Yona had three filters rated for outside air.

She was using one.

One was wet.

The last was still sealed.

There were thirty-six people in and around the abandoned isolation room.

While Elias counted them, Yona put the sealed filter back into her sample bag.

Daniel asked, “You’re not giving it to anyone?”

“To whom?”

“The child.”

“Which child?”

Daniel looked toward Rina’s culture tank.

“That tank can’t move on a filter,” Yona said. “It needs independent circulation.”

“There’s Ian too.”

“If Ian gets it, what about the other thirty-three?”

“Ever heard of children first?”

“Outside, lung condition and walking range matter before age.”

“Is that why you cut your brother’s tether?”

The room went quiet.

Yona did not look at him.

“Yes,” she said.

“Which is why I’ve spent longer than you thinking about whose filter gets taken away.”

Daniel had nothing more to say.

Sarah unfolded the wet filter and examined it. White spores and black mud coated the outer layer, but part of the inner carbon membrane was still dry.

“Can we wash and reuse it?”

“City water would reduce the outside microbes, but it would kill the symbiotic layer in the filter,” Yona said. “Rainwater would add more spores.”

“Heat?”

“Damages the protein membrane.”

“So it’s useless?”

“It could work for a short trip. If you decide who takes the risk.”

Elias searched the city inventory with the remote lock key.

OUTDOOR RESPIRATORY FILTERS: 412.LOCATION: SURFACE-EXPLORATION STORAGE / OUTER-WALL POSTS / EMERGENCY SHELTERS.

“Four hundred twelve is enough,” Daniel said.

Yona looked at the screen.

“How many of those can you reach right now?”

Mira found the nearest emergency shelter on the map. Three hundred seventy meters away, through five doors.

EMERGENCY SHELTER FIVE: RESPIRATORY FILTERS 48.

“There,” Mira said.

Noah checked the route.

“Movement restriction is active at Pressure Door Three.”

“Break it,” Daniel said.

“The legacy port is locked. Even the hinge cover cannot be opened.”

“So this whole city dies because of one door?”

“It isn’t one door,” Mira said. “It’s that every way of opening a door is tied to the same place.”

An air-allocation alarm appeared on the lock key.

SPORE CONCENTRATION RISING IN LOWER LIFE SYNTHESIS SECTOR.AEGIS ISOLATION VENTILATION INITIATED.

The ceiling vents closed.

Old mechanical shutters moved, and the sound of flowing air stopped.

Only the oxygen made by the white breath-stalks remained.

Oxygen concentration: 20.1 percent.

Not dangerous yet.

Carbon dioxide began to rise.

“With thirty-six people, we could see headaches and impaired judgment within two hours,” Sarah said.

“What if the breath-stalks make more oxygen?”

“We don’t know how much carbon dioxide they absorb.”

“When they’re wet, they take it in,” Yona said. “When they dry, they release it again.”

“You’re sure?”

“I measured it outside.”

“It may not be the same organism.”

“Correct.”

Mira enlarged the route to Emergency Shelter Five.

Pressure Door Three had both a modern electronic lock and an old mechanical lock. Aegis controlled the electronic command. The mechanical lock was frozen by the port shutdown.

“There’s a pressure-sensor bypass line beside the door,” Mira said.

Noah asked, “Like the airlock? Put air through it?”

“The opposite this time. If we make the sensor readings match, the electronic lock may release.”

“You intend to feed it false sensor values?” Elias asked.

“We’ll equalize the actual pressure on both sides too.”

“Aegis closed it for spore isolation.”

“If we don’t get filters, people here breathe even more spores.”

“Opening the door spreads them to another sector.”

“Then clear the corridor and close it again after we pass.”

“Have the people in that sector agreed to that?”

“Then ask them.”

Elias used the public channel to send a notice to the surrounding route.

TWO PERSONS FROM LOWER LIFE SYNTHESIS SECTOR WILL MOVE THROUGH THIS AREA TO RETRIEVE FILTERS FROM EMERGENCY SHELTER FIVE.RISK OF SPORE TRANSMISSION DURING PASSAGE.RESIDENTS OF ADJACENT SECTORS: BLOCK VENTS AND CLEAR CORRIDOR.

The response came immediately.

PASSAGE REFUSED: 61 PERCENT.PASSAGE ALLOWED: 28 PERCENT.NO RESPONSE: 11 PERCENT.

