At the time I met Roshina, we didn’t even have a body. Ellia Kazbadi was missing, but we didn’t know if she was dead or had just defected. Either way it was serious.

I did not want a partner on the case.

“As you are fully aware, Ellia was working on new air at the time of her disappearance,” said the suited figure in front of me. “All endeavours in this area, whether scientific or criminal, are subject to cross-national co-operation.”

I looked at her through her face-plate. I already knew Roshina was a senior figure in the High Resenberg police; too senior for my liking, as if they already knew more than they were letting on. I saw what appeared to be a scar on her left cheek; her hair was dark, but I couldn’t see how long it was, and the helmet wasn’t clear enough to show any other features. I already wanted to rip it off her face and see how she coped with some real air. “There’s a treaty you intend to honour?”

“If you are referring to shift in the air border, this is clearly a natural phenomenon. The territory remains yours, even if only we can breathe in it.”

“That’s rubbish. Everyone knows you’ve been working on electrostatic manipulation. You’ve found a way of moving the air around--further into our territory.”

“Come, Inspector. Balaria has also been working on such a device. But that is not our concern. I have warned you of the dangers; perhaps we should return to the matter in hand.”

I wished I had a helmet myself. She would always have the advantage over me, hiding her face within a protective hood, whereas I had to display my every reaction. I tried to control myself.

“Ellia Kazbadi,” I said. “Thirty-four years old, single as far as we’re aware. Senior lecturer in Comparative Biology at the University of Balaria; with a nice sideline in Biochemistry. She made her name improving emergency procedures for people caught in Resenberg air; dozens of people owe their lives to her. Then two years ago she shifted her focus to new air.”

“At which point we appointed a High Resenberg advisor to work with her,” said Roshina.

“As you say, such investigations must be cross-national. Two days ago her advisor, Gravia Dolinda, turned up to work to discover all Ellia’s work destroyed, and Ellia herself missing. She alerted us immediately, but we’ve found no trace of her. In Balaria.”

“You have my assurance that she has not sought asylum in High Resenberg, nor do we know of her presence in our country, legal or otherwise. It may be that your security services intervened, if she was close to going public with her research.”

I wished I could read her face. I had no idea how far she was putting on an act. I suspected she knew exactly where Ellia had gone. “More likely yours,” I said. “Your own scientist is unharmed, after all.”

“I think we should speak to this Gravia first,” she said, and I could not think of a reason to disagree.

*     *     *
Gravia too wore a breathing suit, which made it difficult for me to tell the two women apart. When we arrived, she was packing various mementoes into a cardboard box.

“You’re leaving us?” I said.

“The project is over. Ellia may be alive, but with her work destroyed, the university has closed the project.” She spoke with a thicker Resenberg accent than Roshina, with shortened vowels and without diphthongs.

“You were working with Ellia on new air.”

“Supposedly.”

“You don’t believe in the concept?”

Gravia put her hand to her helmet. It was the gesture of someone who did not live in a suit full time; she had expected to straighten her hair or scratch an itch, and had not expected to find the face-plate in the way. “All our research suggests it’s impossible. The respiration systems of Resenbergians and Balarians depend on two different gases. Simply mixing the two together can keep a subject alive for about half an hour, but after that the impurities in the air, necessary for the survival of one race, become a fatal toxin to the other. This was thought of as insurmountable, until Ellia discovered the secondary respiration system.”

I momentarily concentrated on my own breathing. As far as I could tell, I had a single set of lungs, taking in a single air. I could not conceive of the idea that I might have a dormant system in reserve.

“It is redundant, like the appendix, yes? But Ellia believed that the system could be switched back in the presence of the correct mix of gases. And the system was very similar in both races. If the gases could be identified, Resenbergians and Balarians could breathe the same air.”

I breathed out sharply, aware once again that I was displaying my feelings far more than the other two. Roshina asked, “Did you ever feel any pressure to stop your work?”

Gravia shook her head; her helmet moved far less than she intended, but the gesture was still clear. “The university gave permission for the project. My government gave permission for me to join. If anyone wanted to dissuade us from the research, they could have done so more subtly and at an earlier stage.”

