Professor Dumbledore sent all the Gryffindors back to the Great Hall, where they were joined ten minutes later by the students from Hufflepuff, Ravenclaw, and Slytherin, who all looked extremely confused.

"The teachers and I need to conduct a thorough search of the castle," Professor Dumbledore told them as Professors McGonagall and Flitwick closed all doors into the hall. "I'm afraid that, for your own safety, you will have to spend the night here. I want the prefects to stand guard over the entrances to the hall and I am leaving the Head Boy and Girl in charge. Any disturbance should be reported to me immediately," he added to Percy, who was looking immensely proud and important. "Send word with one of the ghosts."

Professor Dumbledore paused, about to leave the hall, and said, "Oh, yes, you'll be needing..."

One casual wave of his wand and the long tables flew to the edges of the hall and stood themselves against the walls; another wave, and the floor was covered with hundreds of squashy purple sleeping bags.

"Sleep well," said Professor Dumbledore, closing the door behind him.

The hall immediately began to buzz excitedly; the Gryffindors were telling the rest of the school what had just happened.

"Everyone into their sleeping bags!" shouted Percy. "Come on, now, no more talking! Lights out in ten minutes!"

"C'mon," Ron said to Dudley, Dean, Neville and Hermione; they seized some sleeping bags and dragged them into a corner.

"Do you think Black's still in the castle?" Hermione whispered anxiously.

"Nah, if he's smart, he'd be long gone," Dean said.

"Dumbledore thinks he might be," Ron said.

"Why would he try to break in to our dorm?" Dudley asked. "I thought he was meant to be wanting revenge on Dumbledore for beating his boss."

"Yeah, that is weird," Ron said.

"Maybe he wanted McGonagall too?" Neville suggested.

Hermione looked thoughtful.

"What?" Ron asked. "I know that look. You have an idea."

"Well, what if Black and Mr. Malfoy have joined up," she said. "Maybe Mr. Malfoy wants Black to get revenge on Dudley too?"

"Me?" Dudley blinked.

"Come off it," Ron said. "Ok, I get that Malfoy would want revenge on Dud, but why would Black do what he says? Black was You-Know-Who's right hand man. If anything he'd be giving the orders."

"Maybe there's something hidden in Gryffindor Tower that Black wants?" Neville chimed in.

"Like what?" Dean asked.

"I dunno," Neville shrugged. "I can't see why else he'd go there."

"Maybe he just wants to kill Gryffindors?" Dudley suggested. "Black's a Slytherin, right? He's bound to hate Gryffindors."

"Not necessarily," said Hermione.

"Most Death Eaters are Slytherin, but that doesn't mean they all are,"

Ron snorted, disbelievingly. "Corm on, Hermione. The whole house is rotten to the core. Even Snape used to be one, remember?"

"Hey, Hermione—didn't you find out that the Malfoys and Blacks were related last year?" Dean asked, suddenly.

"Ohh, yes, I'd forgotten!" Hermione said. "Lucius Malfoy married Narcissa Black …"

"Who is related to Sirius Black," Ron finished. "Siblings weren't they?"

"Cousins," Dean corrected. "I think Narcissa's sister was someone called Bellatrix?"

At the mention of the name Bellatrix, Neville had let out a small growl of anger.

"What's wrong?" Dudley asked, noticing.

"Oh, er, nothing," Neville asked. All eyes were on him and he blushed. "Gran mentioned her, that's all. She was a Death Eater—one of the worst."

"There you have it," Dudley said. "The Blacks and Malfoys are related. They must be working together on something."

"But why attack Gryffindor Tower?" said Dean.

"The lights are going out now!" Percy shouted, halting all conversations. "I want everyone in their sleeping bags and no more talking!"

The candles all went out at once. The only light now came from the silvery ghosts, who were drifting about talking seriously to the prefects, and the enchanted ceiling, which, like the sky outside, was scattered with stars. What with that, and the whispering that still filled the hall, Dudley felt as though he were sleeping outdoors in a light wind.

Once every hour, a teacher would reappear in the Hall to check that everything was quiet. Around three in the morning, when many students had finally fallen asleep, Professor Dumbledore came in. Dudley watched him looking around for Percy, who had been prowling between the sleeping bags, telling people off for talking. Percy was only a short way away from Dudley and his friends who quickly pretended to be asleep as Dumbledore's footsteps drew nearer.

"Any sign of him, Professor?" asked Percy in a whisper.

"No. All well here?"

"Everything under control, sir."

"Good. There's no point moving them all now. I've found a temporary guardian for the Gryffindor portrait hole. You'll be able to move them back in tomorrow."

"And the Fat Lady, sir?"

"Hiding in a map of Argyllshire on the second floor. Apparently she refused to let Black in without the password, so he attacked. She's still very distressed, but once she's calmed down, I'll have Mr Filch restore her."

Dudley heard the door of the hall creak open again, and more footsteps.

"Headmaster?" It was Snape. Dudley kept quite still, listening hard. "The whole of the third floor has been searched. He's not there. And Filch has done the dungeons; nothing there either."

"What about the Astronomy tower? Professor Trelawney's room? The Owlery?"

"All searched..."

"Very well, Severus. I didn't really expect Black to linger."

"Have you any theory as to how he got in, Professor?" asked Snape.

Dudley raised his head very slightly off his arms to free his other ear.

"Many, Severus, each of them as unlikely as the next."

Dudley opened his eyes a fraction and squinted up to where they stood; Dumbledore's back was to him, but he could see Percy's face, rapt with attention, and Snape's profile, which looked angry.

"You remember the conversation we had, Headmaster, just before - ah - the start of term?" said Snape, who was barely opening his lips, as though trying to block Percy out of the conversation.

"I do, Severus," said Dumbledore, and there was something like warning in his voice.

"It seems - almost impossible - that Black could have entered the school without inside help. I did express my concerns when you appointed -"

"I do not believe a single person inside this castle would have helped Black enter it," said Dumbledore, and his tone made it so clear that the subject was closed that Snape didn't reply. "I must go down to the Dementors," said Dumbledore. "I said I would inform them when our search was complete."

"Didn't they want to help, sir?" said Percy.

"Oh yes," said Dumbledore coldly. "But I'm afraid no Dementor will cross the threshold of this castle while I am Headmaster."

Percy looked slightly abashed. Dumbledore left the hall, walking quickly and quietly. Snape stood for a moment, watching the headmaster with an expression of deep resentment on his face; then he too left.

"Inside help …" Dudley whispered. "I've just had a thought—"

"No talking!" Percy hissed.

Dudley jumped. He didn't know that Percy had been so close.

"Remind me about my thought tomorrow," he whispered to Dean.