For a moment, nothing happened. In the quiet, Stella could hear the wind howling around the tower.

The Professor held the bottle in both hands. It glowed in the candlelight, making flickering silver reflections on the lenses of his spectacles.

Something dark moved, just out of sight. Stella darted a look behind, but saw nothing but wavering shadows.

With a sigh, like a wave breaking over a pebbly beach, a faint wisp of smoke emerged from the bottle, curled into the air and disappeared.

Stella felt the back of her neck prickling.

Another wisp of smoke snaked from the bottle. It seemed to feel its way into the air, coiling and twisting like a serpent. The bottle glowed more brightly, with an intense silver light.

The Professor drew in a breath between his teeth. His hands were shaking and his knuckles were white. More smoke began to emerge from the bottle, curling up into the air, thickening and swirling, forming vague shapes of faces and creatures and twisting figures.

Gert whispered, ‘Flipping heck.’

Shadow hissed. The kitten gripped onto Ben’s shoulder, her ears flat against her head and her tail as spiky as a bottle brush.

There came another sigh, like a gust of wind. Stella felt the air move. The pages of the book on the workbench fluttered. Smoke poured from the bottle and twisted up into the air. It was difficult to believe so much smoke could come from such a tiny bottle. It formed a thickening column in the air above their heads. Faces formed and opened their mouths wide. Fish swam through tendrils of weed. Strange creatures flickered and writhed and disappeared.

Stella’s hair blew across her face. She pushed it back. A hunched figure was forming in the smoke. It twisted its head from side to side. The sighing grew louder and became a roar, like that of crashing waves.

The Professor cried out and dropped the bottle onto the workbench. His fingers were white and blistered. The bottle was covered with ice crystals.

Cold wind whipped around the room. Papers flew up into the air. The candles sputtered and the fire flared wildly.

Stella spied the cork from the bottle, lying on the floor under the workbench. She looked at the Professor, but he did not see her; he was staring up at the smoke.

She crept over behind him, her eyes on his back. She bent quickly and snatched up the cork. She unfolded the cuff of her coat sleeve and let the thick fabric fall over her hand.

The creature in the smoke seemed to be struggling to free itself. Its enormous head and long neck writhed and twisted.

The Professor’s face was alight with hope. ‘Sorcerer. Hear me,’ he said. The figure lifted its head. Its face was still half-formed, smoky and changing. It opened its wide mouth and roared.

Stella darted around the Professor and reached for the bottle, the sleeve of her coat protecting her hand from the freezing cold. At the same time, Gert flung herself at the Professor and grasped his arm. He struggled with Gert, threw her aside with a curse and seized Stella, but his injured hands were weak and she twisted away from him and reached again for the bottle. He pushed her roughly away, turned the ring on his finger and shouted, ‘Boy! Keep them back!’

Ben grabbed Stella’s arm and dragged her away from the workbench. His face was set and pale.

‘Let me go,’ she gasped.

He took a breath, with obvious effort, and whispered, ‘I can’t.’ He tried to say more, but no words came. He shook his head. His eyes were on the ring on the Professor’s finger.

Then Stella understood. The Professor turned the ring on his finger each time he gave Ben an order. The ring with the dark stone. She pulled free from Ben’s grasp, flung herself onto the Professor and clutched his arm. He cursed again and struck her hard on the side of her head, but she clung onto his wrist and managed to wrench the ring from his finger.

‘No!’ he shouted.

She threw the ring onto the floor and stamped on it with the heavy nailed heel of her boot. It made a satisfactory crunch. The stone shattered and a few dark red drops stained the stone floor.

Ben gave a gasp and staggered. He looked bewildered. ‘It’s gone.’ He put his hands to his head. ‘It’s gone,’ he said again.

‘Hold her back, boy!’ shouted the Professor.

Ben stepped back and stood up straight. He took a deep breath and said, ‘No.’

The Professor spoke, but his words were lost as the smoke figure roared again. They looked up. It was enormous. A tall, twisting column reaching almost to the ceiling. The face glared down at them from the shadows, its eyes like gas lamps, its mouth a gaping cave of teeth.

‘Sorcerer. Hear me,’ said the Professor.

The figure twisted its head from side to side and howled again.

‘Sorcerer. It is I, Aloysius Starke.’

