“Blue thread,” Evelyn whispered. “Alicia tied it there. She said no one could tell the drives apart.”

Mara explained what she’d seen in the school gym. At the mention of the new motors, Evelyn went still. At the mention of the four brass notches, she sat down hard on a stool.

“Did they put the floor together?”

“Yes.”

“The complete floor?”

“As far as I could see.”

“And Julian intends to run it in the street?”

“He says it’s a display.”

“He’s lying.”

“Why did you attack my camera?”

Evelyn looked toward the garage door. “Your camera records the sidewalk where the moving floor is being assembled. Brightline’s vans have passed your house every morning. I told them if they moved the machine again, I’d tell the neighborhood. Julian said he’d use your recordings to prove I’d interfered with the work.”

“That doesn’t explain the spray paint.”

“He told me he needed a clear recording of me approaching his van. That he could have me barred from the event if I even went near it. I thought if your camera was blinded, I could get to the van and remove the part that shouldn’t be there.”

“You were going to sabotage a vehicle?”

“Not the vehicle. A mechanical coupling. I made it thirty years ago. I know how to separate it safely.”

“Then why didn’t you ask me to turn off the camera?”

Evelyn gave her a look of exhausted impatience. “I sent you three notes.”

“I never got notes.”

“I put them through your letter slot.”

“I don’t have a letter slot.”

For the first time all day, something close to amusement passed across Evelyn’s face. “Then I’ve been writing to your father.”

Mara nearly smiled, but the feeling faded. “You still vandalized my house.”

“I know.”

“You have to fix it.”

“I will.”

Evelyn took a breath. “But first there’s something you need to see.”

She pointed to the silver drive.

Mara said, “You told me not to plug it into a computer.”

“Not a computer on the internet. And not one connected to anything else. Peter’s old simulator is here.”

She pulled a canvas cover from a squat gray machine with a monochrome display and a row of chunky switches. It looked like a cross between an office printer and a homemade submarine console.

“The drive won’t simply play a video,” Evelyn said. “It contains a set of motion instructions. Peter used the simulator to visualize how a full mechanism behaved before moving one real panel.”

Mara crouched beside the unit. “You kept it all these years?”

“I kept trying to understand what happened.”

“Did you?”

“No.” Evelyn looked at the USB again. “But Peter never let me see his final test.”