s Caw and Lydia climbed down the drainpipe, Caw caught a glimpse through the living-room window of Mr Strickham slumped on the sofa, staring vacantly ahead.

“Maybe we should talk to your mum?” Caw said as his feet hit the ground, but he already knew what the answer would be.

“Please don’t, Caw. It’ll only set them off again. Besides, this is just a fact-finding mission. It won’t be dangerous.”

Caw instinctively scoured the garden for any foxes, glad not to find any watching them. Maybe Velma Strickham really had banished her foxes from her home. Caw felt a bit guilty about keeping things from her. But he’d done plenty for Mrs Strickham already, letting her invite everyone into his house. She didn’t need to know his every move.

“Let me get us a ride,” said Caw. He looked towards the sky and clenched his fists, ready to call the crows.

“Don’t,” said Lydia, touching his arm. “If there are flies about, they might see us.”

“Then how will we get there?” asked Caw. “The asylum is right on the edge of Blackstone.”

“The number sixty-two bus!” said Lydia. “It might be public but it’s under the radar.”

I’m not travelling on a bus, said Shimmer. It’s undignified.

“You three can meet us there,” said Caw. “Just keep low and out of sight.”

Ooh, a secret mission! said Screech, hopping along the top of the Strickhams’ fence. Exciting!

You’d better be careful, said Glum. They’ll probably want to keep you there indefinitely.

Very funny, said Screech.

“Will you quit it?” said Caw.

Lydia grinned. “Hey, it’s good to see you guys again,” she said.

It’s hard to fly in a straitjacket. Just saying, added Glum.

Keep up, old-timer, said Screech.

Unsurprisingly, Caw and Lydia were the only people on the bus. The driver didn’t even seem to notice them as they got on board. Caw found it strange to feel the soft rumble of an engine beneath him, and it reassured him to watch the crows keep pace outside. He’d only travelled by car or bus a handful of times in his life and it was a relief to step off when they reached their stop. The doors snapped closed and the bus pulled away into the night. Caw watched its red rear lights vanish over a hill.

They were right on the outskirts of Blackstone, where the city’s residential suburbs gave way to scattered industrial buildings, factories and farms. Caw had been here only once before, when he was much younger, exploring with his crows. The bus depot was half a mile up the road, according to Lydia, and there was no other traffic and no pavement.

The Blackstone Psychiatric Hospital sign was painted on a rotting wooden panel set just back from the road. The building itself looked more like a spooky old mansion than a hospital, perched on raised ground, its turrets and towers piercing the sky.

Looks homely, said Shimmer. I like what they’ve done with the bars on the windows.

“It’s one of the oldest buildings in the city,” said Lydia with a glimmer in her eye. “It was built in the early 1700s.”

Caw nodded mutely. The psychiatric hospital wasn’t all ancient though. There were a couple of ugly extensions on either side – plain, windowless, single-storey bunkers – sprawling across the grounds. Spotlights cast eerie pale arcs of light through the deep shadows. There was a mesh fence, about three metres high, and beyond that a wall. Caw shivered. If you were a patient here, you were obviously a prisoner too.

There was a large front gate with a guardhouse next to it. Inside, Caw could see a man in uniform reading a magazine with his feet up.

“What now?” said Lydia. “I have a feeling visiting hours are over.”

Caw looked sideways at his friend. “I have a better plan.”

“Disable the fence?” asked Lydia, rubbing her hands together.

Caw shook his head. He’d already begun to summon the crows as soon as he stepped off the bus. Now they started to arrive, a wave of dark shapes flitting overhead, joining Screech, Glum and Shimmer. Caw guessed they could sense the electric current humming through the second fence, because they landed only on the first.

The guard looked up briefly then went back to reading.

“Flies or not,” Caw said, “it’s the best way.”

Lydia held out her arms. “Come on then, give me a lift.”

The crows landed across their shoulders and lifted Caw and Lydia off the ground.

His friend was grinning madly. “I love this bit!” she said.

Caw directed his birds to swoop over the gates and the hospital itself. From above, they could see that the asylum was built round two central courtyards. Too out in the open. The crows carried them across the steeply pitched roof, and then he spotted something more promising. Caw steered them towards a flat section scattered with bulky chimney flues. The crows set them down lightly then gathered on the rooftop. There were no security lights or cameras up here.

A light breeze gusted through his clothes as Caw picked his way between the chimney stacks. The turrets were huge up close.

“I don’t fancy squeezing down a chimney,” whispered Lydia.

