The favor was small. That was the thing Lea would come back to, later, turning it over like a stone she’d picked up on a beach without thinking: it had been so small.

“It’s a sleeve,” Abby said. “It goes around your thigh. You forget it’s there in an hour.”

They were on the back terrace of Abby’s house — a 1920s stone place on the bluff, gutted and rebuilt inside with a great deal of glass — high above the river, the kind of September evening that made the city look like it had been designed rather than accumulated. Abby had poured the wine. Abby always poured the wine; she had opinions about it and Lea had none, and that was the shape of a lot of things between them.

“And it measures what, exactly.”

“Skin conductance. Temperature. Heart rate variability. Micro-movement.” Abby ticked them off on her fingers, bored by her own list. She was forty-two and ran a company with sixty employees that made medical wearables for people whose bodies were trying to kill them, and she had the particular calm of someone who spent her days being the most competent person in the room. “The firmware’s fine. What I don’t have is enough clean data from healthy women in your age range. The IRB study doesn’t start until January. I need a friend with a boring, regular nervous system to wear it for a few weeks so my engineers stop guessing.”

“Boring,” Lea said.

“Regular. Reliable.” Abby smiled over the rim of her glass. “Those are compliments, from me.”

Lea was thirty-seven. She was a senior associate at a firm whose name was four dead men, and she billed twenty-two hundred hours a year, and she had been divorced for nineteen months and had slept with exactly one person since, a man from a dating app whose apartment had smelled like dryer sheets. She did not think of her nervous system as boring. She thought of it as under control, which she understood, in the honest part of her mind, to be a different thing.

“Does it know what I’m doing?”

“It knows what your body’s doing. It doesn’t know why.” Abby reached down beside her chair and set a small matte-black case on the table between them. “You’d be doing me a real favor, Lea. I’m not going to beg.”

She wouldn’t. That was the other shape of things.

The sleeve was softer than Lea expected, a band of something like neoprene that went around her upper thigh and sealed with a magnetic clasp. There was a sensor array on the inside, a slightly raised oval that sat against the skin on the inner face of her leg, high, where her thighs touched when she stood. Abby fitted it herself, kneeling on the terrace flagstones in her good trousers with Lea’s skirt rucked up to the hip, and her fingers were quick and clinical and Lea looked at the river and felt her face heat and told herself it was the wine.

“Too tight?”

“No.”

“You’ll feel it when you sit. That’s normal.” Abby’s thumb pressed the oval flat against her and held it there for a moment, and the moment was exactly one second too long. “There. Pair it with your phone when you get home. The app’s called Halo. It doesn’t do anything on your end except sync. All the readouts come to me.”

“All the readouts.”

“That’s the point, isn’t it.” Abby stood and brushed her knees. “I’m the one who needs the data.”

Lea went home. She paired the sleeve.

The app opened to a wall of grey text — Halo Biometric Participant Agreement & Data Terms — set in the dense, close type she had spent fourteen years reading in other people’s deals. There was a scroll bar on the right. It was very small, which meant the document was very long. At the bottom, a single white button: I ACCEPT.

She stood in the dark of her kitchen with her thumb over it. She knew exactly what she would tell a client. Don’t sign anything you haven’t read. She had said it a thousand times, in a voice she was proud of.

It was eleven o’clock. It was Abby.

She tapped it.

For a moment nothing happened. Then the screen said Calibrating fit… and against her thigh, under her skirt, the sleeve moved.

It didn’t buzz. It tightened — slowly, a smooth mechanical cinch, a few millimetres, the band drawing in around her leg like a hand closing, until the raised oval of the sensor was pressed flat and firm and warm into the soft skin of her inner thigh, high, much more firmly than Abby’s thumb had pressed it. Lea put her hand on the counter. She hadn’t meant to. It held there, snug, a fraction past comfortable. Then it stopped.

Fit confirmed, said the app. And then it settled into a single screen — a slow pulse of white light that meant connected — with one line of small grey text beneath it.

Clasp: unsecured.

She looked at that for a while. She thought about the word, and about what the other setting would be called, and why a thigh sensor for a wellness study would need one. Then she put the phone face-down on the counter, and did not look at it again that night, and did not take off the sleeve.