“What do you think happened to her?” I asked.

“I think she is in High Resenberg.” I could only guess what expression had passed on Roshina’s face. “She was a friend of my country, Inspector. I am sure she loved her own, but she would often tell me she had the wrong lungs. We frown upon such talk here, but I think your ways are more lax. I think she simply walked out of her old life.”

“And how near were you to completing your work?”

She gave a small laugh, an expulsion of Resenberg air that stayed within her suit, to be recycled to something she could breathe. “The new air. It is, how shall I say? A bullshit.”

We thanked her for her time.

*     *     *
Later that day I found out for myself how the border was moving.

Sometimes I kept away from it, taking a slightly longer route home. But today I wanted to walk along the edge. I wanted to watch where the air met.

The two airs did not mix, and specific environmental features kept them apart; local features caused an updraft and a temperature differential that had proved highly robust in forming an atmospheric border. There were always small variations, but it was around the southern edge of the agora. Resenbergian air was slightly darker than Balarian; I could usually tell it by sight if I was paying attention. It also had a slightly different scent; the Resenbergians called it sweeter, but I thought it smelt of compost. I also had the sense to carry a detector with me, and besides, you could tell if you were breathing the wrong air.

But the first sign I had that something was wrong was the crowd. Usually the agora was filled; now people had gathered in the northern half, staring at the air as if it were a rival football team. Some shouted anti-Resenberg insults; I noticed that the few suited figures in the agora stood apart from the Balarians.

There were always small variations where the two air systems met; sensitive detectors displayed the exact position of the air border. But this was more than the effect of the currents. This was a deliberate encroachment of our territory.

Ignoring the alarm that went off on my tablet, I walked into the thicker air. I could smell the rot as it entered my nose and lungs; there was no doubt that I was breathing the wrong air. Nonetheless, this was my country. I felt that I had to walk to the border, the real border, just to assert the boundary that was in force.

I walked further in, holding my breath against the foul odour. I reached down for my inhaler, ready for an emergency blast of Balaria. I fixed my lips around it and puffed.

Nothing. My inhaler was empty. I turned round, looked at the crowd. No one had come any nearer to me. Involuntarily I found myself breathing in. My lungs were already beginning to protest; this wasn’t air, this wouldn’t sustain me. I knew I had to get out of the agora but found myself collapsing to my knees. The toxins were already making their way into my system.

Suddenly I felt hands under my armpits, and a strong suited figure pulled me back to my feet. I tried to walk as she dragged me away, but I probably did more harm than good. “Idiot,” I heard her say, and knew it was Roshina.

She dropped me heavily into a chair in a café on the edge of the agora and went to buy us coffees and a blast of clean air. The barista looked with suspicion but took her money.

I felt recovered enough to speak. “You knew this was going to happen.”

“I knew you needed keeping an eye on. I told you, this is an atmospheric phenomenon. We are studying it closely.”

“I bet you are.” I inhaled deeply, then started on the coffee. I knew I should thank her, but I said nothing more.

“Let us leave the declarations of war to our governments. What did you think of what Gravia said?”

“I agree. The new air is nonsense.”

“I meant about Ellia.”

I collected my thoughts, sipping at the coffee again. Being near the border this café often had Resenbergian clientele, drinking through the filters in their helmets. There were none here now. “I agree about that as well. We’d know if she were in this country. I think we should look in High Resenberg.”

Roshina nodded. “I will make the arrangements.”

“What did Gravia mean about frowning upon talk of friendship?”

She put down her mug and closed her filter. “Have you wondered why you have been paired with a Superintendent Commissioner rather than a mere investigating officer?”

“Well, it had crossed my mind. In the past I’ve worked with an Inspector Devan.”

“Mr Devan made some poor career choices whilst in your country. We may frown upon friendship, but in matters of security we frown very hard indeed.”

“Oh. Where does that leave you?”

“Do not imagine for one second that I am being friendly.”

I didn’t.

