The Doctor has discovered something.
The path leading up to that discovery began with him fixing on finding out a way to stop himself being annoyed by the Master. He'd tried ignoring him, but that had only annoyed the Master, something he didn't want to do again if he valued his eyebrows. He'd tried hiding from him, but that only resulted in the Master crowing over a megaphone that he'd found somewhere, about how he was scared of the Master. He obviously couldn't have that assumption becoming fact, and had therefore had to reveal himself, which resulted in the annoyance starting again.
He knew it was because the Master was bored. Who wouldn't be, stuck in a TARDIS which was floating aimlessly in the Vortex? He'd attempted to get the Master a hobby. He'd chucked some wires and circuitry his way, and asked him to fix a few parts of the TARDIS. It seemed to work for a while, seeing as he made the Master start small, with the heating, and build up to the big stuff. Unfortunately, when the Master was allowed onto the "big stuff" it ended in the TARDIS having no gravity for ten days. It was not something the Doctor wanted to repeat.
The second hobby he'd tried to force upon the Master was that of writing. It was quickly ended, however, by the Master setting himself the target of learning to write swear words in two million different languages, and posting them up all over the TARDIS.
The Master had always been handy with computers, so the Doctor gave him one, along with an assortment of games. Unfortunately, the games were designed for the Doctor's skill level, so the Master defeated them rather quickly.
By this time, the Doctor had arrived at the somewhat deserved conclusion that the Master was annoying him on purpose. When drawn on this, the Master put on such a hurt and innocent expression that the Doctor, naive as he was, just hadto believe him. So it was quite a surprise that evening when he fell over a trip wire strung across the corridor outside his room, and ended up in a disposal chute. The shouting match that followed that incident had not been pretty, and they both resolved never to talk of it again.
But then, the Doctor had discovered the "something" that would make the Master shut up. It wasn't amusement, it wasn't a hobby, it wasn't even threatening to throw him out of an airlock if things got too bad.
The Doctor didn't like to think back to when he'd first used this method, as that led him to the question of how the hell he'd found out it actually worked in the first place, which was quite an uncomfortable subject for him at best.
The Master, who seemed strangely oblivious to this thing that the Doctor could always use to stop him, no matter the situation, was caught completely off guard one morning.
The Doctor had been in the TARDIS kitchen, halfway through deciding whether pancakes or cereal would be good for breakfast, and the Master, dressed in only a dressing gown, was heading towards him with the single aim of annoying him by ruffling his hair.
Except the Doctor, having what appeared to be Time Lord detecting device he could turn on and off at will, turned round just as the Master's hand was being raised. The Master found himself on the receiving end of a deeply disapproving look.
"Don't touch the hair."
The Master scowled, though he didn't really mean it, they both knew that. "Or what?"
"I'll use my secret weapon," threatened the Doctor.
The Master laughed. "And what's that? Cut off my supply of porn? Take away my Ipod? Oh, Doctor, you wouldn't?!"
The Doctor was now in two minds whether to actually use his "weapon". On one mind, he desperately needed some peace and quiet to get some serious reading done, and he would never get that if the Master continued to annoy him. On the other, there would be no going back if he did use the weapon and, more importantly, the peace and quiet would most probably be short-lived.
The decision was made for him as the Master's hand snaked back out and ruffled his hair. Incensed at this blatant disregard for the rules that he'd set the day the Master had come aboard the TARDIS, the Doctor shoved the Master away, carefully smoothing his hair back into the haphazard lifestyle it was so accustomed to.
"That's it!" The Doctor glowered at the smirking Master, who had only just avoided landing in the butter dish. "I hereby invoke the ancient Time Lord right as the owner of this TARDIS."
The Master's eyes narrowed. "You wouldn't."
The Doctor wagged a finger at him. "Oh yes, I would." He put on a stern expression. "You've had plenty of warnings. It's time you paid the price for your actions."
The Master crossed his arms, a picture of defiance. "I won't!" He had the distinct urge to stamp his foot and, even though it probably wouldn't help his cause, he did, simply because he wanted to.
"You will!" The Doctor pointed towards the kitchen door. "You'll go right now. Don't make me count to three!"
The Master glowered at him.
"One," said the Doctor.
The Master continued to glower, pausing only to briefly scratch his nose.
"Two," the Doctor growled, shaking his head.
The Master's eyebrows unknotted, being replaced by a pleading look. "I didn't mean it."
"Don't make me say three."
"Oh, come on. I said I didn't mean it. What more do you want?"
The Doctor blinked, a hundred possibilities running through his mind, some of them sensible, but a few of them definitely unethical considering the Master was effectively his prisoner. "A verbal apology."
The Master scoffed. "You don't get one of them without a lot of work on your part."
"Three," the Doctor thundered, though he could have thundered a lot louder if he truly wanted to.
The Master yelped, moving away from the Doctor, who began to pace forward. They then performed a dance around the kitchen table, the Master managing to evade the Doctor's grip, until he accidently knocked the toaster over. It, being a decidedly vengeful toaster, landed on the Master's foot. While he was hopping around in pain, the Doctor caught him which, the Master thought, was decidingly unsporting.
He continued to think this as the Doctor manhandled him through the TARDIS corridors. He almost escaped near the boiler room but, having nowhere to run, decided on letting the Doctor keep ahold of him. It was completely voluntary, you understand.
Having rather failed at breaking free, the Master fell back on what the Doctor always used in these situations. "I'm sure we can talk about this."
The Doctor tugged him around another corner. "You had your chance. You chose to break the rules."
"Rules?" the Master scoffed. "Get over yourself, you skinny wreck."
This, as it turned out, was the wrong thing to say. The Master couldn't imagine why, but it just was.
"You touched my hair," the Doctor stated. "You therefore will be punished."
The Master felt like banging his head against a brick wall. No, he felt like banging the Doctor's head against a brick wall. Slowly and painfully, if possible.
"It's hair. It's not even alive!"
The Doctor growled threateningly. "It is a part of me. When I became this body, I became my hair. You insult my hair, you insult me. It is a part of me, just as much as that stupid Ipod is a part of you."
The Master wasn't amused by this at all. "Don't insult my Ipod, you non-ginger idiot!"
This was possibly the worst thing the Master could have said in any situation. The Doctor is very sensitive about his non-gingerness, you see, so he was perfectly permitted to turn to the Master and whack him around the head, which he did, before throwing him into the Master's room.
The Master landed on the floor, whipping his head around as the Doctor pulled the door closed. As fast as the Master was, he wasn't quick enough to get up, make his way across to the door, and get it open, while still following the no-running rule the Doctor had set after that unfortunate incident with the goldfish they'd had for a while, before the Doctor had locked the door.
"Oh, you didn't!" The Master kicked the door. Unsurprisingly, it didn't make him or the door feel any better.
"I did," came the Doctor's retort. "Now, I'm going to leave you here for half an hour. Think about what you did."