A tiny noise of discomfort, barely audible and instantly cut off, made McCoy aware that his grip on Spock's injured arm had tightened. He quickly released him, looking around the featureless chamber they were in. The floor, walls, and ceiling were obscured by a rolling fog that covered everything with hazy shadows. Flashes of light flickered constantly from all directions without any seaming pattern, tinting the mist with a rainbow of random color.

"Greetings," Spock said beside him, and McCoy jumped as he whipped his head back around.

A tall being had appeared before them, the fog swirling around it as though it were a cloak. It was bipedal, with long, spindly legs and arms, and McCoy could see nothing that might indicate gender despite its utter lack of any clothing. Something about the way its large, hairless head tilted slowly at them, huge eyes blinking languidly, set McCoy's nerves on edge.

"Greetings, Spock of Vulcan and McCoy of Earth," the alien replied. Its tiny slit of a mouth did not move, and the quality of the voice reverberated in McCoy's head in a way that the doctor knew from experience was telepathy. "My form disturbs you," the alien continued, and even though its facial expression did not change, nor did it make any gesture, McCoy knew it was referring to him.

"Um," McCoy said, knowing it wasn't exactly diplomatic to admit to that, but unable to honestly deny the truth of it. Invisible, icy fingers brushed across his face, but the touch was fleeting and gone before he could do more than give a startled gasp.

"A racial memory," the alien said, surprise coloring its flat voice. "We had not thought your species would remember."

"Remember what?" McCoy asked, but the alien had already turned its attention to Spock.

"And you should not have been able to operate the time device," it added, its oversized, bug like eyes narrowing slightly. Spock suddenly raised a hand to his temple and took one step backwards before completely freezing in place, as if locked into some kind of force field.

McCoy glanced between the alien and the pained look on Spock's face. "Stop it," he said. The alien ignored him, and Spock's shoulders hunched slightly with strain. "Stop it, dammit!" he shouted, stepping between them. "Can't you see you're hurting him?" The alien gave no indication that it had even heard him. McCoy grit his teeth with determination and started forward, but found himself unable to move. McCoy struggled, knowing instinctively that it was pointless even as he did so. He couldn't turn around to see Spock now, but he heard the Vulcan let out a small grunt of effort. "You have no right to do this!" McCoy shouted in fury.

Spock let out a gasp and stumbled into McCoy's back as they were both released from whatever was holding them. McCoy braced his feet to bear the weight, but Spock was upright an instant later. "You all right?" McCoy whispered, taking a step back to stand beside him.

Spock had gone several shades paler, and his mouth was pressed into a stern, grim line, but he gave McCoy a tight nod. "I am uninjured."

"It would appear that your species has advanced more than we thought possible in the time since we last visited," the alien said, and waved a skeletal arm in the air. The bracelet disappeared from around Spock's wrist in a flash of light and reappeared in the alien's hand.

"You should have that thing looked at," McCoy said flippantly, trying to steady his own nerves. "Feels like going through a meat grinder."

The alien studied him for a long moment, its head tilting to such an angle that it looked like the massive, misshapen thing would roll off of the impossibly thin neck supporting it. "It was not designed for your physiology," it replied after a long moment. "You cannot comprehend how astonished we are that the device allowed you access at all." The bracelet glowed blue in the alien's hand, and the enormous eyes grew even larger as it jerked its head down to look at it.

"Who are you?" McCoy asked.

The alien looked up slowly. "We are… custodians." The fog suddenly lit up so brightly that McCoy lifted a hand to shield his face and Spock squinted into the glare. The alien held up the glowing bracelet. "You see as well as I do their past. You see the mark left upon them, and from whom. It is time." The lights went dormant altogether, leaving them in shadowy gloom. The alien slowly lowered its arm and regarded Spock and McCoy in turn. "Long ago, a favor was asked of us, by one for whom we could not deny any request. Ask your questions."

"Custodians of what?" Spock immediately asked.

"Time."

"Why did you bring us here?"

"We did not," the being answered and made a slight gesture with the hand that held the bracelet. "This one did. You do not yet have a full understanding of the nature of time. You could not be allowed to make your next intended transition. Neither you nor the other species involved in this… incident… should have been allowed any transitions. But as with all things, there are always exceptions."

"Why an exception this time?" McCoy demanded. "Why in God's name allow the Romulans access to a time machine?"

A single light pulsed from the fog, and the alien looked at it silently for a long moment before turning back to McCoy. "Time is a flexible entity, but there are some things that cannot be altered. They are fixed points, serving to bind the flow of time together. Whatever happens, the First One will find a way to make certain that those events always occur, without fail. In this instance, exceptions had to be made to ensure the stability of time. Permitting you to retain this knowledge is another exception, allowing for an event that has yet to occur."

"Who is this First One?" Spock asked.

The fog flared at that, and the alien emitted a very human sounding sigh. "Should you, for any reason, begin to speak of any of the events occurring between the moment of your first transition to your past and your return — even amongst yourselves — the entire experience will be excised from your memory." The alien stared directly at Spock. "You have the information you will need. The debt is paid."

McCoy opened his mouth to ask a question, but the breath was knocked out of him as he fell to the floor. He rolled onto his back and bumped into Spock, who was already jumping to his feet. McCoy tried to figure out what was going on, but his eyes had begun to water from the acrid smoke that surrounded him. The scent of scorched electronics assailed him and he realized he was back on the bridge of the Enterprise.