*     *     *
This time I was better prepared when I entered Resenbergian air. We crossed through an official border station, where the air was properly measured and I did not find myself poisoned through my own breathing. For a few minutes, Roshina and I were both in suits. I had to put mine on before it was safe for her to take hers off. Only outside the building, after a technician had taken the air readings, did she remove her helmet, shaking her head to allow her hair out. It was shoulder length, but she seemed to be older than I had realised. Her scar was deep, blemishing what might have been an attractive face, but I did not dare ask her how she had gotten it.

“Your helmet suits you better than mine suited me,” she said.

I looked at her through my face-plate. “I find it stifling. The sooner I’m out of it the better.”

Roshina turned up the corner of her mouth, a gesture I had not seen before. “It can be arranged.” She led me away from the border station. “We will collect your papers from the police station, then I will show you to your room. I take it you will want to visit the university tomorrow?”

I nodded, then realised that my helmet would not have moved so much as my head. “Yes. Ellia visited there often, didn’t she?”

“The arrangement between the two states was mutual. Although most of the work took place in your country, exchange visits here were part of the deal.”

“Did she have contacts with anyone else while she was here?”

“We didn’t spy on her.”

I didn’t believe this. “She didn’t meet with any unificationist groups?”

Roshina stopped and stared at me, ice in her eyes. “We do not have such groups. They are purely a product of your own decadence. We are proud of our national identity.”

“There must be someone.”

“Let me tell you something, Inspector. We do not want new air in High Resenberg. The air border is our only defence against you. You have the wealth, and the ambition. We know that if the new air were released and you could cross the border at will, we would quickly become merely another of your subject states.”

“You’ve built your own empire.”

“Only as a defence against yours. As a Balarian, Ellia already had many enemies. As a researcher into new air, our whole country was her enemy. We offered her protection, but we did not guarantee her safety. Nor do I guarantee yours.”

We got into a marked police car. As we drove off, I was highly aware that a small crowd of people were staring at me in silence. Roshina made no move to disperse them.

*     *     *
The mood at the university was unpleasant. The university was near the border, a foolish move that its founders had hoped would promote understanding between our two nations. I saw a picket on the furthest edge of its playing fields; protestors had converged on the grass, many wearing breathing suits, the others hanging well back.

The air border had moved again, into High Resenberg, annexing the outskirts of the campus.

“It’s fortunate that you haven’t poisoned the air in the buildings themselves,” said Roshina as she surveyed the scene, apparently looking for danger.

“I thought you said it was an atmospheric phenomenon?”

“And you said it wasn’t.”

“Well. Now you know how it feels.” In fact I was glad that we had fought back. I had suspected that if High Resenberg had developed electrostatic manipulation, we would have the same technology. And ours seemed to be stronger. Yesterday Resenbergians had danced in our agora; now they could not breathe in their own university. If we had to push the border back for our own security, so be it.

“You should not spend much time here,” said Roshina.

“I’ll be out of here as soon as I’ve spoken to Gravia again.”

“I meant the country.”

Gravia had returned to High Resenberg soon after our interview. She retained her place at the university and had made a smooth transition to her old post. She seemed surprised to see me. “It is not safe for you here.”

“Are you sure it is safe for you?”

“I am working on a way to protect ourselves from poison air. Society approves.”

I wondered if Gravia did not believe in new air through knowledge, or as a political stance. “Where did Ellia stay when she visited here?”

“With me. Balarian visitors need escorts. You should be aware of that.”

“She had no contacts with any extremist groups?”

She shook her head, a little too much. “I never let her out of my sight.”

I could read her gestures better now. She was hiding something. “I want to see all your work on the subject. All your notes, correspondence, results.”

Roshina was good, but I was aware that she stiffened, that there was the tiniest intake of breath. Gravia said, “Ellia’s work was destroyed.”

“But yours wasn’t. This was a cross-national project. You’ll have your own copy here.”

I noticed Roshina give a slight nod. Gravia opened the drawer on her desk and took out two small storage sticks, which she handed to me.

“That’s all?”

“That’s a lot.”

“Thank you.” I turned to Roshina. “Perhaps you could escort me to my room?”

“Be careful, Inspector,” she said. “My protection may not be good enough. These are ugly times.”

