s mighty impressive.”

“A human turning into a ghost?!” Sally yelled, grinning from ear to ear. “You’ve even frightened the horses.”

“So can I come with you?” Oscar asked.

The detectives glanced at each other. “Fine,” Sally said. “Got a few more questions myself.”

“Thank you,” Oscar said. The thought suddenly crossed his mind that it might be quite dangerous, being taken to the world of the dead. “Er…I mean, if I’ll be back in time for breakfast? It’s just, my mum will worry if not.”

“Certainly,” Sir Cedric said. “You have my word.”

“Right, that’s settled.” Sally nodded. “You ready? Next stop, headquarters.”

Sir Cedric shook the reins, and the carriage sped off down the road. There was no wind even though they were traveling fast. No sound either; but a mist soon spread out of nowhere. “Hold on!” Sally shouted. “You might not like this.”

Almost at once, the mist was so thick that Oscar couldn’t see his hand in front of his face.

The carriage drove on through the fog faster and faster. They’d left Marigold Street, the whole of Little Worthington in fact, far behind. Oscar knew this, even though he couldn’t say how he was sure. Dark spindly shapes whirled past them like huge grasping hands. Sir Cedric muttered something to Sally.

“Where are we?” Oscar asked.

“The void!” Sally yelled. “And these are void horses, descended from the steeds of the Valkyries. They run so quick they can break the phantasmic barrier. That’s why they need to be bony! Can’t have any extra weight.”

“Right,” Oscar said. None of it sounded very reassuring.

“Whoooah!” Sir Cedric pulled up hard on the reins.

There was a bright flash. The fog cleared.

They were on a busy road, lights glaring out from bars, late-night stores, and high office buildings—some ancient, some made of glass, reaching high overhead.

Is this…London? Oscar wondered. That’s fifty miles away!

A pair of bright headlights blazed down on them. Sir Cedric didn’t flinch or steer away.

It was a bus! A big red London bus and it was coming straight for them. Oscar screamed as the light filled his vision. He continued screaming as they drove on through it. The driver and then the passengers sitting in their seats flashed past him. None of the passengers looked up from their phones.

Oscar was crouched in the bottom of the cart with his hands over his eyes.

“You’ll get used to it,” Sally said. She was chuckling.

They drove straight through another bus, then streams of cars and other vehicles. Oscar managed not to cry out but couldn’t stop himself flinching each time. To distract himself, he tried to look about him.

“Where are we?”

“Fleet Street,” Sally answered. “Pretty good driving, Sir C—it’s hard to be accurate in the void.”

Then all pitched forward as Sir Cedric grunted and jerked on the reins. The cart stopped hard, the horses rearing up.

“Blimey!” shouted Sally down to someone in the road. “You watch yourself!”

They’d narrowly avoided hitting a child. A small, shimmering child who’d run out into the street from a tavern to chase a flickering ball. Its mother came running out and picked up the crying little boy. She waved her thanks to Sir Cedric.

“Ought to know better, Francine!” Sally cried as the cart moved off. “You’ve only had four hundred years to teach that child to mind a road!”

“Is she a ghost?” Oscar asked.

“Both of them are. Francine works at the Ancient Mariner inn—best jellied eels in Londinium. They both died in the Great Plague as I recall. Snagged a watching visa to look after her family and they’re still hanging about. Not ready to let go of the living world. Happens sometimes. People stick around for a whole host of reasons. Sir C, for example, he just couldn’t give up on the living world.”

“That’s right,” the knight grunted.

“Some ghosts have a skill they’ve mastered—and they just want to keep doing it forever. They’ve a passion, see? Others get offered a job at the Ministry….”

“Is that what happened to you?” Oscar asked.

“No. I had…unfinished business.” A dark gleam appeared in Sally’s eye.

“Wait! So are there lots of ghosts, then?” Oscar asked, still desperately trying to keep up.

“Why don’t you see for yourself?” Sally smirked.

Oscar looked out the carriage window, and his mouth dropped open in astonishment.

“Wha…whah…”

The usual houses and cars and living pedestrians were there—but they all seemed a bit faded and gray. Moving amongst them, and shining brightly, were dozens of shimmering people dressed in all kinds of historic clothing—some had ruffles and flamboyant waistcoats, another man wore a vintage pin-striped suit and fedora, and a woman in an old-fashioned sari was buying a newspaper from a scruffy urchin on a street corner. The dead seemed more real than the living.

“Why haven’t I seen any of this before?” Oscar whispered.

Sally shrugged. “Maybe you just didn’t look properly.”

“That mist did something to me.”

There were shining ghost buildings too, crammed in the gaps between real houses—or even on top of them. They came in all types. Mud huts, ancient wooden houses, drunken, half-timbered mansions leaning out over the street. They had doors in strange places, and the ghosts were moving in and out of them as if they lived there.