He ducked instinctively as a phaser beam blasted into a bulkhead, but it wasn't anywhere near him. He looked around and noted with some pride that his medical staff were calmly pulling the injured off to the side and tending to them without giving any heed to the combat waging around them. Generally speaking, his team wouldn't have been called to the bridge until the battle was over. But when the captain goes down, medteams get called – no matter the circumstances. His kit was right where he'd left it, and so was Kirk. There must not have been anything left to shoot at or run from, because Sulu and Chekov had both left their posts to join the fight by the time McCoy scrambled to Kirk's side.

An iron grip seized his arm, nearly causing McCoy to drop his tricorder. "What happened?" Kirk demanded through teeth clenched in pain.

McCoy shoved a hypo under his nose. "If you try to sit up, I swear I will drug you senseless."

Kirk scowled at him, but let his head fall back to the deck with small grunt while McCoy started to peel back the charred layer of his uniform to get at the shoulder wound. It was easier to think now, but Kirk knew he must have a concussion. He twisted his neck, trying to check the status of his crew. To his relief, it looked like the last of the Romulans had been taken down and he found Spock hauling the unconscious body of one of them towards the security team near the turbolift. Reinforcements had arrived at some point, for it seemed like the entire bridge was swarming with red shirted officers.

"Spock," Kirk barked.

Spock looked over his shoulder and gave him a quick nod to acknowledge the summons. Another officer quickly stepped up to relieve Spock of his charge, and the Vulcan made his way across the bridge to kneel beside Kirk. "The ship has been secured, sir," he reported. Kirk struggled to sit up, dislodging the square of absorbent bandage pressed to his shoulder. "The Romulan vessel has been destroyed," Spock added as McCoy growled in irritation and jabbed a hypo into Kirk's neck. The captain had time for a surprised gasp before his eyes rolled into the back of his head and he fell back.

"I warned you," McCoy snapped.

"He will be most displeased," Spock commented as McCoy waved down a passing nurse.

"Get me a gurney," McCoy ordered without bothering to respond to Spock. "I want you in sickbay, too," he said as he replaced the bandage, sealing the edges to Kirk's blackened skin. Spock opened his mouth to object, and McCoy snatched up the hypo again to hold it in front of the Vulcan's face. "You're not immune, Spock," he threatened. "Get this mess cleaned up, but I want you down there. God only knows what's wrong with us."

Spock did not have time to argue, for two nurses had arrived with the gurney. He watched as his captain was lifted onto it and carried off the bridge, then walled away his concern and buried it under layers of logic and duty. Four of the surviving Romulan boarding party were unconscious. The fifth was sitting groggily on the deck next to his fellows, with four security officers training phasers on all of them. Perhaps now they would get some answers.

The Romulan looked up as Spock began to approach, and something about his expression stopped the first officer in his tracks. Commander Giotto fired his phaser just as the Romulan snapped his arm upwards, but wasn't in time to prevent him from slapping at something at the collar of his tunic. All five jerked in unison, and became utterly still.

"Sorry, sir," the security chief said, looking away from the corpses in shock. "I didn't …"

Spock waved a hand to quiet him and stared at the dead Romulans with regret. Such a useless waste of life, but not unexpected. "Take them to the morgue," Spock said calmly and turned away from the sight. "Damage report," he ordered as he headed for the command chair. The stunned crew took a moment to gather themselves, then quickly returned to their stations. Some one turned off the red alert. The reports began rolling in while the injured and dead were removed from the bridge.

Spock chose to ignore their confused glances as they returned to their duties. For the crew of the Enterprise, it had been a whirlwind event that, from beginning to end, took less than thirty minutes. No one dared to ask for an explanation, however. A few furtive glances were cast Spock's way, but the Vulcan remained silent, his elbows resting on the arms of the command chair with his fingers laced together and steepled in his manner of deep thought.

Spock had recognized the alien. Their image was scattered through Vulcan's most ancient myths, from a time before they began recording their own history. There were some scholars on Vulcan who believed these images were at least partially true, representing something that had happened far in the planet's past. These scholars were generally ignored, which was the Vulcan equivalent of derision. Spock knew what it was like to have one's theories dismissed out of hand, and strongly wished that he could validate those historians with the knowledge he had obtained. But he could not. He had no doubt that the aliens could and would remove the memory if he did not heed their warning.

Apparently, McCoy had also recognized them.

Spock abruptly got up from the command chair and headed to his science station, ignoring the curious looks of his crewmates. He was plying the consol for information even before he was seated, and did not see Lieutenant Sulu wave people back to their business. The helmsman discretely took over bridge duties, routing various requisitions and reports away from the first officer while Spock worked. Repairs to the bridge were done as quietly as possible over the next several hours, and still Spock did not enlighten the crew.

By that point, however, no one was expecting enlightenment. There was an air of resignation on the part of the crew, as they realized one by one that this was probably going to be one of those classified events that they would never really know about, despite having been present at the time.

Eventually, it was the ship's chief medical officer that broke the Vulcan's concentration. "McCoy to the bridge."

Spock crossed back to the command chair and toggled a switch on the arm without sitting down. "Spock here."

McCoy's voice was laden with exhaustion and annoyance, making his accent exceptionally noticeable. "The captain would like a word, Mister Spock."

"Acknowledged," Spock said and closed the channel.

Spock straitened his shoulders and resisted the urge to sigh. Once he reached sickbay, the doctor would almost certainly remove him from active duty. Spock could feel his own weakness demanding his attention now that the danger had passed. More than the physical exhaustion, it was the mental strain that was forcing Spock to think the doctor would be correct to do so. He desperately needed to meditate, to put his mind to rights. But not before he delivered what he knew would be an unsatisfactory and incomplete report to his captain.

Spock headed for the turbolift. "Mister Sulu," he said as the doors parted for him. He stepped in and turned, his hands clasped behind his back. "You have the bridge."