But Plum was lying. She understood. The ghost did want her. It had always wanted her. She was a Chatwin, and Chatwins lived on borrowed time. Plum wondered if it would hurt.

Bump. The sound came from the wall to her left—something had run into it from the other side. A shower of plaster fell. A man’s voice said something like “oof.”

Plum looked; the ghost in the mirror didn’t.

Boom! The wall exploded inward, throwing chunks of wood and plaster and stone in all directions, and Plum ducked, and a man came crashing through it covered with white dust. It was Professor Coldwater. He shook himself like a wet dog to get some of the dust off, though he still looked like he’d been hit with a sack of flour. White witchery sprayed out of both his hands like sparklers, so bright they made purple flares in her vision.

When he saw what was in the mirror he froze.

“Oh,” he said quietly. “Oh my God. It’s you.”

Plum didn’t think he was talking to her. Did he recognize this thing? It was almost like he knew it personally, which would be pretty weird even for him. He took a deep breath and pulled himself together.

“Don’t move till I tell you,” he said.

That was meant for Plum. She didn’t move, but she didn’t dare to believe that he could actually save her. All she’d done was drag him into the catastrophe too.

With an arm protecting his face Professor Coldwater reared back one of his long legs and kicked in the mirror. It took him three kicks—the first two times the glass just starred and sagged, but the third time his foot went right through it. It got a little stuck when he tried to pull it out. It was a measure of how shocked she was that Plum’s first thought was, I must tell Chelsea that she won’t have to pay for the mirror.

That didn’t dispel the ghost, but it was definitely inconvenienced. It was still watching them, suspended in midair, but now it had to peer around the edge of the hole. Professor Coldwater turned his back on it—the ghost threw something at them, Plum couldn’t see what, and he deflected it with one hand without looking. Then he placed his palms together.

“Get down,” he said. “All the way. On the floor.”

She got down. The air shimmered and rippled, and her hair crackled with so much static electricity it made her scalp hurt. The entire world was shot through with light. At the bottom of it was the dull bass pressure-beat of the door exploding outward out of its frame.

“Now get up and run,” Professor Coldwater said. “Run! Go ahead, I’m right behind you.”