Daniel said, “Did anyone ask our permission to die?”

Elias lowered the screen.

“If we force passage, we choose the risk for other people.”

“If we don’t go,” Yona said, “we choose the risk for the people in here.”

“There is a difference.”

“Which side of the wall you happen to be standing on?”

Ian covered his ears.

At first Mira thought he simply did not want to hear the argument.

Then his nose began to bleed.

She went to him.

“You hear it again?”

“Behind the door.”

“What?”

“People.”

The wrist tags and ventilation systems of residents around Pressure Door Three were vibrating with the same warning cycle. Ian was not connected to the black tissue, but he could still feel the low-frequency signal traveling through the floor.

“I hear it even when I block my ears,” he said. “I didn’t connect to anything.”

“I know.”

“Then why?”

“Because the whole city is vibrating too loudly.”

Mira did not cover his ears with her hands.

“Want to move farther away?”

“Where?”

There was no quiet place in the isolation room.

“Come sit next to me,” Rina said.

Ian sat beside the culture tank. Rina did not put a hand against the glass. Ian did not either.

The two children stayed close without connecting.

Ian’s breathing slowed a little.

Yona watched them.

“A child I saw in City Three was like that too.”

“How?” Mira asked.

“Better farther from walls. Worse closer to the ground.”

“What was the child’s name?”

“Rao.”

“How old?”

“Said thirteen. The record said fifteen.”

“Why the difference?”

“Lost memory or lied. I didn’t ask.”

“Where is Rao now?”

“Inside City Three.”

“Is that why they told you not to bring us a Function Zero child?”

Yona did not answer.

Another air warning appeared on the lock key.

RESIDENTIAL SECTOR SIX OXYGEN SUPPLY: 71 PERCENT.

The sector where Mira and Ian had lived.

AIR PURIFICATION SECTOR NINETEEN: MANUAL MAINTENANCE PORT LOCKED.FILTER REPLACEMENT IMPOSSIBLE.

The same sector where, before dawn, Mira had wiped away a single speck of dust.

“Sector Nineteen’s filter is clogged again.”

“The workers?”

“At the door. They can’t open the access panel.”

Aegis applied air-allocation priorities.

SUPPLY MAINTAINED: MEDICAL SECTORS / CHILD SECTORS / CRITICAL PRODUCTION FACILITIES.SUPPLY RESTRICTED: LOW SURVIVAL-CONTRIBUTION SECTORS.

Three blocks in Residential Sector Six lost air first.

Adults with no assigned function.

Chronically ill residents with low projected recovery.

Elderly residents with no current production function.

Ian looked at the screen.

“What does low survival contribution mean?”

“People classified as unlikely to perform work currently needed to maintain the city,” Elias said.

“People who need less air?”

“That is not what the term was intended to mean.”

“What’s the result?”

Elias could not answer.

ESTIMATED SAFE TIME — RESTRICTED BLOCKS, RESIDENTIAL SECTOR SIX: 47 MINUTES.

Mira closed the filter-retrieval route.

“We go to Sector Nineteen first.”

“What about our filters?” Daniel asked.

“If we open Nineteen, thousands of people in Sector Six breathe.”

“You said Rina would be in danger in two hours.”

“That’s why we split up.”

“Who?”

“Noah and Yona go to the shelter. I go to Nineteen.”

“You cannot move alone,” Noah said.

“I don’t even have a function. Who’s going to stop me?”

“There is a public demand for your trial. Citizens may attack you.”

“Then one perimeter guard comes with me.”

Daniel said, “Don’t reduce the people staying beside Rina.”

Rina spoke first.

“Mira goes.”

“Rina.”

“I’m here.”

“If the power goes—”

“Dad is here. Sarah is here.”

Daniel did not look reassured.

Yona tossed the sealed filter toward Ian.

Daniel caught it.

“Now you’re giving it to him?”

“If I don’t come back, use it on the child.”

“Before that?”

“Don’t open it. Wet cloth is enough for the spores inside the city.”

Elias drew nine marks on the floor.

Yona’s three filters and six filters they believed might remain at the shelter.

“Even if we secure these, only one quarter of the current group can leave.”

Rahim spoke from the other side of the voice tube.

“You want to decide that now?”

“If we wait until departure, people will fight.”

