The Gryffindor common room was very noisy that evening. Harry, Ron, and Hermione sat together next to a window. Hermione was checking Harry and Ron's Charms homework for them. She would never let them copy ("How will you learn?"), but by asking her to read it through, they got the right answers anyway.
Harry felt restless. He wanted Quidditch Through the Ages back, to take his mind off his nerves about tomorrow. Why should he be afraid of Snape? Getting up, he told Ron and Hermione he was going to ask Snape if he could have it.
“Better you than me,” they said together, but Harry had an idea that Snape wouldn't refuse if there were other teachers listening.
He made his way down to the staffroom and knocked. There was no answer. He knocked again. Nothing.
Perhaps Snape had left the book in there? It was worth a try. He pushed the door ajar and peered inside - and a horrible scene met his eyes.
Snape and Filch were inside, alone. Snape was holding his robes above his knees. One of his legs was bloody and mangled. Filch was handing Snape bandages.
“Blasted thing,” Snape was saying. “How are you supposed to keep your eyes on all three heads at once?”
Harry tried to shut the door quietly, but —
“POTTER!”
Snape's face was twisted with fury as he dropped his robes quickly to hide his leg. Harry gulped.
“I just wondered if I could have my book back.”
“GET OUT! OUT!”
Harry left, before Snape could take any more points from Gryffindor. He sprinted back upstairs.
“Did you get it?” Ron asked as Harry joined them. “What's the matter?”
In a low whisper, Harry told them what he'd seen.
“You know what this means?” he finished breathlessly. “He tried to get past that three-headed dog at Halloween! That's where he was going when we saw him — he's after whatever it's guarding! And I'd bet my broomstick he let that troll in, to make a diversion!”
Hermione's eyes were wide.
“No — he wouldn't, she said. “I know he's not very nice, but he wouldn't try and steal something Dumbledore was keeping safe.”
“Honestly, Hermione, you think all teachers are saints or something,” snapped Ron. “I'm with Harry. I wouldn't put anything past Snape. But what's he after? What's that dog guarding?”
Harry went to bed with his head buzzing with the same question. Neville was snoring loudly, but Harry couldn't sleep. He tried to empty his mind — he needed to sleep, he had to, he had his first Quidditch match in a few hours - but the expression on Snape's face when Harry had seen his leg wasn't easy to forget.