She noticed the sleeve, over the following days, far more than an hour’s worth. She noticed it on the train, where the pressure of her crossed legs put the sensor against her and she uncrossed them and then, ten minutes later, found she had crossed them again. She noticed it at her desk, under the long glass table in the conference room on thirty-one, when she shifted in her chair and the band pulled against the skin of her thigh with a soft, deliberate drag. She noticed it in bed, most of all. She had always slept on her side with her knees drawn up and now there was something there, between her legs, a small warm fact, and she lay awake with it.

On the fourth night she touched herself. She had not been going to. She had been lying there thinking about the Brennan deposition and then she had been thinking about Abby’s thumb pressing the sensor flat, the second too long, and her hand was already under the waistband of her shorts. She did it the way she always did, efficient, on her back, two fingers in small fast circles, and it took four minutes and she came quietly with her jaw clenched and lay afterward with her heart slowing and the sleeve warm against her leg, and only then did she think: oh.

It knows what your body’s doing. It doesn’t know why.

She did not sleep well.

Abby texted the next morning at 7:12, while Lea was in the shower.

Abby Interesting night.

Lea read it standing on the bathmat with water running off her. She read it again. She typed What do you mean and deleted it. She typed It’s just data and deleted that too, because it had not been a question, and answering it would be an admission.

She put the phone down. She dried her hair. She went to work and billed nine point four hours and did not look at her phone more than she needed to, and at 6:40 that evening, when the office had thinned to the associates and the cleaners, it buzzed again.

Abby Come by tonight. I want to show you something.

Abby’s house at night was all low lamps and dark wood and black glass, and through the glass the river below was black with the city’s lights floating on it. Abby had her laptop open on the kitchen island. She had not poured wine.

“Sit.”

Lea sat. The sleeve pulled.

On the screen was a graph. Abby did not explain it, at first; she let Lea look. Time along the bottom, four days of it, with the nights shaded grey. A heart-rate line, low and even, with the small daily hills of the commute and the gym. A second line, skin conductance, flat as a table.

And then, on the fourth night, at 11:52, both lines went up together like something lifting off.

“Four minutes and ten seconds,” Abby said. She was standing behind Lea’s stool, close, and her voice was mild and conversational. “Start to finish. You’re efficient. I’d have guessed that about you.”

Lea’s face was burning. She could feel it in her ears. “You said it doesn’t know why.”

“It doesn’t. I do.” Abby reached past her and tapped the trackpad and the graph zoomed, and now the climb was a mountain, and at the top of it was a small sharp cluster of spikes, three of them, close together, and then the long slow slope down. “That’s you coming. Those three. You hold your breath — see, there, the HRV drops out — and you clench everything, and then it’s done. Then nothing for a minute. Then you roll over.” A pause. “You rolled over, didn’t you.”

“Abby.”

“I’m not teasing you.” The hand on the trackpad withdrew and came to rest, lightly, on Lea’s shoulder. “I’m telling you I looked at this for a long time this morning. Longer than I needed to. I want you to know that.”

Lea sat very still. The sleeve was warm. Her pulse was in her throat and she knew, with a clarity that felt like falling, that Abby could see that too, that somewhere on a server the line was lifting.

“You’re not boring,” Abby said. “I lied about that. I needed you to say yes.”

“Why.”

“Because I’ve wondered for six years what you look like when you’re not in control of something.” The hand moved, slowly, from her shoulder to the side of her neck, and stayed there, the way you’d rest a hand on something you’d already paid for. “Now I’ve seen it. And I find I’m not finished. That’s the honest version. You can take the sleeve off and go home and we’ll have brunch on Sunday and I’ll never mention it again. Or you can leave it on.”

Lea thought about the train. She thought about the glass table on thirty-one, and her crossed legs, and how many times she had uncrossed them. She thought about the fact that she had not deleted the app.

“What would ‘more’ mean.”

Abby’s thumb moved once along the tendon in her neck.

“It would mean,” she said, “that you don’t do that again until I tell you to.”

She did not say yes. That would matter to her, later, in the long nights when she tried to find the place where she had agreed. She did not say yes. She said, “That’s insane,” and Abby said, “Probably,” and poured the wine then, finally, two glasses, and they drank it on the terrace and talked about the Brennan deposition and Abby’s board and a restaurant that had closed, and when Lea stood to leave at eleven Abby walked her to the door and said, “One week. Just to see. I’ll know if you cheat,” and kissed her on the cheek, and Lea went down the long stone front steps to her taxi with her hand pressed flat against the sleeve through her skirt.

She did not take it off.

She did not touch herself that night either, lying on her back with her hands flat on the sheet beside her, and that was the first time, and it was the only one that felt like a choice.