“You think they won’t fight now?”

Elias tried to retrieve everyone’s medical records. Detailed records were locked under Aegis protection authority.

“Rina and Ian need filters,” Sarah said. “So does Yona. The guard with respiratory symptoms and Kerin as well.”

“I thought Rina was going in the culture tank,” Daniel said.

“She may be exposed to outside air while we transfer her to a different carrier.”

Someone outside said, “Why does that child already have a reserved share?”

Daniel moved toward the tube.

Rina spoke before he could.

“Don’t reserve one for me.”

“You absolutely need one.”

“So do other people.”

“We should decide by survival probability and likelihood of successful travel,” Elias said.

Rahim asked, “Who calculates that?”

“People will have to, this time.”

“Which people?”

No one in or outside the isolation room offered a name.

Yona erased one of the floor marks with her boot.

“The wet filter may not last to the outer wall. Count eight.”

She erased another.

“My filter has to be used when we reopen the return passage.”

“You’re special?” Rahim asked.

“If the guide collapses, everybody gets lost.”

“That’s survival contribution too.”

Yona nodded.

“We calculate outside too. We just say it to people’s faces instead of hiding behind a machine.”

Elias looked at the seven marks remaining.

“Does that change the outcome?”

“It changes who owns the responsibility.”

“Don’t decide yet,” Mira said. “We don’t even know how many filters actually exist.”

“We should prepare for the worst case.”

“If you start removing people based on the worst case, then even if we find more filters, someone has already become a person we abandoned.”

“What is your alternative?”

“Make more filters.”

“We lack material and time,” Yona said.

“The breath-stalks make membranes that catch spores. What if we layer those with city filters?”

Sarah looked at the white stalks.

“That could increase the spore concentration.”

“Still better than choosing seven people and stopping there.”

Elias erased all the floor marks.

“We will not make a list until we confirm the filters.”

“Record that,” Rahim said.

Elias entered it into an emergency terminal.

PRESELECTION OF OUTER-WALL DEPARTURE GROUP DEFERRED.DECISION TO BE MADE THROUGH OPEN PROCEDURE WITH AFFECTED PERSONS AFTER PHYSICAL FILTER INVENTORY AND ALTERNATIVE-FILTRATION TEST.

Ian asked, “What’s an open procedure?”

“We still have to make one.”

“We’re doing it first again?”

“It appears we have to.”

Rina asked, “What if it fails?”

Yona answered, “Then people without filters have to decide for themselves whether to go or stay.”

“They could die without a filter.”

“They could die if they stay too.”

No one drew the nine marks again.

Yona and Noah headed for Emergency Shelter Five.

Mira left for Air Purification Sector Nineteen with one perimeter guard.

Their routes split at Pressure Door Three.

The residents on the other side had not cleared the corridor.

About twenty people stood behind the transparent window. A woman pressed her wrist screen against the glass.

RESIDENTIAL SECTOR SIX RESIDENT.AIR-SUPPLY RESTRICTION SUBJECT.

“You want to pass through us?” she asked.

Mira answered through the intercom.

“I’m going to open the filters in Sector Nineteen.”

“You’re Mira Han.”

“Yes.”

“The one who put another child in place of your own.”

“Yes.”

“And because of you, every manual door is locked.”

“Yes.”

The woman studied her for a long time.

“If we let you through, can you fix it?”

“I don’t know for certain.”

“Is that all you can say?”

“If I say yes before I open it, I’m lying.”

The woman spoke with the people around her.

They began clearing the corridor themselves.

“Come through.”

“There’s a spore risk.”

“Rain is already inside.”

“Block the vents and move away from the door.”

Mira opened the pressure-sensor bypass line. The port was locked, but a thin measuring tube remained exposed.

The perimeter guard read the pressure on the far side.

Mira adjusted auxiliary ventilation valves on both sectors by hand.

Pressure difference: three kilopascals.

Two.

One.

The electronic lock released for a moment.

PRESSURE EQUALIZATION CONFIRMED.

Mira pushed the door.

People on the inside pulled.

It opened.

Wet air from the two sectors mixed.

Some people coughed.

Others took their first deeper breath in a long time.

Mira passed through them.

A man grabbed her arm.

The guard started to move, but Mira stopped him.

The man did not attack.

“Open our block first.”

