When she saw it she snorted again. It was a code. A moronically simple alphabetical code. Seventeen and three. Q and C. Quentin Coldwater.

It was a signature of sorts, a watermark. A Coldwatermark. Professor Coldwater had set this seal, and when she saw that, she saw it all. He’d wanted a weak spot, a back door in case he needed to undo it later. His vanity signature was the flaw in the pattern. She extracted the little knife from Wharton’s pencil case and worked it into the crumbly mortar around one particular brick. She ran it all the way around the edge, then she knocked on the brick with her knuckles: shave and a haircut. Free and loose, it pushed out cleanly: clunk. Deprived of that one brick, and hence the integrity of its pattern, the rest of the wall gave up the ghost and fell apart.

Why would he have sealed it up? And why him in particular—everybody knew Coldwater was a wine lush. She could always ask him, he was standing about twenty yards away. Or she could get on with what she was here for. It was chilly in the passageway, much chillier than in the cozy Senior Common Room. The walls were unfinished boards over very old stone.

Dead reckoning, it was about one hundred yards from the Senior Common Room to the back of the wine closet, but she’d only gone half that distance before she came to a door, fortunately unlocked and unsealed. More passage, then another door. Like she was going through a series of airlocks. Weird. You could never tell what you were going to find in this place, even after living here for four and a half years.

The fifth door opened onto open air. That was very odd. It was a pretty little square courtyard that she’d never seen before, maybe twenty yards on a side. Mostly grass, with one tree, a pear tree, espaliered against a high stone wall. She’d always found espaliers a little creepy. It was like somebody had crucified the poor thing.

Also she was almost positive that there shouldn’t have been a moon tonight.

“Nutso,” Plum said quietly. She frowned at it. The moon looked back at her blankly, pretending not to care. She hurried across the courtyard to the next door.

It opened directly onto one of the upper floors of the library. That definitely wasn’t right; she was traversing some magically noncontiguous spaces here. The Brakebills library was arranged around the interior walls of a tower that narrowed toward the top, and this must have been one of the teensy tiny uppermost floors, which Plum had only ever glimpsed from far below, and which to be honest she’d always assumed were just there for show. She never thought there were any actual books up there.