Agent Morisato heard the sound of C-Sharp and, at first, ducked. Then she registered the tall glass window next to her, which faced windows from other office buildings. She reconsidered, and bolted down the nearest hallway instead. A moment later, gunshots cracked the glass and pounded into the wall where she'd been.
She stopped, caught her breath, and then carried on down the hall. Now her sidearm was in her hands. If the Coalition knew she was in the building, they'd be locking down right now, and anyone on the floor would be alerted to her position. But they wouldn't know that she had the fire key for the elevator, her simple but elegant escape route. The documents ought to be in the quartermaster's office, which was nearby. A few odd turns to throw anyone off her tracks, and she'd be ready for the ride out.
At the end of the hallway, she slowed to a jog, and tapped the base of her left ear, activating a sub-dermal implant, which sent a signal backward in time.

.-.-

Then she was running again.
Once she had the documents, was reasonably sure she wasn't being followed, and was at the elevator, reaching for the button she heard the sound of C-sharp again. What​? She looked down the halls, but she had been in and out of them with no visuals… and her intel had sworn up and down that the building would be clear. She looked at the elevator again, and frowned.
(There was someone on the other side of the elevator, holding a taser and a pistol. The Coalition knew Morisato was in the building about five minutes before the shots, which was enough time for their second agent to leave the sniper's nest, and make it up the elevator. As soon as the door opened…)
She took the stairs, instead. On the fifth step down, she tapped the base of her left ear.

.-.-

At the bottom, she used a miniature charge to explode through the door's lock, listening all the while for C-sharp and footsteps that never came. The door opened onto the street. She grinned, spun the gun around in her hands before dropping it back in the holster, and booked it across the street to meet her getaway. She'd move fast enough that the snipers on the other side of the building would never know she'd gone.

The implant was a little heavy, and sometimes it itched. Now and then it also made a very low humming sound in her left ear, but she'd gotten used to it. If she turned the sound up in Sea Hero, it just sounded like part of the game. Even among her colleagues, not many people had the implant- after all, it was still relatively untested- but she'd made an effort to stay in touch with a few who did.
After handing off the file and debriefing, Morisato lit a stick of incense and sat in her private quarters in the dark for a while. Then she turned the lights on, microwaved a frozen dinner, powered on her computer, and called up Agent Oberoi while her game loaded.
“How's it going, Hotshot? Back from a mission?”
“Yeah. In the Ice World, did you find the skeleton key in the first dungeon or did you have to wait to beat Pirate Cave?”
“I think I had to wait. You don't actually need it in the mean time.”
“Thanks. How's it going?”
“Aw, not so bad. Been on R&R the last week.”
“Oh?”
“It's the leg again. Nothing too bad, they don't want me jumping on it or anything.”
“Relaxing?”
“Oh yeah. You got to try it, Hotshot. Let's do vacation again sometime.”
“Hell no. I don't know how you do it.”
“I compartmentalize. Come on- Jakarta's nice. We'll get out of the city- I hear they have flying squirrels the size of raccoons.”
“That's your idea of a good vacation? No wonder Minsk was a flop.”
“It's on my bucket list. Hey, gotta go. Take care of yourself. You still seeing the shrink?”
“Well, yeah.”
“Me too. Good. If you need help with Pirate Cave, text me.”
Agent Oberoi worked at another site now. She had the implant too. Oberoi heard C-sharp sometimes, and, well, she just coped better with it. Morisato knew it was an unfair thought- Oberoi worked just as hard to keep her mental state in order as she herself did- but she must just be better at it. How else did you explain the cafe in Minsk? Halfway through their lunch in a small cafe off the main drag, a child banging on piano keys had hit C-sharp, and the noise went straight through her brain. Morisato had cleared the area. Immediately.
Had she hurt anyone? Had she broken anything? Oberoi assured her she hadn't, maybe scraped a chair when she toppled it in her mad dash out. By that point, her breathing at slowed and she was far too embarrassed to go back inside. It was a piano key. It wasn't the computerized tone she knew. It wasn't even the same timbre.
Stupid, she thought as they walked away. Stupid, stupid, stupid.

Another time, she'd been walking along a busy sidewalk in a city she'd never been in, and she heard C-sharp. At first, she backed away from the street edge, then backtracked through the crowd and slipped inside a fast-food restaurant, eyes fixed on the street outside, as she tapped her ear to send the signal back in time.

.-.-

Nothing. No gunshots rent the air. No vehicle ran off the road and plowed into the sidewalk. No screams, no alarming figures in heavy coats walking slowly or with their hands stuffed in pockets to conceal their firearms. Of course, a sniper would have seen her move. She called her supervisors and bought an apple turnover while they scanned the area from afar.
The tentative all-clear meant she should go outside again. She was ready to move, but nothing happened. No high tone rung in her ear for the rest of that trip. What would have happened? Why had the signal gone back in the first place? False positives had never been a problem before. Obviously, her acting had changed the situation. But for the signal to send, it had to be sent back for some reason. Somehow.
Who had sent it?

Site 81, Supervisor's Office, Phone Line

“Agent Morisato, the technology is safe. It absolutely cannot, and I mean mathematically impossible, transmit over five seconds in the past, so large-scale reality alterations or paradoxes are impossible. It's the golden goose.”
…
“Well, the signal is safe. The technology will never be entirely safe, not until we understand it entirely, and it will never keep you absolutely safe. But this signal is safe.”
…
“No, I know you did your reading.”
…
“It's, listen, Hotaru. The signal itself is sent from an alternate future in which your life is threatened. By receiving that signal, you can take action to avert the situation that created that danger, and by sending that signal back, you ensure that your own timeline is stable. Everyone is fine.”
…
“No, I- what do you mean, parallel universes? We don't even know that's how this works.”
…
“What do you mean?”
…
“Who?”
…
“Never mind.”