“If I open the Sector Nineteen filter, air comes back to all of you together.”

“What if you can’t?”

“I’ll open this auxiliary valve manually. It may reduce air to another block.”

“Then do I have to ask those people?”

He looked at the countdown on his wrist.

“Within forty-seven minutes?”

Mira could not answer.

Six maintenance workers were waiting outside the access door to Air Purification Sector Nineteen.

All of them had worked with Mira before.

No one greeted her warmly.

Sena, the oldest, said, “You’re the reason the ports are locked.”

“I know.”

“Even the access-panel hinge is internally locked.”

“Is the duct I opened before dawn still there?”

“Inside the central filter chamber.”

“The bypass duct reachable from outside.”

Sena shook her head.

“The old dust-waste line from when you were an apprentice?”

“That one has no authentication.”

“A person can’t fit through it.”

“Not a person. A tool.”

Mira found the abandoned dust-waste pipe near the bottom of the filter-room wall. Eighteen centimeters in diameter. Hardened dust clogged it.

The workers connected a manual suction pump.

With no power, all six took turns on the handle.

Black clumps came out.

The dust was not dry powder. Rainwater and white threads had bound it into wet masses like soaked cotton, stuck to the inside of the pipe. Each turn of the handle made the pump’s leather diaphragm fold like a cough.

“This wasn’t here before dawn,” Sena said.

“Neither was the rain.”

“It grew this much in less than a day?”

Mira examined the tip of a white thread. There were signs it had fused with the mycelial structure of the existing filter.

“It may not have grown today. It could have been inside the walls waiting for water.”

“Did the city know?”

“If there were no sensors, it didn’t.”

“What about us?”

Mira worked the handle again.

“We never opened the wall either.”

RESIDENTIAL SECTOR SIX SAFE TIME: 31 MINUTES.

When the pipe cleared, Mira attached a cutting hook to the end of a long connecting rod.

“What are you trying to cut in there?” Sena asked.

“The filter retention mesh.”

“Catch the wrong thing and you cut the blower blades.”

“That’s why you tell me which way.”

Sena pressed her ear to the wall and listened to the blower vibration.

“Twenty degrees right. Up.”

Mira inserted the rod.

She could not see the end.

Metal scraping echoed through the pipe.

“Down.”

“This much?”

“More.”

The hook caught something.

Mira pulled.

It did not move.

Two workers joined her.

“Now!”

The retaining mesh tore.

They heard the filter being sucked toward the blower.

“Stop!” Sena shouted.

Mira released the rod.

Part of the filter had caught on a blower blade. The vibration became irregular.

“You ruined it,” the guard said.

Sena pressed her ear to the wall again.

“No. It’s still turning.”

Wet debris that had blocked the filter fell through the torn gap.

Airflow rose from seventy-one percent to seventy-eight.

Eighty-three.

The blower vibration climbed with it.

“If it goes above ninety, reduce output manually,” Mira said.

“The manual port is locked.”

Mira closed her eyes.

She had opened one failure by creating another.

Airflow: eighty-eight percent.

Blower bearing temperature rising.

“How long?” Sena asked.

Mira listened to the vibration.

“Two hours. Maybe four.”

“And after that?”

“The blades may break.”

“Replacement parts?”

“Forge warehouse.”

“Movement restriction.”

They looked at each other.

Air returned to the restricted blocks in Residential Sector Six.

The safe-time warning disappeared.

In its place appeared a new estimate.

SECTOR NINETEEN BLOWER FAILURE: 02:17:44

Mira pulled out the connecting rod.

Wet dust clung to the end.

So did a white thread.

Biological tissue brought in by the rain.

Dust was not the only thing that had clogged the filter.

Tiny openings appeared along the white thread on the rod.

When Mira breathed out, it turned toward her.

Yona’s voice came through the remote lock key.

“We reached the shelter.”

Metal impacts sounded behind her.

“You opened the filter storage?”

“Opened it.”

Yona was breathing hard.

“But there aren’t forty-eight filters.”

“How many?”

“Six.”

“Where are the rest?”

“Removed eleven years ago for a City Three exploration plan.”

Mira dropped the white thread onto the floor.

“That exploration plan isn’t in my team records,” Yona said.

Residential Sector Six had begun to breathe again.

The group had only nine filters with which to leave the city.

And someone had known the route to City Three for eleven years.