Snape had never understood what had possessed The Founders to make them build the school in Scotland, of all places. The weather was unbelievably vile, and having been born outside Manchester, Snape could believe quite a lot. He'd never seen anywhere else where it rained right to left. At least this meant that Quidditch practice was cancelled, which was a temporary relief since he'd soon realised that if he didn't catch the phantom curser, he was going to have to stand watch every time Harry got on a broom.

His best endeavours to discover who had sat behind him had revealed nothing more than a few vague comments about a woman, variously described as "Fat", "Thin", "Young". "Quite old", and "I think I've seen her in Hogsmeade". Which left him just about where he'd started – Quirrell, Dervish and a possibly imaginary woman as suspects. He'd asked Minerva but she'd soon admitted that her eye-sight was so poor that she'd been unable to see the broom at all, it was so high, and couldn't even confirm that it had been bespelled in the first place. In any event she'd been too busy casting cushioning charms to look about her.

It was utterly infuriating, made more so by the fact that his House was playing up and he was having to spend far too much time reducing it to order. Someone was playing silly devils and while they were welcome to put as much ground nettle in Malfoy's underwear as they liked, he needed them to leave Janice Mulciber and the eldest Moriarty alone. Both of them came from families that had formerly supported the Dark Lord, and if Dumbledore was right and He was slated to return, they both ran the risk of being recruited as Deatheaters as soon as He showed what remained of His face. Snape had spent a considerable amount of time and effort making sure the two young idiots were aware of options which didn't involve the Mark and he'd been reasonably confident he'd steered them away from it. Now some blithering nincompoop was sabotaging his efforts with a selection of nasty pranks which some other blithering nincompoop had ascribed to the Griffindors, inciting a wave of anger and self-righteousness which just might drive his two escapees back into the bosom of the Dark.

It had taken him two days to discover the culprit was the younger Mulciber boy and three to drill it into Slytherin's hotter heads that he was to blame, though how they thought the Griffs had managed to get access to Malfoy's underwear he did not know.

Any more than he knew what Julius Mulciber thought he was doing attacking Malfoy – admittedly Draco was as irritating as hell, but their fathers were allies and the power was all on the Mafoy's side. Detention was likely to be the least of young Mulciber's problems when he went home for Christmas.

On Friday he left breakfast to find Malfoy picking a fight with a Raven twice his size, probably because the girl was muggleborn. Of course he had to intervene, and of course he had to take a disproportionate number of points off her – he'd never keep control of his House if he didn't. He was just sorry that he could only let her get one good kick in where it would do most good before he intervened. He'd sent Malfoy off to the Infirmary with a note to let excuse him from his next lesson. The little sod could limp up there, he wasn't going to heal him.

Then to cap a truly beautiful week, he'd been walking through the main hall, half an hour before dinner and came across the Griffindor Quidditch Team, Granger and the youngest Weasley, soaked to the skin and blue with cold, coming in from practice.

There was, he reflected afterwards, something deeply satisfying about achieving multiple goals at once with a single action:

he (Snape) was relieved of guard duty in the pouring rain;

the older Griffindors would think he was sabotaging the team and be wildly indignant for days;

Harry was kept safe; and

Harry would be forced to admit that Snape was acting in his best interests – even if he didn't particularly like being prevented from practising. Although judging by his shivering, he might not be that bothered.

However, somewhat to his surprise, Quidditch practice was not what Harry wished to talk about when they met up on Saturday afternoon. They had their usual hot chocolate and, in Harry's case, éclairs and Snape was all set to begin an encouraging conversation about the place of originality in potion brewing, when Harry swallowed an enormous bite of his confectionery and said, "Professor, why did you take all those points off Joanne yesterday? It was all Malfoy's fault – he was being horrible about her parents."

