The lift to the Strata took ninety seconds, and Nadia Quispe-Halloran spent eighty of them watching Gus Hadley-Mbeki sweat through a rented suit.

"It's holding," she said, without looking up from her phone. "Stop checking your wrist."

"I can't feel it," said Gus. "Should I feel it?"

"You should feel like a man who is deeply in love with the woman standing next to him. Which, if you remember, is the whole transaction."

Femi Laurent, the woman standing next to him, laughed once, too loud. Her hands were wrapped around a paper cup of lift-station coffee she hadn't drunk. Both of them smelled, to Nadia's eye, exactly like what they were: two people who had paid four thousand dollars for a lie and were now, forty storeys above the river, discovering that a lie has to be carried.

The lift opened onto the Strata, and Tessarine fell away behind the glass. From here you could see all of it: the Floor down in the canyon's throat with its flood walls and its brown braid of river, the terraces stacking upward in sodium orange and laundry-line white, and far above, hard against the sky, the Rim, where the glass was so clean it looked like absence. Nadia had grown up at the bottom of that view. She still checked it on the way up the way other people checked a stove.

The Assay Office occupied the whole of a building called Marrow House, which was a joke only the Registry had ever been proud of. Inside, a waiting hall of pale ash wood and humming air filters. A wolf at the desk, a young man with the carefully blank face of someone trained not to react to anything he could smell, which was everything, took their appointment slip.

"Hadley-Mbeki and Laurent. Bond verification, first renewal." He glanced at Nadia. "And you are?"

"Witness." She slid her ID across. It said Nell Owens, which was not her name but was a very good piece of paper. "Humans co-sign, wolves test. Isn't that the rule?"

"It's a rule," he said, and stamped her slip.

That was one of the first things you learned in Tessarine, before your own phone number: the Assay was a nose, and a wolf nose could not be bribed. Anyone could swear to a bond. Anyone could post a ring photo, write a vow on a bridge. None of it counted until two people stood in a cold room and breathed into the same little glass chamber, and a trained Assayer decided whether their scents had braided into the third note: the shared one, the one that appeared only when two lives had truly grown into each other, by fate or by years. Without that note you were not bonded, and without a bond you were not eligible for a Strata lease, a Rim tax code, or a vote on the Tenancy Council. In a city with a housing list nine years long, the third note was the most valuable smell on earth.

Nadia had been selling it for two years.

They were called in at ten past two. The Assay room was small and white, a glass chamber at its centre the size of a bread bin, two stools, and an overhead vent that hissed like a held breath. The Assayer was a wiry older woman with a grey plait and a tablet. Behind her, in the corner, in a charcoal suit and a posture that suggested he'd been issued it, stood a man Nadia hadn't seen on the sheet.

"Spot audit," the Assayer said briskly, noticing her look. "The Registry sends someone for every third renewal. Don't mind Mr. Dastgheib."

Mr. Dastgheib inclined his head one centimetre. Mid-thirties. Dark hair cut by someone who respected it. The kind of stillness that wasn't calm, exactly, but discipline: she'd seen it on bomb-disposal officers and the better sort of landlord. His eyes moved across Gus, Femi, and then to Nadia, and stayed there a half second longer than the others.

She felt, as she always did, the small cold click of being looked at by someone who then found nothing.

It was the one thing she had never told anyone, not even Toby, who knew everything else: wolves could read people the way other people read a face. Nerves, appetite, grief, lies, all of it, every body leaking its news into the air. Nadia leaked nothing. Some fluke of skin chemistry she'd inherited from a father who had never explained it. To a wolf she was a room with the furniture removed. Mostly they took it for soap. She had spent twenty-nine years letting them.

Gus and Femi put their heads to the chamber. They breathed. The vent hissed. The Assayer leaned in, nostrils flaring, tablet forgotten in one hand.