Caw stopped by a metal hatch in the rooftop, with a simple looped handle. It looked newer than the rest of the building, and a couple of modern air vents had been fixed beside it.

“Hopefully we won’t have to,” he said.

He reached down and tugged on the loop. It opened half a centimetre, but then snagged. Caw pulled harder but it didn’t budge.

“It’s locked from the inside,” he said.

“Oh well, good try,” said Lydia. “I guess we go through the courtyard.”

Caw peered down over the edge. There were more security spotlights mounted on the walls, but it looked like they were switched off.

“Shimmer, do a sweep for any guards,” he said.

The crow took off, diving down over the edge of the roof. As she approached ground level, several of the lights blinked on, casting the courtyard in silvery light. Caw heard an electronic hum and saw cameras rotating to focus on the empty space. Shimmer banked and flapped skywards again, rejoining them just as a guard wandered into the courtyard. Caw and Lydia ducked out of sight with their backs against the chimney flues. The crows were dark silhouettes, unmoving.

“I don’t think that’s an option then,” said Lydia.

Caw chewed his lip. There was no way they could break open the roof hatch without some serious metal-cutting equipment. And Lydia was right about the chimneys – they were too small.

Too small for a human anyway …

Caw scrambled to his feet. He edged back to the vents beside the hatch. They were about a foot across.

“A crow could fit down there,” he said.

“Good thinking!” said Lydia.

Caw turned to Screech, who looked away as if he was suddenly very interested in something in the distance.

“Screech,” Caw said, “can I borrow you for a moment?”

The crow plodded over. Why me?

“You’re the smallest,” said Caw. “Glum wouldn’t fit.”

Yes, I would! said the old crow indignantly.

Shimmer sniggered. Too many French fries. Hey – I’ll do it! She hopped excitedly from side to side.

But Screech shook his wings. Go on then. Bring me back in one piece, OK?

“Of course,” said Caw. He closed his eyes, and concentrated on his mental image of Screech. He felt his spirit detach from his body as it searched for the young crow. For a moment he floated on nothingness, then the crow body drew him in with a sort of feral gravity.

As Caw felt his talons touch the ground, he opened his eyes and found himself perched among the other crows. Several regarded him with curiosity, as if they sensed a different aura about him. He saw his human body lying motionless beside Lydia, eyes rolled back in his head. He took a few steps, getting used to the new configuration of limbs. He opened his beak – Screech’s beak – and squawked.

In truth, he’d chosen Screech because he was the easiest crow to control. Caw wasn’t sure why – perhaps because he was the youngest, or perhaps it was just because their connection was stronger – but Shimmer was definitely harder, and Glum almost impossible.

“Are you in there?” said Lydia, crouching to look into his eyes.

Caw bobbed his head up and down in answer.

“That’s so cool,” said Lydia.

Caw flexed his wings and hopped up to peer over the edge of the vent. Below lay a black abyss – even with his enhanced crow vision – curving downwards.

Good luck, said Glum.

I’m coming with you, said Shimmer. Her talons rattled on the steel alongside him.

Caw stepped forwards and felt his claws skid for purchase. He tumbled into darkness.

He flapped his wings in panic, but there wasn’t any room to extend them as he fell. He heard Shimmer cry out and felt her body buffeting against his. They crashed down on to more metal. Shimmer landed beside him in a tangle as dust filled his beak.

You OK? Shimmer asked.

I think so, replied Caw.

He turned in the gloom, and saw a dim light to his left. He skittered down the shaft towards it. Three slats crossed the opening, but by flattening his wings he popped through to the other side. Caw was in a narrow stairwell with bare walls of patchy plaster. He guessed it was there to give access to the roof, for repairs. Shimmer flapped through too, scattering a loose feather. She was covered in dust and cobwebs.

Up there, said Caw.

At the top of the steps, a vertical ladder led to the underside of the metal hatch they’d seen on the rooftop. On this side, a simple rusty bolt was drawn across. He and Shimmer flew up. Caw twisted his head and took the bolt in his beak. He strained his neck and managed to shift it a fraction.

Help out, will you? he said.

Shimmer joined him, fastening her beak on the bolt as well. Together they succeeded in moving it across.

Lydia, it’s open! Caw shouted, forgetting for a moment that he was talking in crow.

But his friend must have heard the sound of the bolt shifting. The hatch swung open from above, and she grinned down. “Nice work, guys!”

