Constance had imagined the grey substance would be thin and taut to the touch, but it was softer than that, putting up an almost fleshy resistance that made her stomach turn. She could feel it coating her fingers and collecting under her nails, as if she were trying to scrape apart a wall of heavy clay or mud. Beside her, Mildred was making small, disgusted noises, but kept doggedly tearing at the surface all the same, peeling away layers and layers of grey stuff with no result. After a few moments, she stopped, shaking her hands briskly in an attempt to clean some of the muck from them.

"I don't think we're getting anywhere, Miss," she said. "When you try to tear it apart this way, it just goes on forever. We need to come at it straight on, the way we did with the pencil and things." She jabbed her forefinger at the barrier, and it made a dent, sank in, and then breached the surface with the same soft pop Constance remembered from earlier in the evening.

"Oh!" Mildred's eyes went wide.

"What is it?"

"It's pulling," Mildred said in a strained voice. "It feels as if it's trying to suck my whole arm in."

"That's the vacuum." Constance hurriedly checked the knots she'd tied in the magical rope to be certain they were secure. "We've got to make the hole larger now you've broken through." She stretched farther over the windowsill and sank two of her own fingers in just above Mildred's, until she felt the suction catch hold of them and tug. It was impossible to tell what the atmosphere on the other side was like, except that it was chilly rather than hot. At least they weren't about to be burnt to a crisp by a roaring wall of flames, she thought.

"Help me pull it wider," she said to Mildred. "Like this, in opposite directions. And brace yourself."

With her combined strength and Mildred's, they wrested open a hole the size of a tennis ball, and as they did, the ever-present whispers surged, becoming louder and clearer at the same time, knitting the snatches of words and phrases together into complete sentences.

If you…

...come

...here...

Where?

In here. It's opening.

Do you think it's someone coming to help?

I don't know yet. Go and get her.

I can't… a voice began, but was cut off as Mildred lost her grip on the edges of the hole they'd made and it snapped shut again, turning the rest of whatever the voice had been going to say into a muffled murmur.

"Oh no," Mildred groaned. "I'm sorry—I'm sorry—it just slipped!"

"It's all right, Mildred." Constance leant her forehead against the stone surround of the window, trying to catch her breath. "We'll rest a moment and then give it another go. It may be easier now we know what to do."

"Okay." Mildred flexed her fingers as if they ached. "Did you recognise that voice? The one that said 'it's opening?'"

Constance nodded. "It was Miss Drill."

"That's what I thought," Mildred said. "And the voice answering her sounded like Ethel. Do you think they're really there on the other side?"

"I hope so," Constance said. A wash of giddiness swept over her, and she deliberately fixed her eyes on the grey barrier in an attempt to avoid hallucinating that bottomless chasm again, or something even worse. She could feel magic fizzing and sparking and itching in her fingertips, her body's automatic response to what it thought was an imminent threat. They needed to get to a safe place before it built up to the point where it began spilling out on its own; she hadn't experienced a Tesla discharge like that in a long time, but she hadn't been this agitated and off balance in a long time, either.

"Very well, Mildred," she said. "We'll try again now, and you mustn't let the voices distract you. Whatever happens, keep pulling as if someone's life depends on it, because it may."

"Yes, Miss Hardbroom." Mildred's frightened face reshaped itself into firm, adult lines, and for the first time, Constance saw a pure, crystalline glimpse of the witch that Mildred would be in just a few more years, sure of herself and confident in her powers. She'd seen many girls make that transition over the years, but had never expected Mildred Hubble to be one of them. Perhaps, she thought, she might yet be proven wrong. If so, she might even be glad to admit it.

"Now," she said to Mildred, and as one, they pierced the barrier again and wrenched it in opposing directions, expanding the gap past its previous size to the diameter of a dinner plate, then a cauldron's rim. Constance could see nothing clearly through the opening, only vague shapes, some hard and vertical, some just coloured blurs; it made her uneasy, but she kept pulling anyway, knowing she couldn't trust her own perceptions at the moment.

Come quick, said Miss Drill's voice on the other side, I think it's almost—

Constance! said another voice, and Constance knew this one too, knew it so well and had yearned to hear it for so long that the sound brought hot, stinging tears to her eyes. The hole in the barrier was wide and ragged now, with its edges fluttering limply as the last of the pressure equalised. She pushed Mildred through first and flung herself after, landing hard on an unyielding surface with a painful jolt. Her teeth came together in a sharp snap that left her tasting her own blood.

"But it's the castle," Mildred was saying, somewhere to her left. "We left the castle. How can we be back in it?"

"What?" Constance forced her eyes open and sat up, only to discover that Mildred was right; they were back in the Great Hall, but a Great Hall empty of its seething masses of cats, lit by a brilliant light that streamed in through the stained-glass windows and cast dancing kaleidoscope patterns on the floor.

"Constance!" that familiar voice said again, so close now that it could only be in the same room with them. Constance turned, still dazed, to greet the Headmistress, but instead of Amelia's untidy grey hair and bright blue eyes and impish smile, she saw a monstrous thing: a misshapen creature looming three times the height of a witch, with glistening skin the colour of a bruise and writhing tentacles where its limbs should have been; a Great Old One from a story; an abomination.

"Constance," the creature said in Amelia's voice, and its tentacles squirmed eagerly. "Oh, Constance, at last. I am so glad to see you."