The amber alert meant the platforms were shifting.
Riku learned this from Yuki in the first thirty seconds of the amber light, delivered in the clipped, information-dense style of someone who has processed an emergency many times and no longer wastes words on transition.
"Amber means electromagnetic recalibration," she said, moving fast through the holding corridor with Riku behind her. "The platforms adjust position every fourteen hours to maintain optimal load distribution. During adjustment, there's a thirty-minute window when the security grid in the lower zones drops sixty percent — they have to redirect power to the electromagnets. Upper rings stay covered. Middle rings are partial. Lower rings —"
"Lower rings are open," Riku said.
"Not open. Just less watched."
They moved through the holding corridor the way Yuki moved through everything — with the ease of someone who has mapped a space so many times it lives in her body rather than her memory. She knew which panels in the walls could be depressed to reveal maintenance corridors. She knew which maintenance corridors connected to which transit shafts. She knew how long each transit shaft segment took to travel at a walking pace, and how the timing aligned with the security rotations, and which operators would look away for the right kind of favor and which ones couldn't be reached.
Three years, Riku thought. She's been doing this for three years.
"The children," he said.
"Already gone. I sent word through the network before they brought us in — the moment you agreed to take liability, I had twenty minutes of expected processing time. My people had the kids out of juvenile intake in fifteen."
He absorbed this. "You planned for me to say yes."
"I planned for either outcome. If you'd said no, we'd have handled the kids differently and I'd have had to find another way to make contact." A brief pause as she checked a junction ahead, then waved him through. "But I hoped for yes."
"Because of the verification window."
"Because of that. And because —" She stopped at a panel, pressed three specific points in sequence, and it swung open into a shaft that smelled of machine oil and recycled air. She went in first, reached back to steady him. "Because three years is a long time to do everything alone."
She said it with no self-pity, which made it worse.
The transit shaft was narrow and vertical, with rungs set into the wall for descent. Yuki moved down them with the automatic confidence of a climber who no longer needs to look at her feet, and Riku followed, and below them the city opened up in layers as they descended.
He could feel the altitude dropping. The air changed — losing the crisp, maintained quality of the upper rings, becoming thicker and more complex, carrying the smells of cooking and machinery and the particular density of many people living close together in spaces not quite large enough for them. The lights changed too: the uniform white of the upper levels giving way to the patchwork of the middle rings, and then, as they passed through the cloud layer, to the amber glow of a city that had built itself in the space between two levels of better-lit world.
"Zone 29 is four levels below the cloud floor," Yuki said. "That's where the trial environment interface is, according to everything I've been able to map."
"You've been looking for the trial interface for three years?"
"I've been looking for a lot of things for three years. The interface is one of them. Mei is the other." She paused on the rungs. "I have reason to believe they're connected."
"How?"
"Because Mei shouldn't have been able to come through without a portal. The portal opens in response to the individual's resonance frequency — it's like a lock and key. If no portal opened for Mei, it should mean she didn't come through. But she's not in our origin timeline. She's not dead — I've verified that through three different timeline monitoring systems, none of which I'm supposed to have access to, long story. She's somewhere in the trial infrastructure. But she didn't have a portal."
"So someone else's portal took her."
Yuki stopped climbing and looked at him.
"Yes," she said. "And for that to happen, someone else would have had to — not use theirs." A beat. "Someone would have had to give her their portal instead."
Riku held on to the rungs and thought about this. "Who?"
"Someone who loved her more than they wanted to go home."
They continued down. Four levels below the cloud floor, the shaft opened into a space that looked nothing like the rest of Polaris City. The engineering was different — older, the materials predating whatever had built the upper city. Raw struts and polymer panels rather than the seamless metal and glass of the levels above. And woven through all of it, running along every surface like the nervous system of something enormous, visible light-pulse conduits — not the traffic data of the upper city, but something else. Slower. More deliberate.
"The trial infrastructure," Yuki said quietly.
"It's built into the city."
"The city was built around it. Polaris City is approximately two hundred years old. The trial infrastructure is older. The city grew up around it the way a forest grows around an old ruin — not knowing what it was, just knowing it was solid, and building on that solidity."
Riku ran his hand along one of the pulse conduits. It vibrated slightly under his fingers, the way a string vibrates when the instrument it belongs to is being played in another room.
"And my portal is here?"
"The portal interface should be somewhere in this level. I've never been able to go further than this —" She stopped. "Every time I try, the city has a reason to need me elsewhere. An amber alert, a zone conflict, a child in intake. I've never been sure if it was coincidence or if the system was keeping me away."
"The trial system?"
"Managing me. Making sure my trial stays active by keeping me occupied with things that look like my trial but aren't."
Riku looked at her in the low amber light. She met his look with the steady eyes of someone who has been underestimated by systems larger than herself before and has learned to find it motivating rather than discouraging.
"So today you have backup," he said.
"Yes."
They went further in.
The portal interface announced itself before they found it: a change in the air pressure, a deepening of the vibration in the conduits, and then the light — familiar now to Riku, the blue-white that had taken him from the train, that he associated now with impossible passages and the choices that preceded them.
It was not a door. It was a membrane — vertical, floating, with no frame and no support, sustaining itself in the middle of the infrastructure space with the serene self-sufficiency of a thing that didn't need external validation. On its surface, a pattern that shifted when he looked directly at it and stabilized when he looked slightly away: the specific resonance signature of his own psychology, rendered in light.
His portal. His door home.
Beside it, less stable, flickering at the edges: a second membrane, smaller, with a different pattern. Not his. Something that had the quality of an image copied one too many times — close to the original but not quite aligned.
"What's that one?"
Yuki was very still. "That," she said, with the controlled voice of someone who is working very hard to keep themselves functional, "is Mei's portal."
"But she didn't have —"
"No. Someone went through hers, so she came through theirs. Which means their portal — wherever they ended up — still has her resonance on it. And it left an echo here." She pressed her hand flat against the flickering membrane and it steadied, briefly, under her touch. "She went through this. This is the trail she left."
— ✦ —
Riku looked at his portal and then at Mei's echo-portal and then at Yuki, who had her hand pressed against the membrane with the look of someone touching evidence of something they've been searching for long enough that finding it is almost more than they can process. "If I go through mine," he said slowly, "and the verification window opens — can you follow Mei's trail? Through the echo?" Yuki turned to look at him. "That's theoretically possible," she said. "But the echo is unstable. If I go through it without anchoring it first, there's a significant chance it collapses midway." "How significant?" "Very." He looked at both membranes. "There has to be a way to anchor it." "There is one way," Yuki said, and her voice had gone careful, the way voices go when people are about to say something they know will change things. "I've known about it for two years and I haven't done it because it requires —" The city shook. Not a recalibration. A real shake — the deep, structural vibration of an electromagnetic platform failing, somewhere above them, the sound of Polaris City lurching in its suspended position above an enormous drop. And through the infrastructure's pulse conduits, a new signal started running: not the city's standard amber alert but something red, something that had no color code in the language Yuki had taught him, something that pulsed in the specific rhythm of a system that has just been activated by someone who was not supposed to have access to it.