I bowed to Gravia, and I made sure that Roshina went ahead of me.

*     *     *
As I studied the sticks in my hotel room, one thing slowly became obvious about the project. Ellia was the junior partner.

She was bright, and clearly capable. She had designed most of the experiments and done most of the actual work, but it had been Gravia who had proposed the directions of the research, made the hypotheses, and suggested which results were promising and which were dead ends. For someone who did not believe in new air, she seemed very keen to discover it.

That discovery had not been in sight. Ellia had not been removed for stumbling upon the secret; the research was no nearer a result than when it had started. I could not understand why it had been put to an end so suddenly, either by herself or by persons unknown.

I was interrupted by a knock at the door. I went to answer it and found the hotel manager, standing in front of two large men. From their aprons I assumed them to be kitchen staff. I felt immediately threatened.

“It is with some regret that I must ask you to leave our premises,” said the manager. He was a small man, his face hidden behind a black beard. I saw no trace of regret on his features.

“I’m booked here for a week,” I said.

“You will receive a full refund.”

“But why? I’ve not broken any rules.”

“I fear that I cannot guarantee the security of my property so long as you are a guest.”

The larger of the men behind him began to move forwards.

“I’ll pack,” I said.

*     *     *
I was not sure where to go. I did not want to call Roshina, for fear of making myself look as if I was completely unresourceful. But I also doubted that any other hotel would take on Balarian guests. This was a cosmopolitan area, near the border, and the hotel had been chosen because of its friendliness to our people. If the mood had changed, it had changed everywhere.

Fortunately I had travelled light and carried only a single bag. I wandered for a bit and made my way into one of the parks, where I hoped I could keep out of the way while I decided what to do next.

I saw a figure on a bench ahead of me, slumped over. I hoped it was a drunk, and that he would cause me no problems. As I approached I saw that he was wearing a breathing suit, and I was relieved that there was another Balarian here.

As I was about to call a greeting I saw that the face-plate was smashed, and the suit had been torn open.

It was impossible to assist while I was in a suit myself. There was too great a risk that I would cut my own suit on the broken glass, and mouth-to-mouth resuscitation was out of the question. However, I soon convinced myself that I was far too late. He had been murdered hours ago and left here, to be ignored by the local population. Belatedly I realised that this was just as well; if his attackers were still around, I had made myself their next target.

I rang Roshina on her private number. “I’ve found a body,” I said. “A Balarian body. In Antika Park.”

“The authorities have been alerted,” she said. Her voice over the airways sounded as if it had come from inside a suit.

“You are the authorities.”

“Your authorities. The body is one of yours; your own police will have to come and collect it. When they dare.”

I felt cold inside. “Roshina, are we at war?”

“We have never been at peace. I have arranged for your visa to expire tomorrow. This is for your protection, do you understand? I would prefer you to leave our country intact.”

“I see.” I knew that I was no longer safe. But I also knew that a young woman was missing, and it was my job to find the truth. “I’d like to speak to Gravia once more before I leave. Apart from anything else, I need to return the sticks.”

“Okay. I grant you that.”

“Where should I go in the meantime?”

“Out of sight.”

She hung up, and I looked for a corner of the park in which I could hide.

*     *     *
I did not sleep, and I was also very aware of being unwashed when I met Roshina at the university the next day.

The students were still protesting. “We will not be able to control them for long,” she said.

“Perhaps the air border will move again.”

“It might be best if you told me what was on your mind, Inspector.”

We walked across from the grounds of the car park. “Gravia was the senior partner in the research,” I said. “She told Ellia what to do. But every time, it was a dead end. She wasn’t there to help, was she? She had no intention of inventing new air. She was there to stop Ellia from doing so.”

Roshina shrugged. “Perhaps that is for the best.”

“But she couldn’t have done it. She’d have no idea what would work and what didn’t—unless she already knew.” I looked directly at her. “You’ve already got new air, haven’t you?”

She stared back at me, as if she was trying to find my eyes through the face-plate. “This is ridiculous. If we had invented it, why would we not use it? Why keep it a secret?”

