The Master enters the castle through the impressive double doors, and follows the corridor to the Great Hall. That's why no one was at the front gate, he thinks, they're all stuffing their faces!
Everyone stops their meal as this stranger strides in. Harry, Ron and Hermione exchange anxious looks at one another. This man looks like trouble. The Master saunts up to Dumbledore at the head of the Hall. The Headmaster stands, his face stern.
"Everybody, remain calm! I am The Master. This castle is the grounds of magical power UNTOLD. I wish to make an alliance on behalf of the Time Lords"
Dumbledore raised a brow and spoke softly but firmly, the calm in his voice showing his higher power; after all, words are our most inexhaustible form of magic.
"And what is it that Time Lords wish for these grounds. Everyone here is aware of its vast abilities, the creatures that reside here and the opportunities they have just by being in this castle. No headmaster has ever made an alliance since the four founders of this school."
All eyes were on the stranger at this point. He clearly did not like the Headmaster's rejection. Harry was, as usual in times of uncertainty, in his pocket ready to whip his wand out at the first sign of danger. He saw that Ron, Hermione, Ginny, Fred and George had also developed this habit, and his heart sank a little that his closest friends were so accustomed to danger.
At the rejection of his offer of allegiance, and the failure of his oh so straightforward plan, The Master's anger grew. He pulled out a glowing blue orb, held it up for the Headmaster's view and bellowed
"this device has the ability to wipe the minds of every living creature on this ground! Making them perfect vessels to become my army of magical slaves!" he laughed maniacally.
"Accio." the shimmering ball flew out of the Master's grip, an astonished look on his face, and into the hand of Albus Dumbledore. HHe examined it and chuckled.
"Nox" he said, the power in his still calm voice echoing throughout the hall. All light left the globe, save a small blue flame. Dumbledore smashed the ball on a table and uttered another small but powerful spell to extinguish the flame. "Aquamente"
He smiled at The Master.
"you didn't really believe that would work did you? You see, when The Doctor heard you were plotting something against Hogwarts, he informed me at once. He's very fond of this place, and our puddings" He gave a small wink at Luna Lovegood at the Ravenclaw table at this comment before continuing.
"our grounds keeper, quite possibly the most knowledgeable man about every aspect of the grounds, besides myself of course, out of loyalty and love for this castle, fed you only the good about this land." Dumbledore gestured for The Master to turn around. When he did so, there were several centaurs and thestrals blocking the entrance to the Great Hall, and a snarling black dog in front. Harry smiled as he knew exactly was under that form, and was thankful that no one else did. Dumbledore continued once more.
"he did not tell you, however, the lengths they would go to in order to protect their home. Nor did he give you the correct information on how to erase anyone's memory and take over Hogwarts. A few dazzles of light and a little trickery will get you nowhere Master."
As if on cue, wind filled the Great Hall and the sound of a wailing engine filled the ears of the students, and a blue box materialised directly behind The Master. The doors opened and out popped a smiling young man; the Tenth reincarnation of the Doctor. He beamed up at Dumbledore and mouthed "did I miss the pudding?"
Dumbledore smiled and shook his head, inviting his friend into the castle for some well deserved treats. Dumbledore was his only confidant in his escapades to save as many people as he possibly could. He was always welcome upon these grounds.
The Doctor put his arm around the Master playfully
"foiled again I'm afraid"
The Master, looking gobsmacked, disappointed and livid bellowed in frustration, no words coming to him. After all these years he knew the drill. He trudged begrudgingly into the TARDIS and shut the door.
He walked through the control room to the staircase and followed the corridor to a bedroom, where he leapt to the bed with a pounce to grab the white board marker he had left in a hurry this morning, bounded to the whiteboard excitedly drawing a list of other fandoms he would like to jump to. This was his and the Doctor's new game!
Meanwhile the tenth Doctor stepped gracefully out of his beloved vehicle and strode up to the teachers table where room was being made for him by the Headmaster, a plate of jam roly poly and custard laid for him in wait.