The bookshop was called Sora Books.
Riku noticed the name first because he was seventeen and the words sora and books in proximity made him think of the girl in the forest with the code under her skin, who had chosen to carve her name in bark and walk through a portal and go home. He noticed it and did not mention it. Sometimes the world rhymed and all you could do was register the fact.
He noticed the man in the bookshop's rear section second, because Reiko had said he was there and Riku was therefore looking, and because the man had their father's posture: the precise, unoccupied stillness of someone doing nothing that is actually the maximum-awareness stillness of someone doing everything.
He was pretending to look at a book about bridge engineering, which Riku found both predictable and sort of touching.
"He's there," Riku said to Hana, who was beside him, pretending to look at a display of notebooks near the window.
"I know. He's been there since we arrived. He looked up when we walked past the window and then looked back down."
"Did he recognize —"
"I don't know. His face didn't change, but Dad's face doesn't change when he's recognizing things. It changes slightly later, when he decides what to do about the thing he recognized."
This was so accurate that Riku briefly couldn't speak.
Inside the house, their father and Reiko were having a conversation about what to do, which Riku had left because the answer seemed obvious: do nothing, wait, eventually the other-dad would either give up or force the issue and it was not currently their move. But Hana had come with him to the bookshop window, ostensibly to see what the neighboring block looked like and actually — Riku suspected — to look at the man in the back of the shop.
The man turned a page in the bridge engineering book without having read anything on the previous page.
"He's not reading," Riku said.
"He's thinking. There's a difference with Dad."
"There's a difference with everyone."
"With Dad specifically —" Hana paused, finding the words. "When he's reading, he's very still. When he's thinking, one finger moves. Very slightly. On the edge of whatever he's holding." She nodded toward the glass. "Look at his right hand."
Riku looked. The man's right index finger was moving: a minute, rhythmic tap on the spine of the bridge book. The specific movement of someone who is internally solving a problem and whose body is keeping the beat of it.
"He's got as far as we have," Hana said quietly. "He knows something arrived. He knows it's related to the bridge site. He's trying to connect those two facts and he doesn't have the piece that joins them."
"The portals."
"The portals. He's a structural engineer in a world without portals. He can feel that something is using the ground beneath his city but he has no framework for what it is."
Riku watched the tapping finger and thought: that's our father. In a version where there was no earthquake to motivate three years of secret research. In a version where he was just a man who was good at his job and went home to his family in the evenings and woke up three weeks ago feeling something wrong under the city and started, alone, quietly, investigating.
He thought: we inherited the best parts of him and the worst. The watching-everything-from-a-distance. The solving-it-alone. The finger tapping while the face stays still.
The man looked up.
He looked directly at the window, directly at Riku, and Riku had exactly one second in which to look away or hold the gaze. He held it — not intentionally, not by decision, just because looking away hadn't occurred to him in time. And in that second the man's face did the thing their father's face did: went fractionally stiller, then fractionally less still, then moved into an expression that was not quite recognition and not quite its absence.
He was looking at Riku the way you look at someone you think you know and then decide you don't and then aren't quite sure.
Riku smiled. The automatic, small, deflecting smile of a teenager who has been caught staring and is pretending it was nothing. The same smile their father used. He had never noticed before that they shared it.
The man looked back at his book.
Riku turned away from the window and pulled Hana by the arm.
"We need to go back in," he said.
"I know. But —"
"He's going to come out of the bookshop and walk past the house. He's been watching long enough, he's going to test the next thing: whether there's anything in the house he can detect from the pavement. We need to be inside and below the resonance threshold before he does."
Hana looked at him. "When did you figure that out?"
"Thirty seconds ago. It's what Dad would do."
They went inside.
The family was assembled in the main room, arranged around the table with the binders and the documents and cups of tea that had been hot enough to hold but were now cold enough to ignore. The other-Riku — who Reiko had decided to call Keisuke for administrative purposes, after the street he'd first arrived on in his own branch — was sitting at the far end of the table, separate enough to be clearly not-quite-of-the-group and close enough to be clearly trying to bridge the distance. Ren was beside Mei. Mei's hands were flat on the table in the position of someone who has decided to be present without performing presence.
"He's about to walk past," Riku said.
Everyone looked at Kenji.
Kenji looked at the window — the thin paper blind that Reiko kept drawn, the strip of street visible beneath it. He thought about the man with the bridge engineering book and the tapping finger and the three reports to the geological survey.
"Everyone below the first floor," he said. "The kitchen has a basement — it's the lowest resonance zone in the building. Reiko will answer the door if he knocks."
"He won't knock," Kenji said.
"Then no one will answer and Reiko will watch him through the peephole and we will know where we stand."
"What's below the first floor is literally the basement," Riku said. "It's fine. What's the worst that —"
The worst thing happened, which was that the doorbell rang.
Not a knock. The doorbell. Deliberate, specific, the ring of someone who has decided to ask a direct question and is not going to stand outside pretending they're just passing.
Reiko looked at Kenji.
Kenji looked at the door.
"Everyone else downstairs," he said quietly.
"You're not going to —" Aiko began.
"I'm not going to open it. But if I'm here and he can feel the resonance through the door, staying in the room may be more informative than —"
The doorbell rang again.
Riku herded everyone toward the basement stairs, which opened off the kitchen. Last through the kitchen door was the other-Riku — Keisuke — who paused and looked back at the older version of himself standing in the main room, and something passed between them across the distance and the fact of their impossible identical existence. Keisuke nodded, once, and went through the kitchen door.
Kenji stood alone in the room.
From the other side of the closed front door, very quiet, barely audible: a voice. His voice, in the register of someone speaking not to be heard by anyone inside but to the air, to the building, to the vague disquiet that had been following him for three weeks.
"I know something's here," the voice said. "I don't know what. But I know it's not nothing. And I —" A pause. "I know the frequency. I've felt it before. In my own house, years ago. In my study, from the middle shelf."
— ✦ —
Kenji went very still. The middle shelf. Between the foundation engineering textbook and the poetry collection. His study, in this timeline — in the version of his life where there had been no earthquake to find, no notebook to keep, no frequency to monitor. And yet. The voice on the other side of the door said: "I had a notebook, years ago. I filled three pages of it and then I stopped, because the numbers didn't add up to a conclusion and I didn't want to follow them to one. I put it away. I told myself it was a miscalculation." A pause. A longer pause. "I don't think it was a miscalculation." Kenji put his hand flat on the front door, the same gesture Dr. Saito had used on the portal membrane, the same gesture you make when you need contact with a surface because the air is insufficient. On the other side of the door, on the other side of everything, his own voice said, very quietly: "If there's someone in there who knows what I was calculating — I need to talk to them."