That same Tuesday, Mara got to Shelter 131 more than an hour late.
The shelter was in the basement of a primary school. Mr. Barrett was waiting at the bottom of the school stairs with the drawings rolled under his arm. His eyes went to her knees, which were soaked through, and then to her face.
"You're never late," he said.
"A man collapsed on the bridge road. Outside a bakery. I stayed until the ambulance came."
"Is he all right?"
"No. He died."
Mr. Barrett nodded. He didn't say anything kind. If he had, she might have cried in front of her boss.
She took out her own tape measure.
So Mara did the stairs. She did them properly.
Her hands weren't steady at first. She could still feel the fourth push in the heel of her right hand. The tape wandered against the handrail, and she got one number, then a different number. She let the tape snap back into its case, took a breath, and measured a third time.
The third time, the number matched the first one. After that, every number matched.
Two flights of stairs. Eleven steps each. One meter and ten centimeters wide between the handrails. And four hundred people would have to use them.
She wrote it all down.
"Anything I should worry about?" Mr. Barrett called from the top.
"Not yet," Mara said. "Ask me when I've done the doors."
Every first-year student at the Polytechnic, the city's engineering college, heard the same fact in the very first lecture. It was about the Second Incursion, years after the wall went up. That was the night the monsters got over the wall and into the city itself. On that night, more people died on stairs like these than died from the monsters.
They fell. And the people behind them kept coming.
Most of Mara's class wrote that down and moved on to bridges. Mara thought about it every time she walked down a flight of stairs.
At the bottom of the stairs was the shelter itself, a long, low room with a heavy sealed door. Paintings from the children's safety drills were taped along the wall. Stick families, holding hands under a green running man.
At the far end stood a gray steel cabinet, as tall as Mara, with a Medical Service seal on its door and words stenciled across the front.
PROPHYLAXIS. AUTHORIZED PERSONNEL ONLY.
What was inside it, medicine of some kind, wasn't Mara's business. Her business was whether the seal on the door was unbroken.
It was. She ticked the box.
'Good. A seal I can check. A number that stays the same when I measure it twice.'
For an hour, she thought about nothing but stairs.
***
At lunch, Mara sat on a low wall in the school yard. Her phone rang. Dina.
Mara let it ring.
A minute later, a voice message arrived. Mara held the phone to her ear and played it.
"Okay, so... news," Dina said. "The group next to us is doing bridges. Everybody does bridges! Also, you didn't answer about the form, so I need to know if you're alive."
Mara typed back.
--Mara--
Alive. Busy. Tell you tomorrow.
The answer came fast.
--Dina--
you sound weird
--Mara--
I'm typing.
--Dina--
you type weird
She started a second message. She got as far as two words.
--Mara--
A man
She deleted it. This wasn't a conversation for a phone. And what was the rest of the sentence, anyway? She didn't have it.
***
By evening, Mara had seven explanations.
Her tests that morning had already ruled out four of them. On the tram home she typed the list into her phone, standing by the second door as always. This time she kept her back to the window.
--Notes--
1-4. Migraine, no sleep, a screen, dirt on the glass. Tested this morning. OUT.
5. Chance. A count runs out over the one man on the street who dies? OUT.
6. Memory. I followed him because of the numbers, before anything happened. I didn't make it up afterward. OUT.
7. My eyes. Not tested yet.
Six explanations were OUT. If the seventh went OUT too, only one answer was left. The numbers were real, and they had known he was going to die.
The fact from that first lecture had stayed with her since her first year because of one thing. Those people on the stairs didn't have to die. Somebody could have made the stairs wider. Somebody could have checked.
If the numbers were real, she could treat them like those stairs. Something caused them, and she could find it.
'So find out what they are... and after that, find out if a count can be changed before it gets to zero.'
First she had to rule out the instrument. That meant her eyes.
She opened the Polytechnic clinic page on her phone and took the first free slot with the eye doctor. Wednesday, 9:40. It meant missing Hydraulics, a class she had never once skipped.
She booked it anyway.
***
When Mara got home, Theo was at the kitchen table behind a wall of newspaper, glue, and the lower half of a volcano.
"You took my notebook," he said.
"Good evening."
"You did, though!"
"Why would I take your science notebook?"
"Because you take things and put them in places that make sense to you. And then nobody else can find them."
Without turning around, he pointed at the bookshelf.
'Don't say it. Whatever it is, please don't let him be right...'
The notebook was on the shelf, between the atlas and the first aid manual. Mara had put it there last night, because it had been lying on the floor.
"That isn't the point," she said.
"That's the whole point."
Theo was twelve. Lately he'd perfected the face Dad made when he thought he was being patient with her. Mara didn't like it on either of them.
He had glue in his hair.
Theo's lunch money was still on the shelf too. He'd forgotten it again. Mara put it in his school bag for tomorrow.
He watched her do it and had the sense not to say a word.
At seven, her phone buzzed.
--Dad--
Later than late tonight. Sorry. Eat without me.
So Mara cooked. She washed her hands before she started, and then she washed them again.
"There's a boy in my class," Theo said, "who can turn his eyelids inside out."
"Really?"
"Both of them! At the same time. He charges people to watch."
"Then what?"
"Then he got sent to the office."
She said "Really" and "Then what" in all the right places. Theo didn't notice she wasn't listening.
The whole time, she could see the bakery window, and the moment the numbers weren't there anymore.
***
Before bed, Mara stood in front of the bathroom mirror for almost ten minutes.
She turned the light off and on. She leaned in close. She stepped back to the door. She stared at the space above her own head for so long that she started to feel stupid.
'If anything shows up there, I'm moving out. Theo can have the apartment. He can have the volcano, too.'
Nothing showed up. There was only her.
Her hair was tied back badly. It was the same color her mother's had been. Her eyes looked tired. And next to her left eyebrow was the small white scar she'd had since she was eleven.
She touched it. She did that sometimes without deciding to.
Touching it never brought back the crash. She didn't remember the crash at all.
What came back was a hospital ceiling, white, with a stain in one corner. Her father's face, coming into view from the side. His mouth moving for a while before there was any sound.
Mara took her hand down. She watched the empty space above her head for one more second.
"What are you looking at?"
Theo was in the bathroom doorway in his pajamas, with his toothbrush in his mouth and glue still in his hair. In the mirror he stood just behind her shoulder.
Mara checked the space above his head before she could stop herself. Nothing. She checked it again. Still nothing.
"Nothing," she said.
"You keep looking up there." He took the toothbrush out. "You did it to me too. Just now, in the mirror."
"I'm tired, Theo."
She turned off the light, fast, before anything could change.
In the dark, Theo didn't move from the doorway.
"Mara," he said. "What's supposed to be up there?"