The Professor’s voice was drowned in the rushing wind. Glass jars fell from the shelves and shattered on the floor. The candles flickered and went out.

Stella saw another chance, snatched up the cork again and darted forward. The Professor lunged at her and dragged her away. She struggled free from him and seized the bottle. The billowing smoke filled her mouth and eyes. She coughed. She could not see. Even through the thick fabric of her coat sleeve, the bottle froze her hand. She gritted her teeth and tried to force the cork back in. The Professor grabbed her. He clasped her around the chest, pinning her arms, and lifted her off the ground. She struggled and kicked. But he was too strong. He started to prise her fingers from the bottle.

Ben and Gert flung themselves at the Professor, making him stagger. His grip loosened. Stella wriggled free. She darted around the workbench and ducked down into a narrow space behind a tall cabinet. She crouched there, hidden, gasping, struggling to push the cork into the bottle. It was difficult; her hands were frozen and her eyes were full of smoke.

She heard scuffles and thumps. Gert cried out. Something fell and shattered.

Stella gritted her teeth, ignored the freezing ache seeping into her fingers and forced the cork into the bottle.

From the darkness above came a deafening howl. The door broke open with a crash, and the room was full of rain and wind and the sound of waves.

‘No!’ screamed the Professor, his voice cracking with fury. He spun around.

Stella shrank back into the shadow, desperately hoping he would not see her. She felt her head swim, as if she were fading and dissolving into the darkness. The Professor lurched towards her. She froze. His gaze seemed to focus beyond her. As if he could not see her. As if she were not there at all.

He reached for her, his fingers clutching blindly at the air. His groping hand met her neck. She felt his fingers pass through her, scrabbling against the wall, clasping only air.

He recoiled and gasped. His wide eyes stared right at her, but somehow he did not see her. She felt as if the room were spinning and she were fading away. He reached out towards her again.

Ben thumped into him. The Professor fell and struck his head on the workbench.

Ben grabbed Stella’s arm and pulled her to her feet. He had a canvas bag under his arm and he was tucking Shadow into the neck of his coat.

‘We got to get away,’ he said.

‘But —’ She felt dizzy.

‘Come on,’ said Gert.

Ben pulled her towards the door.

The smoke serpent howled again.

The Professor lurched to his feet, arms outstretched, fingers clutching.

Stella ducked under his grasping hands.

‘Come back,’ he groaned. ‘Give it to me.’

Stella pushed the silver bottle into her coat pocket and buttoned the flap as she turned and ran.

Outside, the icy wind and rain knocked the breath out of her. It was almost dawn. The sky was a leaden grey with streaks of yellow. The causeway stretched away towards the shore, gleaming in the murky light.

Gert was very pale and shaking with cold. Ben pulled off his coat and helped her into it. She winced as she pushed her injured arm into the sleeve.

‘We got to run,’ said Ben, looking out at the causeway. He held Shadow safe under his shirt with one hand, the other clutched his canvas bag under his arm. ‘There ain’t much time. Tide’s turning.’

‘Can you do it?’ Stella put her arm around Gert.

Gert bit her lip, but nodded resolutely.

They scrambled down the steps and started out along the causeway. The flagstones were broken and uneven, draped with seaweed and spiked with clusters of mussel shells. Deep, dark water swirled at either side. Stella’s boots slid on the wet flagstones. The rain plastered her hair to her face. The wind buffeted them. She and Gert clung together. It was difficult to keep upright.

A huge white-capped wave broke across the causeway. Water swirled around their ankles. Stella tripped and almost fell. Ben slung his bag over his shoulder and took her hand. They splashed through the icy water, slipping and stumbling.

Ahead, through the spray, Stella could see the shore, a dark blur on the horizon. Gulls sailed past, crying. The air was full of flying spray. Salt burned her throat. She coughed, choked and gasped for air as she ran.

Another wave broke over the causeway. The swirling water reached their knees. They waded through, struggling to keep their balance. The water pulled at Stella’s legs. Her boots felt as heavy as stones, and her wet clothes clung to her, weighing her down. They ran on.

Stella stopped for a moment, panting, and looked back towards the tower. She felt her insides lurch. The Professor was coming. He was on horseback, riding quickly, galloping along the causeway, a dark shape crouched low over the horse’s neck. Spray flew up from the horse’s hooves.

Ben looked back. ‘Run,’ he gasped.