They did, presumably.

Oscar was suddenly struck by how very, very old the city was. So many people had died here. So many houses had burned down, or fallen apart, or simply been built over. But it hadn’t gone away. Nothing went away, it seemed. “This is amazing!” Oscar said.

Sally nodded. “You’d be surprised what goes on under your nose, eh? Londinium, a city of ghosts, right on top of the living city. And much more interesting, if you ask me.”

She was right. What if his dad was here? The thought was almost too exciting to grab hold of. His dad! He might actually see him again.

“Does my dad live here?” Oscar asked.

“We can look him up,” Sir Cedric said. “He’ll be on the record.”

Oscar couldn’t believe any of this. How did it all work? What would he say to his dad if he found him? How would he explain this to Mum?

He stretched his bad leg like he always did when he was confused.

But that ended up being a big mistake.

The cold, tingly feeling inside him disappeared, and Oscar turned real and solid. He fell through the cart, thumping down into the road. Pain jolted through his bad leg. The horse and cart trotted away in front of him. Sally’s head was craned out the side, her wide eyes fixed on him with utter astonishment.

Around him, the living world had turned back to full color. The noises of car horns and rushing traffic became loud, and Oscar realized that in his ghost form the sounds of the living world had been muted too. Oscar could still see the ghosts, though they had lost some of their shimmering moonlit quality.

“Blimey,” said a bearded ghost wearing a cravat, turning from a line at a fruit stall to stare at Oscar.

The fruit vendor, a large woman in an apron, gawked. “Did that ghost just turn into a fleshy?”

“Won’t be a fleshy for much longer,” a thin woman in a ball gown said, looking past Oscar.

A horn blared in Oscar’s ears. His head snapped round, and he was blinded by headlights. Two tons of black taxi skidded toward him. The taxi driver swerved to the right and crashed into a lamppost. A bicyclist smashed into the taxi and went somersaulting over his handlebars.

He landed beside Oscar.

A crowd gathered. Ghosts and people. The humans had no idea ghosts moved around them. Now Oscar was in his human form, the living regained their color and pointed at him, some taking out their phones to record videos.

“…just appeared from thin air, I swear!”

“…almost got hit!”

The taxi driver was screaming at Oscar. The bicyclist was screaming at the taxi driver. Sally was screaming too, but only Oscar and the ghosts could hear her.

“You idiot!” she yelled. “Do you have any idea how much trouble you’re making? Turn back into a ghost right now!”

The cart had rolled on ten paces without him.

“Um…ah…”

It was impossible. He could feel all the people staring at him. His bad leg hurt where he’d landed awkwardly.

“I don’t know how,” Oscar said.

“You what?” the taxi driver growled.

“Just think!” Sally shouted.

But it was hard to think—with all those eyes and all that noise and confusion whirling around. The taxi driver and the cyclist were standing nose to nose and looked like they were about to fight.

Oscar shut his eyes.

“What are you doing, you idiot! Hurry up!” Sally yelled.

The first time he’d done it, he’d seen the picture of his dad. That was when he changed.

The second time he’d been slipping through the cart, and then he’d thought about his dad and then…

Oscar pictured his dad in his head. It wasn’t hard. He’d stared at that particular photo so many times. He knew every detail of it. His dad’s little bald spot. The faint smile on his lips. The careful way he was holding the little boy in his lap—as if he would never let anything bad happen to him.

“You can do it, Oscar.”

Oscar vanished.

“Thank Mortis! You didn’t look like you knew how to turn back.” Sally chuckled.

“Don’t do that again,” Sir Cedric said. “This is a level-three breach.”

“Three?” Sally shook her head. “There’s more than thirty people who’ve seen him. It’s a four at least! You’re more trouble than a train full of soul feeders, Oscar.”

“You’re too much paperwork,” Sir Cedric grumbled. Then he stiffened, like a dog catching a scent. “Hear that?”

A wailing screech of a siren was coming closer and closer.

“Uh-oh,” Sally said. “Here comes the awkward squad.”

The siren was getting louder.

“Who are they?” Oscar asked. “They don’t sound good.”

“They’re not. They’re the wipers,” Sally started. “They’ll make sure no one remembers this little accident.” She cocked her head to one side. “Course, not sure what they’ll do about you. Me? I’d want you to talk, but the wipers do like everything to be tidy. And you’re a mess.”

“I’m not a mess! I’m in danger,” Oscar pleaded. “My mum’s in danger. There was a ghost that attacked our mortuary. And I can meet my dad! I haven’t seen him since I was tiny. I need to find out what’s going on. Please don’t let them wipe me!”

Sally and Sir Cedric exchanged a look. Because of his helmet, it was hard to tell what the knight was thinking, but Sally’s eyes widened.

Oscar was worried too. If his memory got wiped, he’d forget all this amazing new stuff.

Worse, he’d never meet his dad.