Nadia kept her eyes on the floor and counted. Braid was a two-stage compound: the first dose set the base, the second, taken ninety minutes before the test, brought the note up. She had timed Gus's and Femi's by the minute. It would hold to the hour. It had always held to the hour.

"There," said the Assayer. A real smile, pleased. "Oh, that's a good one. That's a lovely one, that. You two been together long?"

"Eleven years," said Femi, who had been briefed on this. "Not that it feels like it."

"It never does."

Nadia let her breath out through her nose, very slowly, like a woman at a bus stop. Done. The couple's lease on a one-bed in Glassrow would clear by Friday. It was a cheap, ugly flat. She had seen the listing. It had a window.

"Verified," the Assayer said, tapping the tablet. "Congratulations to you both."

And Mr. Dastgheib said, "May I?"

He had crossed the room without a sound. He bent, not to the chamber but to the stool Femi had just left, and breathed in. A long, quiet, professional inhale. Then he held it, the way a man holds the last of a sentence he doesn't like.

"Mr. Dastgheib?" said the Assayer.

"The note is clean," he said. "It's exceptionally clean." He straightened. His face hadn't changed. "I would like a second sample from each of them, please. Outside the chamber. And a sample of anything either of them has ingested in the past twelve hours."

Femi's cup of coffee, still full, was sitting right on the ledge. Gus made a very small noise.

Nadia did the thing she did best, which was to look as bored as the situation permitted.

"Is there a problem?" she said. "They've been verified."

"There's no problem," said Mr. Dastgheib. "A clean result is the opposite of a problem." He turned to her for the first time directly, and his voice was mild, almost courteous. "It's only that in six years, I have never smelled a braid so tidy. Real bonds are messy. They carry things. Old arguments, a bad winter, an ex. This one carries nothing but the note itself. It's as if someone wrote it."

"Maybe they're just very happy," said Nadia.

"Maybe." He tilted his head. "You're their witness."

"Nell Owens. Friend of the family."

"Mm." He looked at her, and she watched him do what wolves did, a small instinctive lift of the chin, a brief drawing in of air, the reading. She watched him find nothing. She watched the nothing register: a flicker between his brows, there and gone.

"You use no scent at all," he said.

"Allergy."

"To what?"

"Everything," said Nadia pleasantly. "It's a very dull life."

Behind her, Gus was being walked out by the Assayer toward a separate room with a polite hand at his elbow. Femi followed. Nadia heard the tail end of Femi's voice saying something bright and desperate about a dentist appointment, and then the door closed and she was alone in a white room with a man who couldn't smell her and was, with great courtesy, wondering why.

"I'll need to take a statement from you as well, Ms. Owens."

"I've a lift to catch."

"Then I'll walk you to it."

It was not a request. They went out through the ash-wood hall together, past the young clerk with the blank face, past a family of five holding numbered slips like raffle tickets, and into the corridor by the lifts. Neither spoke. A cleaning drone hummed past. Her phone buzzed in her pocket. Toby, almost certainly. Toby had a gift for texting at the exact wrong moment. She did not look.

At the lift doors, Dastgheib pressed the button and said, to the doors, "I should explain why I was assigned this audit."

"You don't have to."

"For eight months, forged bonds have been passing the Assay in this city. Each with the same tidy note. The Registry calls the forger the Chandler. We have had no name, no face, no lab." The doors opened. He did not step in. "We do have a signature. I would know it anywhere."

She kept her face precisely as it was. It was the best face she owned.

"Good for you," she said.

"Last night, I pulled my own bond file from the archive. Three years, registered to Liesel Ostergaard-Banda. Verified at three renewals. Considered, by everyone who has ever looked, a gift of fate." He turned his head and met her eyes. His were an odd, level brown, and absolutely without triumph. "It has the same signature. Someone forged my bond, Dr. Quispe-Halloran. I think it was you."

The lift stood open between them. Below, forty storeys down, the whole canyon waited.

She had given the clerk a false name. It was a very good piece of paper.

He had never needed the paper at all.