Morisato wasn't a physicist. But she'd read a lot about time, since then, and knew that even if using the implant didn't cause universe-destroying paradoxes, cause and effect probably still didn't just go away. The cause was the danger, the effect was triggering that signal and heartless C-sharp. She read books and articles about multiple universes, branching timelines. Who was sending the signal back in the first place?
She didn't think of them as separate people, but as one person; not individual selves cast aside throughout time, but a single copy of her- a tortured shadow that followed her at all times, ready to die for her at a moment's notice.

Morisato trailed along the underground corridor, following the soft light of a laser beacon through the winding halls to the source of the anomaly. As she passed under a smoke detector on the wall, she heard a faint hissing.
After three of her six fellow agents passed below the detector, it rotated 180 degrees in its socket and exploded. A hazy yellow gas filled the air, and the agents fell to their knees. Morisato's skin was burning, her nose and lungs were burning. She reached up and touched her left ear.

.-.-

As she passed under a smoke detector on the wall, she heard a faint hissing and the sound of C-sharp.
She held up one hand in warning, then tested the knob on a side door and hurried her team inside. A minute later, one of them identified the trap from Foundation records, and found an alternate route through the compound. Morisato kept her eye on her watch, and after she heard their new plan, reached up and touched her left ear.

.-.-

At last, they made it to the Coalition's research space. It had been cleared out in a hurry before they arrived, but the strange emission was still there, meaning its source was either immobile or hadn't been evacuated. It would be a great day for Morisato if they'd just left it behind, because it would save her from having to secure the whole compound. Well, if not, she'd had worse days too.
The laser beacon was malfunctioning underground, but their map prediction put the signal's source dead ahead of them, so Morisato signaled to her team and opened the door at the end of the hall.
The laboratory inside was lit by dim purple emergency lights alone. Morisato squinted as she cleared the immediate surroundings. It was hard to see into the lab's corners, of course, but nothing especially unusual appeared. As the team spread into the room, the hint of a dancing red spot of light caught her eye.
The bullet bore through Morisato's head in the span of a millisecond. Blood spattered across the concrete behind her. The implant in her head recorded that her pulse had stopped, and sent a signal back through time.

.-.-

The laser beacon was malfunctioning underground, but their map prediction put the signal's source dead ahead of them, so Morisato signaled to her team and opened the door at the end of the hall. A step into the doorway, she heard the sound of C-sharp. Instantly, she gave her team a hand signal, waited until she heard the faint thump of bodies finding cover, then crouched and dived behind the doorframe. What's the most likely threat? Somebody inside, of course. She drew her sidearm and watched. There was a flash of movement in a far, darkened corner.
Morisato took down the enemy with one shot, and touched her ear.

.-.-

It wouldn't keep her alive forever, of course. One day, its plus-or-minus three seconds wouldn't be long enough, and the signal would send itself backward for the last time. That was where she couldn't rely on poorly-understood technology, she just had to be good. But it had kept her alive damn well so far. She never meant to keep track, but somehow the shadow in her mind became a Wound Man of sorts- here a bullet wound, there a slit throat, there an electric field, amorphous stains where she couldn't have known what exactly would have killed her.
Most of the scenes that stayed in her mind from the field missions, she tried to keep back. But the Wound Woman, the shadow she had never seen in real life, never quite left. It was a ghost in her privileged timeline.
How was she supposed to explain to her supervisor why she no longer took time off unless forced?

“How's therapy going?” asked her supervisor, strolling by while she was gearing up.
“Good,” said Morisato. It was helping with the nightmares, anyway.
“Good to hear. Did you hear about your project?”
“What?”
“The implant. It's been cancelled.”
Morisato put down the ammo sling she was holding. “Am I-”
“You're still cleared for fieldwork, thank god. No change there. But they're cancelling the implant program going forward. Still don't understand the tech, so they say it's too risky and they're shutting it down."
Morisato thought about that. It would just be her and Oberoi, and the others, the dozen or so who'd had the anomalous technology shuttled into their brains on a whim. They'd be hearing C-sharp for the rest of their lives.
“When I said you were cleared for fieldwork,” her supervisor continued, “Well, they are offering you a chance to turn your gun in. Get a desk job. What do you think?” He paused. “You're our biggest asset, Morisato. We need you here.”
“I'll finish the mission,” she said.
“Good.” Her supervisor sighed with relief.

It was a roaring success. The local branch of the Coalition was either in custody, or had fled, and the Foundation had been able to seize their assets. In the final move, Morisato led the charge to recover every single one of their anomalies from that location, and hadn't even heard C-sharp once.

Back in her room again, Morisato lit a stick of incense in her room and turned off the lights. For a while, she just sat, breathing in and out, taking in the weight of darkness. Then she turned the lights back on, and opened her laptop and started a blank email. She addressed it to Human Resouces, and then, slowly, struggled to find the words.
I am informed that I have the opportunity to transfer into a non-fieldwork position, and would like to accept. I have a history of contributions to the Foundation and the following skills…
Her shadow settled in behind her, like an old friend, to read over her shoulder.