Snape sighed. "Malfoy is always being horrible to someone, and there is little chance he will ever stop. Sooner or later he will say something to someone much bigger and much angrier and will end up in the infirmary – possibly with something irreparable. I am merely attempting to ensure that it is well known throughout the school that attacking him is not in anyone's best interests." No matter how tempting it is, he added to himself. Not the whole truth, of course, but possibly enough to satisfy a child as young as Harry, at least for the time being.

"Oh is that why?" said Harry, taking another éclair. "I did wonder. Lots of people think he's your godson and that's why you protect him so much."

Only a lifetime's practice of self-control saved Snape from a nose-full of hot chocolate; as it was he was coughing for several minutes. When he finally regained control, he made haste to clear up that little misconception. "Harry," he said, "What is the first thing you found out about Draco?"

"Um... that he's very snobby?"

"And what is he er...snobby about?"

"His family?"

Harry was starting to have the distinctly hunted look of a pupil surprised by a test, so Snape took over. "The Malfoys are a very old magical family, every member of which has been magical for many generations back." Apart from the squibs they had no doubt disposed of through the years, though there was no point distressing the boy about that just yet. "Such families call themselves "pure bloods" and are ridiculously proud of the fact. My father was a muggle, that makes me a "half-blood" in their terminology, and they would no more give Draco a half-blood godfather than they would give him a muggle one."

Harry looked disgusted. "That's just racist," he said.

"What does 'racist' mean?"

"Oh we talked about it a lot at my old school. People who hate other people because of the colour of their skin."

"How peculiar," said Snape. "Amongst magical people, snobbery is based on magical ancestry, the longer your family has been magical, the better. So far as they are concerned of course."

"But it doesn't make a difference to people's magic, does it? Because Hermione is ever so good." The boy looked positively indignant, which Snape found oddly touching.

"No, it makes no difference."

They sat in comfortable silence for a moment and Harry took another éclair. "By the way," he said. "Thanks for stopping the practices. I love Quidditch but it's hopeless trying to play when it's raining like this. I was wet right through to my vest and pants. I think Oliver is a bit mad making us play in this."

"You are quite welcome, Harry." Now what was the right thing to do? Did he frighten the boy by putting him on his guard or leave him ignorant but vulnerable. Oh hell! "It wasn't just because of the rain I stopped the practices. You do know that brooms are not supposed to act like yours did during the match?"

Harry nodded.

"I'm very much afraid that someone was cursing it while you were playing."

Harry looked unconcerned. "Yes," he said, "I know. Hermione stopped him." He stopped and stared at Snape, his eyes wide. "I mean..."

Snape pinched the bridge of his nose. "Harry, I have spent most of this week attempting to find out who was cursing your broom. Are you telling me you already know? Why didn't you tell someone?"

Harry was actually wringing his hands, smearing chocolate from his éclairs all over them. "It was a teacher and Hermione set fire to him!"

Snape conjured a damp cloth and handed it to the boy to wipe his hands before he smeared chocolate all over the sofa. "Harry, we are a group of teachers not a... a... band of robbers. If a teacher is trying to kill a student, we're not going to try to protect him, of course we want to know."

"But Hagrid didn't believe us, we didn't think anyone else would either and Hermione set fire to him!"

He looked so young, so small and so scared that Snape was conscious of a strange hollow sensation under his sternum. He vanished the wet cloth with a wave of his wand and, leaning forward, took the boy's damp hands in his. "I would have believed you, I promise." He shook the hands until Harry reluctantly nodded. "And I suppose that is why Miss Granger and Mr Weasley were out there in the pouring rain? You have two excellent friends there, Harry." He sat back. " Now, the headmaster is in London until Monday on business for the International Confederation of Wizards. I will speak to him as soon as he returns. In the meantime, I will walk you back to your common room when we finish here and I want you to promise you won't leave it again without at least three or four other people."

"I promise," said Harry, looking relieved. He settled back into the sofa and took another éclair: Snape hurriedly vanished the rest of the cakes. His evanesco had never been very good with vomit.