Caw flew out on to the roof, and landed next to his motionless human form. He concentrated hard on letting his aura split from Screech’s and wobbled slightly as he reassumed his normal body.

Screech blinked then pecked him on the ear lightly. You got me all dirty.

“Sorry,” said Caw. “And thank you.”

Lydia started to climb down the ladder into the service stairwell. Caw followed her, instructing all the crows to stay on the roof apart from Glum, Screech and Shimmer.

At the bottom they came to a plain metal door. Caw turned the handle slowly and opened it up a crack. The corridor on the other side took him by surprise.

The exterior of the hospital may have looked ancient, but the inside was definitely brand-new. The corridor was painted pristine white, with glass doors set into the sides every ten paces or so, each with a number above. Caw pushed open the door, checking both ways, and saw a security camera high up on the wall. It was scanning slowly across the corridor then reached its limit and began to turn back towards them. He quickly closed the door.

“There’s a camera,” he said to Lydia. “We’ll have to time it exactly right.”

Caw waited a few seconds, then peered out again. The camera was facing away. He beckoned to Lydia and they left the stairwell and crept along the corridor.

As they reached the first door – number thirty-four – he sucked in a breath. A skeletal man stood perfectly still behind it, dressed in a beige hospital gown, eyeing them with pale blue eyes. He didn’t flinch or blink, even when the crows came into view. Behind him was a simple room with a perfectly made bed. A table contained a tray with an empty, clean plate.

“This place gives me the creeps,” said Lydia.

The next door revealed a similar room, but in this one the bed was occupied by a tiny figure curled under the sheets. The food on the bedside table was untouched.

“Poor people,” said Caw.

He glanced back and saw that the camera would soon be on them.

The final cell was dimly lit, but Caw could make out someone moving inside. There was no noise at all – he guessed the glass was soundproofed. He hurried past.

The corridor approached a T-junction and Caw pulled Lydia into it, just as the camera swung round to face them. Lydia ducked, tugging him down too. She pointed to a glass-walled office just beyond them with a bank of monitors inside. A woman in uniform was eating a sandwich in front of the screens, with her back to Caw and Lydia. On the wall beside her were several plastic cards hanging from pegs.

“We need to find Cynthia Davenport’s room,” said Caw.

“And I bet those are the key cards,” said Lydia, pointing at the office.

Shimmer hopped on to his foot. My turn, she insisted.

It’s too dangerous, said Screech. That security guard will see you.

I’m not going in there, sparrow-brain! Shimmer said. I’ve got a better plan. She took off, flying low through the corridor.

“Where’s she gone?” whispered Lydia.

“I have no idea,” said Caw.

Sparrow-brain? Screech muttered.

Not a compliment, lad, chortled Glum. I once met a sparrow who was scared of rain.

One of the monitors in the office began to blink, and the footage became shaky. The security guard reached up to the bottom of the screen. She turned a dial, but nothing happened. The image flickered again.

The guard stood up, unclipped a holster at her belt, and walked towards the door, speaking into a walkie-talkie on her lapel.

“I’ve got a camera fault in D Block,” she said, then paused. “Nah, it’s probably just these old electrics. Looking into it.”

She left the office.

Good old Shimmer, Caw thought. “Back soon,” he said to the others. He tiptoed quickly into the office.

The key cards were numbered and named, and Caw scanned them as quickly as he could. His reading had come along a lot in the last few months, but he knew he wasn’t as fast as Lydia would be. Then he saw it: CYNTHIA DAVENPORT. NUMBER EIGHT. He grabbed the key card off the peg. Hopefully the guard wouldn’t notice it was missing when she returned.

Then he spotted a cable leading from the back of the monitors to a plug socket. It gave him an idea.

He took a half-drunk coffee mug and sloshed the contents over the plug. With a fizz and a pop, all of the screens blacked out at once. That should buy us a few more minutes.

By the time he got back to Lydia, Shimmer had returned.

“Got it!” said Caw, showing them the card. “We need to find cell eight.”

Expecting an alarm to sound or another guard to cross their path at any moment, they hurried along the corridors, turning corners until Caw lost his bearings. The numbers were counting down, and soon they reached number twelve, then eleven, then ten. Caw slowed his steps. These cells were empty.

Number nine was vacant too.

Caw felt his neck prickling. He reached instinctively for the Crow’s Beak under his jacket, and Lydia looked alarmed.

“What are you going to do?” she said.

“It’s just for protection,” Caw said. “If anything happens, run. OK?”

Lydia raised her eyebrows.

“I mean it,” said Caw.

