Like all old castles, Cackle's Academy had its share of legends. There were a hundred chilling tales of wailing ghosts and gory murders that put Sir Walter and his Wet Week to shame, and that everyone felt duty-bound to repeat to the gullible first-years, just as they had been told themselves when they were new girls.

Schools being what they were, Cackle's was also host to a multitude of juicy rumours about its staff, and about one member of it in particular. If you had been a pupil there at any time in the last twenty years, you had heard, variously:

– That Miss Hardbroom only slept two hours a night, and they were always two different hours, so you could never be quite sure when it was safe to slip out of your room.

– That Miss Hardbroom had not slept since 1987, and then it had only been a ten-minute nap.

– That Miss Hardbroom never slept at all.

– That Miss Hardbroom was a vampire who was specially adapted not to sleep during the school term, but spent all the holidays unconscious in a coffin below the sub-dungeon, from which she rose fully dressed and coiffed just before the first returning pupil's broomstick made its way into the courtyard. (Some versions of this story held that if you failed your first-year exams, Miss Hardbroom would be allowed to drain all your blood to gain strength for her long summer hibernation, and the school would put it about that you had been sent home in disgrace.)

The truth, of course, was that Constance Hardbroom was as human as her gossipy young charges, and she did sleep "most nights," as she would have put it. Every now and then, after an especially trying day, she went so far as to retire before midnight, and this was one of those times. At half past eleven, she was tucked up in bed—not the stone slab or wooden casket that generations of Cackle's pupils had imagined, but a proper double bed with a blanket and pillow—and reading a book that Miss Drill had forced upon her some weeks ago. The descriptions of magic in it were quite inaccurate, and anyone with a brain could tell that the twitchy professor in the purple turban was up to no good, but she was halfway through and thought she ought at least to see how it ended. Perhaps then Miss Drill would stop pestering her about whether she had liked it or not.

As she was perusing the rules of Quidditch and thinking that it sounded a more interesting sport than any of the ones the girls at Cackle's were forced to do, she heard the patter of teenage footsteps in the corridor and huffed out a sigh. Of course they would have to get up to mischief on one of the rare nights when all she wanted was to read for a bit and go to sleep. She laid the book open and face-down on her chest, keeping her place, and waited to see which direction the steps would go in. If the perpetrators were already on their way back from whatever unauthorised midnight expedition they had been on, she might ignore it, just this once.

To her surprise, the steps went neither upstairs nor down. Instead, they shuffled to a halt outside her door and hesitated there, as if whoever owned them were uncertain of what to do next. With simmering irritation that was beginning to approach a rolling boil, Constance got out of bed, went to the door and jerked it open to reveal the familiar and infuriating shape of Mildred Hubble, one hand lifted as if she had been about to knock.

"Mildred Hubble, what are you doing out of bed at this hour? More importantly, what are you doing outside my door? Shouldn't you and your little gang be downstairs raiding the kitchen or creeping about the …" Constance trailed off as she registered Mildred's expression—not one of guilt at being caught out, but of desperate worry, verging on terror.

"Is there something I ought to know, Mildred? And if there is, tell me quickly, because I was in bed and I would like to return there as soon as possible."

"Listen, Miss Hardbroom. Do you hear it?"

"Hear what? Really, Mildred, this is not the time for riddles—"

"Just listen," Mildred said. Her voice was so tense and anxious that Constance bit back the rest of her reprimand and did as the girl had commanded.

"You do hear it, don't you?" Mildred's eyes were wide, her face pale between the dark curtains of hair that fell to either side. "Please say you do."

Constance nodded slowly. She wasn't quite certain what she was hearing, but she did hear something—a soft and but insistent chorus of whispers that rose and fell and occasionally burst out in a quick sharp exclamation. It was like standing in the Great Hall when the whole school was gathered for assembly, hearing the babble of a hundred voices all together, but not being able to pick out what any individual voice was saying. The sound seemed to come from everywhere, just low enough to escape notice if you weren't listening for it, but impossible to ignore once you were.

"I hear it," she said, and Mildred let out a breath she had clearly been holding. "What is it? How long has it been going on?"

"I'm not sure, Miss Hardbroom. I was still awake—with my candle out, of course," she added, although enforcing the lights-out rule was not top of mind for Constance at the moment. "I was just lying there in the dark, thinking, and then I began to hear it little by little, until it was like this. I thought it was my imagination at first."

"You are prone to flights of fancy, Mildred," Constance said dryly, "but it seems not this time."

"No, Miss. We have to do something, we..."

"We are not going to do anything, Mildred. You will go back to your room at once and stay there, and I will go and fetch Miss Cackle, and she and I will get to the bottom of it."

Mildred shook her head. "You can't, Miss."

"Don't presume to tell me what I can and can't do, girl. Now go back to your –"

"You can't," Mildred insisted. She drew herself up, frightened but defiant, and Constance realised that standing face to face in bare feet and slippers, she and Mildred were very nearly the same height. When had the girl grown so much, and how had she not noticed until now?

She planted her hands on her hips to create a more imposing silhouette.

"Why not?" she asked. "And before you answer, let me remind you that it had better be a very, very good reason."

"Because she's not here," Mildred said. "I already checked, and she's gone. And so are Miss Crotchet and Miss Drill. And –" Her voice quivered. "And so are all my friends. Maud and Enid and Ruby and everyone. They're all gone, Miss Hardbroom. There's no one in the school but you and me."