"Interesting theory," Nadia said, and stepped into the lift.

She did it because standing in a corridor with a Registry auditor was worse than standing in a box with one. The doors closed. The car dropped. It had mirrored walls, which was a design choice she had always hated, because it let her watch her own face being calm.

Kaveh Dastgheib pressed the button for the Floor, not the street level, not the ground floor of Marrow House where a guilty woman might make for the exit. The Floor. He had known where she was going.

"Walk me through the theory," she said. "Since you've got one. I'm a doctor of something?"

"Biochemistry. University of Tessarine, Floor campus. Your thesis was on volatile organic compounds in human sweat. It was withdrawn in your fourth year, when your father's clinic lost its licence." He said it like a man reading a menu. "You were born at Saint Aldric's, which closed. You have a registered address above the laundromat on Cutbank Lane, with a second tenant named Tobenna Eze, whose Chirrup account has been posting pictures of the same laboratory sink for eleven months."

"That's a normal sink."

"It has a fume hood above it."

Nadia looked at the ceiling of the lift for a while. The numbers counted down. Thirty-one. Thirty.

"Toby's an idiot," she said, finally.

"Noted." He did not smile. She found, with some annoyance, that she could tell he was not smiling in a way that was deliberately fair. "I was careful not to say anything in front of the Assayer. I want you to understand that. The Hadley-Mbeki bond stands as verified. If I wanted you arrested, I would have asked for a room."

"Then what do you want?"

"I want you to explain how you made my bond."

"I didn't."

"The signature is yours."

"Auditor." She said it with all the patience of a person talking to a very expensive appliance. "If I were this Chandler, which I'm not, and I'd begun selling two years ago, which I haven't, then your three-year-old bond file couldn't have my formula in it. It would be impossible. So your file is wrong, or your nose is. Pick one."

The lift hummed.

"I didn't say when you started," said Kaveh.

She closed her mouth.

"I said the signature is yours. You've just told me two years." He looked, for a moment, almost apologetic. "You lie well. I have been trying to find the seam, and there isn't one. But you also use a great many numbers, and numbers are what you do when you're frightened."

"That's a huge assumption about my body language."

"I can't smell your body," he said. "It's all I have."

That landed somewhere under her ribs, a small cold weight, and she stood very still so that nothing would show on her. There was no wolf in this city who could have said that and not been lying, or hurt, or both. She would have heard it in his pulse if he'd had one she could read. Instead his voice came out flat and level, and she realised he was telling her something true and had no way to know it was a gift.

"Here is my position," Kaveh said, turning to face her. "My bond is forged. I don't yet know how, and I don't know whether the other party knows. If I report this, three things happen. My bond is voided and my record is audited. Liesel's Tenancy is investigated, and the Ostergaard seat on the Council is suspended pending review. And you are found by people who are much less polite than I am."

"That sounds like an excellent reason for you to report me."

"It's an excellent reason to understand first." The car slid past the Strata's last level. Beyond the glass the canyon wall rose in layers of rust and cream, studded with window-lights like a cliff of low stars. "I've worked in the Registry for six years. I have never falsified a form. I have never been late. I believed, every day of those six years, that the work mattered because the bond mattered, because the note could not be faked." He paused. "I need to know if I was wrong about the work or only about my life."

Nadia had a quick, vicious answer ready. She found it had evaporated.

"What's in it for me?" she said instead.

"Immunity from me. For now."

"That's a very small sandwich."

"It's a very large sandwich, Doctor, if you consider the alternative." He let the sentence sit a moment. "There is a second thing. The Registry isn't the only organisation that wants the Chandler. Three weeks ago an Assayer died in Glassrow. A woman who had been writing to the Director of the Assay Office about irregularities in a batch of filings. It was recorded as a fall."

The words took longer than they should have to arrive. Nadia heard the lift, the faint plasticky ticking of the floor indicator, the thud of her own heartbeat that no one on earth could hear.

"I don't kill people," she said.

"I didn't say you did."

"You said an Assayer died."

"I said she'd written to the Director. I said it was recorded as a fall." He held her gaze. "I'm telling you because someone is cleaning up the same trail I'm following, and I'd prefer you were in a position to help me before they reach you."

Twelve. Eleven.

"Prove it," she said. It came out harsher than she'd planned. "Your bond. Prove it's my signature, not some copycat. I'm not admitting anything. I'm saying if somebody wants me to look at a sample for, hypothetically, a professional opinion, then I'd need the original."

He reached into his jacket and brought out a small flat case, the size of a deck of cards, sealed with a Registry stamp and the date in blue ink. His thumb passed over the stamp, once.

"My Assay swab from the first verification. Three years old, cold-stored, chain of custody unbroken." He held it out. "I took it from the archive last night. I should not have done so. It is the first thing I have done in six years that I would be unable to explain."

She did not take it. "You're giving me evidence. In a lift."

"I'm giving you a chance to tell me I'm mistaken."

"And if you're not?"

"Then I'll need you to tell me what to do about it," said Kaveh, in the tone of a man who had discovered, at thirty-three, a sentence he'd never planned to say aloud.

The doors opened onto the Floor.

It hit her like a smell she could still enjoy: wet concrete and frying onions and river and the sweet chemical bloom of the laundromat vents. A tram bell. Someone arguing in two languages over the price of mangoes. The Floor was loud and low and brown and nobody here ever wondered what she smelled like, because nobody here had the leisure.

She took the case. Her fingers brushed his. His hand was warm, and slightly damp, and not quite steady.

"Don't follow me," she said.

"I'm going to follow you."

"I mean it."

"I know," said Kaveh. "I'm going to follow you anyway, at a respectful distance, because you are the only person who has ever lied to me without leaving a trace, and I find, regrettably, that I want to know how it's done."

She walked. He walked, thirty paces back, hands in his pockets, a man in a charcoal suit threading between fruit crates and wet cardboard on a street that had never seen charcoal. People noticed. A child stared. Old Mrs. Bakr outside the pharmacy actually crossed herself. Nadia would have found it funny if she hadn't been counting.

Her phone buzzed against her hip. Then again. Then four times in a row, the exact rhythm of Toby having a thought.

She stopped at the corner of Cutbank Lane, under the flickering neon sign of Suds & Co, and looked.

TOBY: ok so TOBY: dont panic TOBY: do NOT panic TOBY: theres a lady in the lab TOBY: she says shes from the landlady TOBY: she's looking at the fume hood TOBY: nadia she knows what a fume hood is

Nadia read the messages once. Somewhere behind her, thirty paces back, Kaveh Dastgheib stopped when she stopped, exactly, like a man on a leash held by someone else.

Imelda Reyes-Whitlock's Tenancy owned the building. It had been collecting rent on that lab for two years, in cash, with no questions. Imelda had never once climbed the stairs.

Nadia put the sealed case in her coat pocket, next to her keys, and went up.