Nobody spoke in the vehicle.

The driver, a boy of twenty with a sunburned neck and the rigid shoulders of someone who has been told to look at the road and nothing but the road, took them out of the market square, around the clock tower, and down Customs Lane at exactly the speed limit. Vosk sat in the front passenger seat with her unlit cigarette. She did not turn round. She spoke once, to the windscreen, to ask whether the water in the cooler had been changed, and the driver said yes ma'am, and that was the whole of it.

Sefa watched Tuzlag go by through glass. The sun had climbed. The sailcloth over the market had turned the colour of old bandages. On every second lamp post was a poster in blue and white: BRINEBANK: LIGHTEN YOUR LOAD. Somebody had drawn a moustache on the smiling woman in the picture. Somebody else had drawn a second moustache over the first.

The garrison stood beside the customs gate, behind a wall of sandbags that nobody had needed for eleven years. It was a low, square, sand-coloured compound with a flagpole in the middle, a motor pool at the back, and a canteen smell of boiled cabbage that reached the street. Above the gate the Orlei bell tower threw a short black shadow. Beyond it, across a strip of no-man's pan, stood the Concord's knotted-cord mast with its pennants limp.

They did not take her to a cell, or a room with no window. They took her up two flights of concrete stairs to a corner office with a fan on the ceiling, a kettle on the filing cabinet, and a wall-sized screen showing the live board of the Gatepass app: green dots for approved crossings, amber for pending, one red. The red one blinked in the lower left corner, over a name Sefa could not read from the door.

"Sit," said Vosk. "Tea?"

"I've had a lot of tea today."

"Then you'll know what you want." She put the kettle on regardless, and lit the cigarette from the ring of the little gas stove, and then, as if remembering something, put it out in a saucer. "Sorry. I'm trying to stop. I've been trying to stop since the river went. It makes me very patient with other people's habits. Do you mind if I open a drawer?"

"Is that a joke?"

"It's a courtesy." She opened a drawer.

It was the bottom one of her steel desk, and she drew out of it a plain brown envelope, soft with handling. From the envelope she took a single sheet of grey customs paper, folded in three. She unfolded it and laid it flat on the desk between them, on the green blotter, in the light of the window.

"What do you see?" said Vosk.

Sefa looked.

"Nothing," she said. "It's blank."

"That's what I see." Vosk sat back. "I have looked at that sheet every Friday for two years. I have held it up to the light. I have had the lab in Karabel run it for chemistry, which took six months and a lot of forms. I have had a man in a white coat tell me, very politely, that there is no ink on it, there never was, and there is nothing physically on the page except cellulose and a faint trace of salt." She tipped her chin. "A conscript named Tomas Rhee walked out of this garrison on it. Two summers ago. Past the gate. Past my sergeant, who read it, and saluted, and told the sentry to wave him through, and thought he had seen a perfectly valid discharge signed by me. I have never signed a discharge for Rhee. I would have shot him first, and I like Rhee."

Sefa said nothing.

"He came back once. For a funeral. I asked him how he'd done it. And he said, with a perfectly straight face, that he'd had it written by the girl at the print shop on Quay Road. He said she was very good. He said she charged by the page." Vosk's heavy-lidded eyes did not move. "I've kept that paper because it is the only piece of evidence I have that something is wrong on my border, and the only piece I cannot read. I do not like being the only blind person in a room."

"You've known for two years."

"I've suspected. Teodor Ilves-Nakamura confirmed it this morning, in the way he confirms things, which is by trying not to." She poured the tea. "I should arrest you. I'd like that on the record. Forgery of a military instrument is a hanging offence in the capital. Out here, it's a long time in a very small room. But I have eleven years' worth of reasons for not doing the paperwork."

"What reasons?"

"My border has a river that doesn't exist and a treaty written for a river that did. Everyone I can trust is under thirty or over sixty. My budget is cut every spring. If I put the one forger in Tuzlag in a cell, then every man, woman and goat in this town who has ever needed a permit that wouldn't come will walk past my gate and spit." She pushed a cup across. "I'd rather you were an asset than an example."

"That's a very kind way to say blackmail."

"I'm not kind. I'm tired. Drink your tea."

