Prologue

I can’t remember ever feeling this tired! Dr. Ellie Henderson stood by the exit as her hand-picked team of scientists filed out of the lab for the final time. In truth, tired did not come even close to describing the fathomless depth of her fatigue. She and her departing staff had been working eighteen-hour days for over two weeks, and she was nearing the end of her endurance. She hid her weariness as best she could, unwilling to allow fleshly shortcomings to prevent her from extending a personal thanks to each of them. Despite her best efforts over the years, some of her coworkers persisted in viewing her as no more than a colleague. These few acknowledged their parting with impersonal gestures; handshakes for a few, a simple nod for another. But many had become close friends, and knowing they would never see one another again, most of this larger group could not bear to leave without a final embrace. 
The moment was bittersweet. Everyone had known from the start that they would never see any direct benefits from their efforts, but the project had given them each a temporary sense of purpose in a shattered world defined by chaos and decline. It was a purely intangible reward, Ellie granted, yet sufficient for a group made up of the top scientists from multiple disciplines. Intellects such as theirs rebelled against inactivity. They required a direction to explore, a mystery to unravel, and she had provided them with one for almost a decade. Now that time was up, and they had no choice but to descend one of the world’s few remaining ivory towers and find their own purposes amid the chaos outside its sheltering walls. The final pair to leave was a married couple, both specialists in the relatively new field of temporal mechanics. She thanked them for their work, gave them each a hug, and then they were gone.

The second the door thudded closed behind them, Ellie collapsed heavily into the nearest chair. She slumped back, pressed her palms to her face, ran her fingertips in small circles over her eyes, and let years of accumulated tension begin to drain from her body.

My damned body! she thought. She was exhausted, yes, but at least the painful condition that had plagued most of her adult life had been in remission during these all-important final weeks of the project. For that, she was profoundly thankful. She had no idea how long this current reprieve would last. She might have days still to go—weeks, even—but lately, her periods of strength and vitality seldom exceeded a month. With her work now done, she might get the chance to enjoy this interlude of normalcy for a while, however long it lasted. 
Ellie dropped her hands to her lap and cast her eyes over the tightly packed workstations surrounding her. Much of the technology she could see had been purpose-built, was unique in all the world, and none of it would ever be touched again after today. The lab was unusually quiet now, the room all but empty, and every computer and piece of monitoring equipment powered down for the first time in many months. Whenever the room was occupied, the buzz of multiple conversations, the rapid-fire clicking of fingers on keyboards, and the whirs of more than two dozen CPU cooling fans combined to create a constant mental pressure. It was a subliminal effect, one that revealed itself chiefly through the profound relief imparted by its rare absence.

Ellie was not alone in the stillness. Ryan crossed the room, stopped behind her, and laid his hands on her shoulders. He remained standing even though many of his recent days had been longer than hers. She was not surprised by this; he had always preferred to stay on his feet while he worked no matter how tired he was. 
“Well, that’s finally done,” she said, breaking the silence at last. “Not exactly how I originally intended, but… it’s done.” 
“Hmm.” 
Ellie was not fooled by his neutral tone. Willing her body back to a standing position, she rose and stepped around the chair to face him. “Thank you for sticking by me all these years. I know you’ve never fully approved of the plan, but your support made all the difference every time it mattered most.” She wrapped her arms around him, and he responded by pulling her close. 
He let out a long sigh and spoke into her hair, restating his position for perhaps the hundredth time. “It’s not that I disapprove, you know that. This project means as much to me as it does to you. But you also know how I feel about using her. Besides, we’ve chewed our way through a lot of valuable resources that might have been used on a project with a much greater chance of success.” 
“No point in beating a dead horse.” 
Ryan’s body went stiff at what he heard as a rebuke. He released her, took a step back, and when he spoke there was real pain in his tone. “I remember you once saying you always wanted my opinion.” 
“I wasn’t saying you… Of course I do, Ryan. I only meant that given how bad things are, it’s too late for anything we do here, now, to make the situation any better. It’s been too late practically from the day we were born, and to act as if it’s otherwise—that’s what would be lashing the deceased equine. You’re absolutely right—this project won’t help any of us in the least. But I could make this happen, and I felt I had to. And maybe, in the long run, this will make a difference to them.” 
Ryan’s laugh was a single, humorless cough. “The very long run! Do you really think she’ll figure it out?” 
Ellie considered the old question one last time. Over the years-long course of the project, she had often wondered if she should provide specific instructions, but even in the case of two closely parallel timelines, quantum variations, random and unpredictable, guaranteed there would be differences between them. Knowing that she could never accurately anticipate exactly what conditions would exist or what would need to be done argued strongly against doing anything that might only serve to cause confusion. Or worse, risk exposure, thereby rendering all of all their efforts wasted. In the end, she decided to disguise the truth against casual discovery as best she could and place her faith in her own youthful ingenuity and intuition. Above all, every decision made so far had been guided by her insistence that whatever course of action was eventually taken, the choice would be completely up to the one for whom the project was ultimately intended. Ellie would provide the tools, nothing more, and trust that events would play out for the best.
Still, the project was a huge gamble, one that had taken an incalculable toll on everyone involved, and there were many times when self-doubt had threatened to overwhelm her. As of ten minutes ago, though, her misgivings no longer mattered. They had just let their own metaphorical horse out of the barn, very much alive, and any lingering doubts or second guesses were thereby made pointless. 
“That’s the thing, isn’t it, Ryan? We’ll never know.” She placed her fingers on his chest and looked up into a face deeply lined with age. “But she has to. For Sam’s sake… for Aaron’s. She has to!”