A/N: Hello, everyone! Thank you for the awesome reviews! Just a heads up, I'm going to jump around in time a bit throughout this fic, so it's not always going to be streamlined like when I had Kirk at a medical conference after McCoy threatened to drag him along in a previous chapter. Just so no one gets confused! This chapter takes place during the episode "Return to Tomorrow". I used direct quotes in here (un-italicized), but I don't own Star Trek!

Hippocratic Oath

Most non-medical personnel who knew of the Hippocratic Oath in the 23rd century could only about recite the 'do no harm' part. There were other concepts in there that certainly bore examination.

For McCoy, he found himself dwelling a lot on the part "above all else, I shall not play God". He knew, rather intimately, the consequences of crossing that particular line. Despite all his success as the Enterprise's Chief Medical Officer, the death of his father still haunted him late into the night.

And so he took care to ensure that his staff knew where that line was and that they never, ever crossed it. For the most part, things ran smoothly.

Then there were instances like these where some cough-uninvited-cough guest came aboard and tried to muck with it. McCoy had not seen the true form of this particular "guest" save for a faintly glowing sphere and the body of Dr. Ann Mulhall.

"I require your silence. Only you and I would know that Dr. Mulhall has not returned to her body." Then the clincher: "Isn't that worth your captain's life?"

He looked back at Jim- Jim's body, being kept alive by machines as his consciousness sparkled beside him. God forgive him, but he would do just about anything to get him back on his feet. What would the Enterprise do without Captain Kirk? McCoy didn't follow other commanders nearly as well as he did Jim.

He… was tempted.

Thalassa stepped closer to him. "Doctor," she spoke. "We can take what we wish."

That brought McCoy back to the reality of her words. Super-evolved beings his ass, they always wanted something, they always let their superpowers go to their heads. He'd known from the beginning that Sargon and his trio were dangerous, whatever fanciful promises of technology and benevolence they spouted. He narrowed his eyes.

"Neither you, this ship, nor all your worlds have the power to stop us," she continued.

Still, super-beings on a power trip reminded him of how small he was. How primitive and insignificant.

How humble and human.

"Neither Jim nor I can trade a body we don't own. It happens to belong to a young woman."

"Who you hardly know. Almost a stranger!"

That had been about what did him in earlier. He barely interacted with Ann Mulhall. It would have been… easy… to look the other way. But her point wasn't actually a point, just a reminder of why words like I shall not play God existed.

"I will not peddle flesh," he said sternly. "I'm a physician."

God help him, he was. He was a doctor. It wasn't his place to restore Jim's life at the cost of Ann's. He didn't need two people keeping him up at night, pleading for him to correct irreparable mistakes.

He'd made his decision. He wasn't going to tolerate this megalomania any further. McCoy started to leave because he had to get out of that room, away from that shameful moment of temptation.

He didn't make it. Why did no one ever listen to his warnings about god-like beings? Sooner or later they wound up hurting somebody, and in this case it was him.

He kept it to himself as the mission concluded. Jim was restored, Spock unbelievably spared, and Ann was herself. No one else was badly hurt, cases of "poisoning" aside. McCoy gave them all a healthy rant on why the dangers of possession alone were enough to make an idea bad, and reiterated I told you so on a number of occasions.

Jim took it with an apologetic smile and a promise to be more wary next time ("there won't BE a next time, Jim!"). Spock took it all silently, only raising an eyebrow at his choice of curses. He couldn't tell if Ann Mulhall understood the real reason behind his ranting or if she was painfully oblivious to Thalassa's offer.

McCoy took an oath. He had broken it once. He had vowed never do it a second time.