"Who do you suppose that man was who was wandering around by the kitchens this morning?" Elen of Mercia wondered aloud.

Raghnall MacDhòmhnaill glanced up from his porridge. "What man?"

"Didn't you see him?" said Elen. "Tall, grey hair, not particularly handsome. He passed me in the corridor while I was coming down to breakfast; none of the house-elves seemed to know who he was."

"Well, that's odd," said Raghnall. "What about his robe? Was he wearing any of the Heads' colours?"

Elen shook her head. "No, just an ordinary grey robe," she said. "And I don't think he was working for that new Audiencer of King Constantine's; he didn't have the Dùn-Eidann air about him. Mother didn't know who he was, either," she added. "I asked her."

Raghnall rolled his eyes. He liked Elen of Mercia – she had an active mind and a mischievous soul, and both made a pleasant contrast to his own personality – but her pretensions could be rather trying sometimes. Going around calling herself "Helena Ravenclaw" and referring to Professor Ravenclaw as her mother, just because Her Ladyship had agreed to take care of her during the summer: it had been endearing at one time, but now it was getting old. (Still, he understood it. If she didn't talk that way, she would be forced to face the fact that Professor Ravenclaw had taken her in out of sympathy, because Elen, like her, was an orphan – and Elen was proud; she would never remind herself that she was accepting charity if she could possibly help it.)

"Well, I'm sure there's some explanation," he said. "Maybe Professor Slytherin's planning something, and this man's one of his assistants."

Elen started, and glanced at the perpetual sundial in the middle of the Great Hall floor (a construction of Professor Gryffindor's, which always had a shadow pointing the appropriate hour, even on grey, overcast days like today). "Speaking of Professor Slytherin, we're supposed to be in Transfiguration in about five minutes," she said. "I can't imagine how I lost track so badly. Are you about finished eating?"

Raghnall nodded glumly. In fact, he had deliberately been eating as slowly as possible, since he didn't want to enter the Transfiguration classroom until he absolutely had to. He knew perfectly well that Professor Slytherin was contemptuous of him: of his diffidence, his abysmal lack of skill at Transfiguration, and (not incidentally, from the old Parselmouth aristocrat's vantage point) his Earth-Born ancestry. The three other Heads were all willing to acknowledge that his magical clumsiness was a purely personal characteristic, but Slytherin, who believed more firmly in the existence of an independent wizarding race descended from Väinämöinen than most people believed in the sun, persisted in regarding his failings as the typical inadequacies of one not born to magic. It was quite unpleasant, and Raghnall had come to dread Transfiguration classes as a gastritic dreads mealtime – which, he was well aware, only made matters worse.

Still, it had to be faced sooner or later. Wearily, he stood up from his seat at the long table – and then nearly collapsed onto the floor as a sudden wave of dizziness and nausea came over him. He felt as though he had swallowed a toadstool – or perhaps one of Professor Hufflepuff's emetic potions.

"Elen?" he said faintly. "I think I'm going to be sick."

Elen sighed. "Now, honestly, Raghnall," she said, "I know Professor Slytherin's a bit intimidating, but there's no need to..."

"No, really," said Raghnall. "It's not Professor Slytherin, it's just... I..." His voice trailed off as he felt a sudden warming sensation in the back of his throat. Hastily, he leaned over his empty porridge bowl, just in time to refill it with the material he had just eaten out of it.

Elen paled, and took a step backwards. "Oh, ick," she said with feeling. "What did you do to yourself, Raghnall?"

Raghnall shook his head dumbly. His mind, never quick at the best of times, was now focussing all its attention on staying abreast of the next upheaval; he had no mental energy left over to search for root causes of his malady. (Even if he had, the search would have been fruitless. He hadn't eaten anything that morning except that one bowl of porridge, and he'd never heard of porridge doing this before unless it was thoroughly rancid – which his most certainly hadn't been, though it had admittedly tasted a little odd.)

"Must be grippe or something," said Elen. "Stay there; I'll go find Professor Hufflepuff."

She darted from the Great Hall, and Raghnall was left alone. For a moment, the semi-gratifying thought crossed his mind, Well, that's one way to get out of Transfiguration, but then this and all other thoughts were driven from his mind by a second eruption of semi-digested oatmeal. He moaned softly and braced himself for what promised to be a long siege in the sickroom.

Andrew Harlan walked beneath the shadow of the Castle on the Hog's Warts, his nondescript grey robe fluttering about his legs as he headed back to the track-shifter. He had done what he came to do; there was no sense in staying around longer than necessary, and possibly letting 10th-Century wizards – even fictional ones – trip over a piece of 111,394th-Century technology.

He reflected, as he had many times before, that Noÿs and her fellow-Temporals were remarkably generous souls. Despite the horror and detestation with which they viewed the very concept of Eternity, they had recognised that an Eternal's habits die hard, and had given him a playground of non-existent Realities, derived from classic works of Primitive literature, to keep him from getting restless in the new, Eternity-free history. He could Observe, calculate Reality Changes, execute them, and feel the accustomed warm glow of satisfaction at having made the world a better place, all without affecting the Basic State in the least.

This, he thought with satisfaction, was a particularly elegant calculation on his part. A few flakes of emetic powder sprinkled on a single young wizard's breakfast porridge, and the young wizard in question was prevented from showing up at Transfiguration that morning, which meant that Rowena Ravenclaw would never have her life threatened by the reptilian monstrosity that he was supposed to accidentally turn a sparrow into. This, in turn, meant that Salazar Slytherin's contempt for Muggle-born wizards would remain a private prejudice, rather than being enflamed by his unconfessed love for Ravenclaw into an obsession that would eventually drive him from the school.

The four Houses would still be instituted eventually, of course (at least, there was an 87.4% probability that they would be), but they would be houses, not rival philosophies – and, in particular, the existence of Slytherin House would not serve to legitimise an extreme racialist theory. As a result, the whole notion of "blood purity" would be relegated to the sidelines of wizarding history, until eventually it went the way of Wager of Battle and other quaint mediæval notions – and Tom Riddle, though he would still murder and make Horcruxes, would be unable to turn his quest for personal immortality into a mass movement threatening all of wizarding Britain. A good day's work, all in all.

He had reached the track-shifter, and slipped into its cockpit with a sigh of contentment. I wonder what my next task will be, he thought as he set a course back to Reality. Star Wars, maybe; there must be some way of keeping Anakin Skywalker from becoming Darth Vader...

Disclaimer: J. K. Rowling: Harry Potter. Isaac Asimov: The End of Eternity. Me: zip.