"What happened to Rhee?" Sefa asked. She was surprised to find she cared.

"He went home. He has a goat farm outside Karabel and a wife who writes to me every Christmas to tell me what a good man I am. I've never had the heart to correct her." A flicker of something like humour. "He was nineteen. His mother was dying and the leave form was in a drawer in the capital, under a stack of other drawers. I'd have signed it myself if I'd had the authority. So you see, Ms Tamarak, I'm not here to lecture. I've had two years of resenting you and being grateful, and it's been exhausting."

"I'm sorry."

"Don't be. Be careful." She tapped the blank page. "What did he pay you?"

"Forty coins and a jar of pickled lemons."

"That's an outrage. They were my sergeant's lemons."

Sefa drank. It was strong and sweet and tasted faintly of tin. Her hand shook a little on the cup, and she saw Vosk see it, and not say.

"The man on the Quay," said Vosk. "Achterberg."

"You know his name."

"His name came up on a Concord bulletin eleven days ago, an hour after he crossed my gate. Foreign Office bulletin, the sort they send us in four languages and a hundred pages. Person of interest. Do not detain. Report movements." She frowned. "A man you don't detain is a man somebody else wants to follow. I took it that he was a courier. A man with something to carry."

"I don't know what he was carrying."

"You did. You sponsored his entry." It was said flatly, without heat. "And I have your face on the counter-camera at Counter Two, with your hand on a sheet of paper that Teodor does not remember seeing. I looked at the footage this morning. You wrote something on the counter, Ms Tamarak. In front of the camera. In front of the whole queue. And not one person in that room remembers it."

Sefa's tea went cold in her hands.

She did not remember it either. That was the horror of it. She had written, in broad daylight, in the plainest possible way, and paid for it, and the price had been a day she could no longer find.

"I didn't know that," she said.

"I think that's the first completely honest thing you've said since you sat down."

Outside, a truck revved. Somewhere below them, a whistle blew for the changeover of guard. The fan turned. On the wall, the red dot on the Gatepass board pulsed, once, twice, like a heart.

"One more thing," said Vosk. "The tip. Four fifty-one. BARGE NINE DEAD MAN." She took the cigarette out of the saucer, looked at it, and put it back. "We couldn't trace the number. We traced the mast. It came through the guild's tower, but the handset was a Brinebank clinic handset. Registered to a nurse. Odile Fenn. Thirty-four years old. Works the evening clinic on the ground floor of the Hall."

"Have you asked her?"

"She's on leave. Has been for ten days. Her flat is locked. Her mother says she's gone to stay with a cousin in Karabel." Vosk's mouth was a thin straight line. "Her mother is lying, but I haven't decided how much. I thought you might like to know there's a woman out there who knew about your dead man before you did."

Sefa put down her cup. Her fingers had gone numb to the wrist.

"Why are you telling me?"

"Because it's the only trade I have." Vosk leaned across the desk. For the first time there was something in her face that was not exhaustion: a flat bright flicker of fear, quickly put away. "I want you to understand where we are. There's a woman in Tuzlag who can write a thing true. There is a guild that is buying people's bad days with their rent. There is a dead man in a chapel with a Concord flag on his file. And at four this afternoon, in the Orlei Rest House, there is going to arrive a woman I would very much rather not meet."

"Who?"

"Dame Lwazi Mthembu. Appraiser of Contracts, Tamarind Concord. Sixty years old, laughs at everything, has never lost a negotiation, and has requested a meeting with a person who, as far as the Foreign Office is aware, does not exist." She watched Sefa's face. "She asked for you by name."

"I've never met her."

"No," said Vosk. "But somebody has told her all about you."

"Who?"

"That," said Vosk, "is what I was hoping you would tell me."

She stood up and went to the window, and for a long moment she stood with her back to the room and her hands behind her, looking out at the strip of white pan between the two flags, where the heat was beginning to make the air wobble. Sefa could see the line of her shoulders under the field jacket. They were square and stiff and very tired.

"I'm going to release you," Vosk said. "You're free to walk out that door, and I'll give you a driver if you like. But I'll tell you what I tell my recruits on their first night. Every border is a story two people agree to believe. The minute one of them stops, you've got a war. And I think someone in this town has just stopped."