“That’s the problem.” I resumed walking. “Unless—” Everything suddenly became clear to me. “We’ve got it as well. Good God, it’s the ultimate deterrent. If either side uses it, the border dissolves, and the nations unite whether we like it or not. It’s like the nuclear button, but the other way round. Instead of mutually assured destruction, we have mutually assured peace. And both sides have such a vested interest in the conflict that—”

I did not see the blow coming. It didn’t hurt, of course; didn’t even touch me. But in less than a second, she had smashed open my face-plate.

I turned to her, about to cry out. She held a knife in her hand, and expertly slit open my suit.

“Some secrets are not for discovering,” she said. “I tried to warn you.” I sat down, wanting to conserve my breath. From my side she removed the emergency inhaler; I had no spare air.

“Please,” I said.

“Your authorities will be informed.”

I could only watch her walk off as the poison air entered my lungs. I choked immediately and tried to hold my breath. I had had no warning, had not been able to hyperventilate or even breathe in before my air supply was cut off. I had only a minute or two.

The students no longer seemed to be watching. I knew none of them would try to rescue me.

I still had my phone, but there was no one I could ring. No one from Balaria could get here in time to save me; no one from High Resenberg would try.

I climbed to my hands and knees, still trying to think of a plan. Again I found myself unable to stop breathing in; again my lungs screamed at the toxins I inhaled. I felt my lungs tightening up, asthma as a defence against a mortal threat. But I had nowhere to go. I remembered Roshina saving me and knew she would not do so again.

But then there were arms beneath me. I felt myself placed in a fireman’s lift as a strong figure carried me towards the university buildings. I was too weak to resist, and I thought I was under water. I realised this was a hallucination, and that I was falling unconscious.

I woke to a hissing sound. Groggily I opened my eyes. I recognised the room; I recognised the figure of Gravia sitting opposite me, looking at me with no real concern. I realised that the hissing was coming through vents in the walls. Gravia checked some readings on her laptop, then nodded with satisfaction.

I took off my broken helmet and could only stare at her helplessly as we both breathed the same air. It smelt a little artificial, but I knew that it was as good for me as it was for her. We breathed new air.

It was a minute or so before she spoke.

“We grew fond, Ellia and I. But one of us always wore a suit. It was futile. Do you understand?”

I could only nod.

“She visited me here. I did not expect her. She said she had the wrong lungs. She wanted to kiss me, Inspector. That is all. She did not know about this room. I could not get her here quickly enough. If I could have breathed her air back into her lungs, I would have done. But she died in my arms.”

She was better at hiding her emotions than Roshina.

“I hid her body,” she continued. “You will not find it. I did not wish to be frowned on.”

I nodded again.

“I do not wish for your company, Inspector. I will find you a new suit. Then you must leave. Roshina will not want to leave a job half-finished.”

I still said nothing. I only continued breathing, more softly now. This was not Balarian air, but I breathed it nonetheless.

*     *     *
I returned to Balaria alone, always looking behind me, terrified of the knife slitting my suit again. But I returned, and now I can breathe again, without a helmet.

I reported Ellia as having died through misadventure. She had an estranged mother to be informed, but no further family. There was little grieving on this side of the border.

My superintendent accepted the report. “And there is no evidence that she was anywhere near discovering the secret of new air?”

I shook my head. “I’m inclined to agree with her co-researcher,” I said. “I don’t believe this air is possible.”

But I know otherwise. I know that our governments have stockpiled the air, could release it today, chase away the poison air and replace it with an air we can all breathe. No one need ever have the wrong lungs again.

For now, things are stable. The border returned to its usual place; clearly we are both using electrostatic manipulation, but neither side quite dares to provoke the other. The threat of the new air keeps us both in place.

But I have made a vow to Ellia.

I do not care what changes I have to make in my life and career. I do not care what risks I take.

No politician will ever use new air. Our warring governments will never release the gas that can bring about the peace we all dread. The rulers of Balaria and High Resenberg will never finally think the unthinkable, and press the button that forces our hateful nations to face each other, with no advantages and no defences.

I will.