They waited until midnight—Jack fidgeting with impatience the whole time—and then Ellini led him out of the window and onto the Faculty roof. She was barefoot, a fact that Jack was still struggling to get to grips with.

"You don't even run in shoes?" he asked her, as they climbed the sloping slates to the chimneystacks.

Ellini shook her head. "It's counterproductive. Bare feet are better for gripping, and, anyway, it's harder for the gargoyles to hear. Shoes on tiles and slates are extremely noisy."

"But what happens if you cut your foot on a loose nail or something?"

"It hurts." She gave him a look of playful concern, and added, "If that worries you, then by all means, go back inside."

"Hah," said Jack, with an expression that was half-smile and half-glare. "I'll be fine, thank you. Just worried about the lady."

She sat down by the chimney, with her knees drawn up to her chest, out of sight of the crowds that were already beginning to gather in the street below. There were placards this time—some urging her on, some calling her 'the Bride of Satan'. Nobody was scared, that was the strange thing. Despite the deaths of Sergeant Hawthorne and that apple-seller, nobody seemed to want these rooftop chases to end.

Jack was ridiculously excited. He felt like a child on Christmas morning, or Danvers at the beginning of the cricket season. He couldn't believe he'd been living quietly at the Faculty, taking pills and giving blood samples, for so long.

Ellini was now wearing the ragged black Charlotte-Grey-dress—which, if she had been any other woman, would have explained why his heart was racing.

If you stripped all the lace and ribbon off Alice Darwin's mourning dress, you'd have something a little like the Charlotte Grey dress. It was basically just a corset with a slim, simple skirt—except that the skirt had been slit up both sides to the level of her thighs, which was probably what had incensed the man with the 'Bride of Satan' placard. The black ribbons had been twisted haphazardly all the way to her upper arms. It looked as though she was wearing long black gloves that had been partially dissolved by acid.

She looked exquisitely vulnerable with her bare feet and bare shoulders, yet the Charlotte Grey costume seemed to give her confidence. Her steps were surer up here on the slates, her shoulders less slumped. The breeze was blowing about little strands of hair that had come loose around the nape of her neck, and it looked as though the tickling sensation was making her smile.

Oh god, he wanted to want her so much in that moment. Everything seemed more real up here. There was even some proper darkness in the shadows of the chimneystack, away from the lamplight. He felt as though he was breathing more deeply than he had done for years. And she was part of all this, maybe even the source—he just couldn't reach her.

Ellini fumbled in the cloth bag she'd brought with her and pulled out two thick leather straps. They looked like those ammunition belts worn over one shoulder by American gunslingers, except that, instead of bullets fixed every half-inch along the length of the strap, it was little glass phials.

"The ones with the red tops are sandalwood oil," she explained. "That's for when you need a distraction. They'll send the gargoyles off in whichever direction you throw them. Try not to use them when there are people nearby, because if someone in the crowd gets splashed with sandalwood, they'll be marked out as a target for the rest of the night. The ones with the blue tops are peppermint oil. It overwhelms their sense of smell—a bit like a blinding flash of light to someone who can see. Use the peppermint oil if you need to get away. It disorientates them, and they won't be able to follow your scent again for a good five minutes."

Jack watched her eyes while she said all this, a slow smile spreading across his face. "It's all business once you get up here, isn't it?"

Ellini frowned and prodded him in the chest. "Listen, or you'll die. This isn't a game."

"Maybe not, but it still sounds like fun." He pointed to the cloth bag, which was still bulging in a promising kind of way. "What else have you got in there?"

And he was so glad he'd asked, because the next item which emerged from the bag was hilariously out of character. It was a large, evil-looking grapple, with a long length of rope tied to the base. It was so solid, and so cruelly pointed, that anyone who had the slightest concern for Oxford architecture wouldn't have been able to see it up here without wincing.

Jack, on the other hand, burst out laughing. "That's a nasty-looking grapple for such a sweet little girl."

Ellini didn't rise to this. "Essential for moving between buildings when they're too far apart to jump. You can have the big one," she said, pressing it into his hands, and pulling another, slightly more genteel grapple from the recesses of her bag.

"So I take it this dastardly scheme of yours means more to you than the preservation of historic architecture?"

"More than anything," said Ellini.

"More than me?"

She gave him an exasperated smile. "Well, fortunately, I haven't had to choose."

This was an impressively evasive answer, and Jack respected it—mainly because he dreaded the real answer.

They were silent for a moment, listening to the crowd massing in Broad Street. They had taken up some kind of a chant. He couldn't make out the words, but it didn't sound promising.

"Do they ever put you off?" he asked.

Ellini shrugged her bare, bony shoulders. "I don't pay any attention once I start running."

"You know, I think half the problem is the dress..."

Ellini looked down at the black corset. "I didn't choose it. It's Charlotte Grey's. With the name goes the costume. Now, what else?" she muttered, brushing one of those loose hair-strands away from her eyes. "Oh, the gargoyles aren't very good with sudden changes of direction—I suppose because smell is a less immediate sense than sight. You don't have to be very quick to lose them. Just take sudden and unexpected routes. Watch me for an hour or so before giving it a try yourself."

"Yeah," said Jack slowly, "that's probably not going to happen, but thanks for the concern, all the same."

Ellini sighed and told him to hold out his wrists. He did so, fully expecting her to slap them, and was quite disappointed when she took a perfume bottle out of her bag, removed the glass stopper, and trailed it across his veins. He was acutely aware of the cold, wet glass against his skin.

She dabbed her own wrists, and then her neck. Clearly, if he wasn't going to hang back and watch for an hour, she was going to make damn sure that she was the more tempting target for the gargoyles.

They stood there looking at each other for a while until a low, guttural howl unrolled like thunder across the sky, silencing the outraged chanting in Broad Street.

"Good luck," she whispered, and started running.