There had been a time when Constance would have disbelieved Mildred's story on principle, and they would have spent the next quarter-hour touring the castle, checking every room, until she was forced to admit that Mildred hadn't been lying. After four years of dealing with one Mildred-related drama after another, she simply accepted it to save the effort. It would turn out to be true in the end anyway.

"Perhaps you should come in," she said.

Mildred recoiled as if Constance were the spider from the poem and she were the fly. "Into your room, Miss?"

"Yes, yes," Constance said impatiently. "Come along, don't dawdle."

Mildred stepped over the threshold with the expression of someone going to her own execution, and Constance shut the heavy iron-studded oak door behind her and locked it tight with a casual zap of magic. Inside, the whispering voices were muted, but she could hear them still, nagging away at the very edge of her awareness and putting her nerves on edge. She turned from the door to find Mildred gazing about the room in wonder, her eyes flickering from sofa to rug to dressing table to bed as if she had never seen such objects before in her life.

"What on earth are you gawping at, Mildred?"

"Nothing, Miss Hardbroom," Mildred said hastily. "It's only—you have such nice things."

"Well, what did you expect, a rack and an iron maiden? Never mind, don't answer," she said as Mildred opened her mouth, most likely to put her foot right in it. "Sit down and tell me everything you know. You can begin with when you last saw anyone other than me."

"Before lights-out." Mildred perched on the very edge of the sofa as if she thought she might need to leap up and flee at any moment. "Maud came into my room to talk for a few minutes when we'd finished getting ready for bed, and then we said goodnight and she left. I think I heard a few other people walking past after that, but I can't be certain. I wasn't really paying attention."

A sharp remark about how Mildred rarely was paying attention rose to Constance's lips, but she held it back and sat down in a chair facing the girl. "Very well. And at what time did you leave your room again, and where did you go?"

"About eleven, I think. I went to Maud's room to ask if she could hear the sound, and when she wasn't there I thought she might have gone to Enid or Ruby or Jadu, so I checked their rooms and they were empty, and so were all the other rooms on my floor. I was scared there had been some sort of emergency and no one had told me, so I tried to find Miss Drill and then Miss Crotchet and then Miss Cackle, and when they were all missing too, I came here."

"For goodness' sake, Mildred, did you not think I might need to know about it sooner? Why did you go to the rest of the staff before me?"

"I didn't want to bother you unless it was really important." Mildred hesitated. "You do get a bit cross about things, you know."

Constance had a sudden strong memory of sitting at a table in the dungeon two years ago, with the ink still drying on her letter of resignation, insisting to Mildred that she did not shout. It seemed they had made little progress since that day. Why did this girl, of all the hundreds she had taught, always throw her so off balance and make her think about things she would rather not? Why couldn't she be in this situation with Ethel Hallow, who was as ordinary and predictable (and, even Constance had to admit, rather dull), as she was talented at charms and spells?

"Never mind, it can't be helped now," she said, recovering. "So you spoke to Maud at nine and went looking for her again at eleven. I made my evening rounds at half past nine and saw no one in the corridors, but I did stop at Miss Cackle's room just after ten to speak to her. She asked me to stay and have a cup of hot chocolate, but I said no because I wanted an early night. By half past ten I was in my own room reading, and I neither saw nor heard anyone after that."

Mildred looked wistful at the mention of hot chocolate, which the girls were not allowed except on very special occasions, but she made an effort to follow along and put it all together. "So...whatever happened, it was sometime between half past ten, when you went to bed, and eleven, when I got up?"

"It would seem so," Constance said.

"But what could it have been?"

"I've no idea, Mildred. We shall have to see what we can find out. What is the correct magic to use when you need to find something that is lost?"

"A Restoring Spell," said Mildred promptly, and Constance nodded in approval. "But that's for things, not people, and only one at a time. Maybe scrying?"

"We can try," Constance said.

They tried both separately and together, but the glass, when asked where the missing teachers and pupils were, first darkened and then showed only a flat, featureless grey, without even the subtle variations of clouds or fog. Frustrated, Constance terminated the spell and set the mirror down with an angry thump, facing the wall.

"I'm sorry it didn't work," Mildred said tentatively as she watched Constance pace the room.

"You had the right idea for once, Mildred. It ought to have worked. I don't know why it didn't." She stopped abruptly on her third pass in front of the girl. "I don't suppose you've gone outside at all since you noticed people were missing?"

Mildred shook her head. "No, I was afraid whatever was making that whispering sound would be out there. It seemed safer in the castle, even with everyone gone."

"I wonder...Help me with these shutters." Constance was already at the casement and undoing the latch as she said it, and Mildred came too and dragged the left-hand shutter open—they were heavy, and tended to catch on the stone sill—while Constance took the right. With a tortured screeee sound from the hinges, the shutters parted, and both Constance and Mildred drew in a sharp breath of shock so perfectly synchronised they might have rehearsed it.

The evening had been cool and rainy, but instead of looking down into the wet courtyard, they found themselves staring at the same solid grey nothing they had seen in the mirror. It began at the edge of the outer sill and seemed to go on, flat and uniform, into infinity. Mildred put out an exploratory finger to touch it, and Constance seized her wrist to stop her.

"Not with your hand, Mildred! Go and fetch a pencil from my desk."

Mildred ran to do it and came back in a few seconds with a red pencil, which Constance took by one end and gingerly extended through the window. When it reached the beginning of the grey area, it met with a springy sort of resistance, as if she were prodding at an inflated balloon. She pushed a little harder, and all at once there was a pop—something more felt than heard—and the pencil was sucked away from her grip and vanished. For a split second, the whispering voices surged, swelling until she could almost make out what they were saying, and then faded again.

"Oh, crumbs," said Mildred.

"Exactly," said Constance.