“I said we don’t usually have children down here,” said the librarian sternly. “I didn’t say you couldn’t come in. All epiphanized people are welcome down here. We only have three rules. No chewing gum. No talking. And if you die, it’s your own fault. Furthermore, there’s no catalog, so don’t come here asking me where a particular book is. If you don’t already know where the books you want are, then I can’t help you. All right? Good. Now I just need your names.”

Wolf knew he had to hurry. He had to enter this building by midday. But how did you enter a building with no door?

He walked around the perimeter again. This time he looked more carefully for anything that could be a door. Of course, it wasn’t until he got to the third side of the rectangle that he found it. Just a faint outline in the wall. Only visible in exactly the right sort of light. Wolf touched it and an old-fashioned digital keypad came up on a touchscreen. It had numbers from one to nine laid out like on old phones. Each number had a blue circle around it. Clearly Wolf was going to have to put in an entry code to get the door to open.

But the only code Wolf had was in letters. MMXIV. Had he missed something? Should he go back? Then something flashed into his mind. Something from a book on Napoleon. A date. It had been written with an M at the beginning like this, but it was longer. The M actually meant one thousand and then there were lots of other letters that spelled eighteen hundred and something.

Of course. The letters were not letters at all, but numerals. Roman numerals. He’d skimmed through a book on the subject in his early days in Leonard Levar’s shop. One M was a thousand. Two Ms were two thousand. X was ten, and . . . Wolf worked out that together these letters stood for the number 2014. That had to be the entry code.

Wolf tapped it into the keypad.

Nothing happened.

He looked at the keypad again. There were only two other buttons on it. One had a big C and the other had a big hashtag. Wolf tried pressing the C. His numbers disappeared from the display. So C obviously stood for “clear.” What was the hashtag for? Wolf tapped in the numbers 2014 again and then tried pressing the hashtag.

The metallic door slowly opened. It slid sideways, like doors used to do on TV programs featuring spaceships. But Wolf wasn’t sure this wasn’t a spaceship. Though he had no idea what it was. He knew that walking through a door like this was probably a mistake. No great military leader would ever do it. Napoleon wouldn’t. Sun Tzu wouldn’t. But presumably neither of them had ever lost their ten-year-old sister.