“So!” said Snape, his long nostrils quivering. “Is this another treasured gift from Mr. Weasley? Or is it—something else? A letter, perhaps, written in invisible ink? Or—instructions to get into Hogsmeade without passing the dementors?”

Harry blinked. Snape’s eyes gleamed.

“Let me see, let me see . . . ,” he muttered, taking out his wand and smoothing the map out on his desk. “Reveal your secret!” he said, touching the wand to the parchment.

Nothing happened. Harry clenched his hands to stop them from shaking.

“Show yourself!” Snape said, tapping the map sharply.

It stayed blank. Harry was taking deep, calming breaths.

“Professor Severus Snape, master of this school, commands you to yield the information you conceal!” Snape said, hitting the map with his wand.

As though an invisible hand were writing upon it, words appeared on the smooth surface of the map.

“Mr. Moony presents his compliments to Professor Snape, and begs him to keep his abnormally large nose out of other people’s business.”

Snape froze. Harry stared, dumbstruck, at the message. But the map didn’t stop there. More writing was appearing beneath the first.

“Mr. Prongs agrees with Mr. Moony, and would like to add that Professor Snape is an ugly git.”

It would have been very funny if the situation hadn’t been so serious. And there was more . . .

“Mr. Padfoot would like to register his astonishment that an idiot like that ever became a professor.”

Harry closed his eyes in horror. When he’d opened them, the map had had its last word.

“Mr. Wormtail bids, Professor Snape good day, and advises him to wash his hair, the slimeball.”

Harry waited for the blow to fall.

“So . . . ,” said Snape softly. “We’ll see about this . . . ”

He strode across to his fire, seized a fistful of glittering powder from a jar on the fireplace, and threw it into the flames.

“Lupin!” Snape called into the fire. “I want a word!”

Utterly bewildered, Harry stared at the fire. A large shape had appeared in it, revolving very fast. Seconds later, Professor Lupin was clambering out of the fireplace, brushing ash off his shabby robes.

“You called, Severus?” said Lupin mildly.

“I certainly did,” said Snape, his face contorted with fury as he strode back to his desk. “I have just asked Potter to empty his pockets. He was carrying this.”

Snape pointed at the parchment, on which the words of Messrs. Moony, Wormtail, Padfoot, and Prongs were still shining. An odd, closed expression appeared on Lupin’s face.

“Well?” said Snape.

Lupin continued to stare at the map. Harry had the impression that Lupin was doing some very quick thinking.

“Well?” said Snape again. “This parchment is plainly full of Dark Magic. This is supposed to be your area of expertise, Lupin. Where do you imagine Potter got such a thing?”

Lupin looked up and, by the merest half-glance in Harry’s direction, warned him not to interrupt.