The cell marked EIGHT was dark inside, but he could make out a woman hunched at the end of the bed. If she had seen them, she didn’t show it.

Caw lifted the key card towards the sensor then paused, his heart racing. The last time they had fought, the Mother of Flies had created a swarm of her creatures, a solid giant of insect flesh. She had almost crushed him in a fist made of flies. If this was a trap, then opening the door in front of him might be the last thing he ever did.

Lydia pressed a switch beside the sensor. Inside room eight, the light blinked on.

The figure on the bed shielded her eyes, shuffling backwards. Cynthia Davenport seemed to have aged ten years in the last two weeks. Her hair was grey and messy, her pasty skin covered in blotches. Her once cruel eyes gazed at her visitors with a vacant stare from hollow, dark sockets. It was hard to tell from her hunched posture, but Caw even thought she had shrunk a little. Dried white spittle lurked at the corners of her mouth, and her hospital tunic was covered in food stains. There was no sign of any flies.

Caw looked at Lydia, who stared back sadly.

The crow talker searched Cynthia Davenport’s vacant eyes, looking for some hint of the Mother of Flies.

“What now?” said Lydia.

Caw breathed deeply. He had to find out what had happened to Selina.

“Stay back,” he muttered to Lydia. Then he held the key card against the sensor. The red light flashed green and the door slid open.

The patient in room eight didn’t move.

“Hello?” said Caw, his heart beating so hard he thought he could hear it. “Commissioner Davenport? It’s me, Jack Carmichael.”

The patient blinked once. “Fly, fly away,” she muttered in a weird sing-song voice, her head jerking. “Fly to the sky for his webs reach high. Spiders can’t fly, but their spirits don’t die.”

Caw frowned. “We need to talk to you,” he said.

Cynthia Davenport continued mumbling as if he hadn’t spoken. “I spy his eight legs dancing nigh. You can try, you can try. But a crow can’t fly from his spider eye.”

Her lips barely moved, but the strange chant made Caw shudder. He had been wrong after all. There was no way this woman could have kidnapped her daughter.

He took a step closer, and she shrank away, pressing herself against the wall.

“Please, we won’t hurt you,” said Lydia, entering behind Caw. “We just want to know about Selina. She’s gone missing. Have you seen her?”

Cynthia’s head whipped round at the sound of her daughter’s name. Her eyes gleamed manically. “Oh, yes, she’s here,” she said. “She’s always here with me.”

Caw’s pulse quickened. He lifted the point of his sword. “What have you done with her?”

Cynthia reached out a hand in front of her, lightly pushing the point of the Crow’s Beak aside. Her nails were filthy, but she moved her fingers up and down as if she was stroking something tenderly.

“My girl is here,” she said. “My pretty girl. Come to Mummy, darling. Let’s go to the old well and draw the water together … Yes, we can pick flowers, sweetheart. All the flowers you want … No, no spiders there, petal. Just flowers and water so clear you can see the past and future in its depths.”

Lydia touched Caw’s shoulder. She shook her head.

Caw lowered the blade. And as Cynthia Davenport continued to stare at the empty space where she could see an imaginary girl, the tension left Caw’s body. In its place, a dull sadness throbbed. He wasn’t looking at an evil feral any longer, just a broken mother longing for the daughter she had lost.

Screech gave a sudden warning squawk.

Caw spun round, expecting to see a guard.

Then Lydia cried out in alarm as a hairy shape leapt through the air and hit Caw in the chest.

He felt something sharp scratching his forearm. “Ow!”

Caw dropped the Crow’s Beak, staggering in pain as a small pale-furred monkey leapt off his arm. It bared its needle-like teeth and Cynthia Davenport began to wail as the monkey hopped around the room. Three more appeared, their horrible shrieks filling the air.

The crows flapped wildly, but soon both Screech and Shimmer were held in monkey paws, trapped on the ground. Glum tried to stay airborne, but the remaining two primates knocked him down and held him to the floor.

Get off me, you stinking furball! said Screech.

Caw started towards the Crow’s Beak, but backed away sharply as a panther stalked into view in the doorway, followed by the hulking form of Lugmann.

A scrawny young woman with her hair in ratty strands stood behind him. The monkey that had bitten Caw leapt on to her shoulder, hissing.

The panther snarled and Lydia pressed up against Caw, her face pale.

Last of all, Mr Silk sauntered into cell eight. The moth feral’s lips twisted in a cold smile.

“What a pleasure to see y’all here,” he said.