Kreacher, it transpired, had been lurking in the attic. Sirius said he had found him up there, covered in dust, no doubt looking for more relics of the Black family to hide in his cupboard. Though Sirius seemed satisfied with this story, it made Harry uneasy. Kreacher seemed to be in a better mood on his reappearance, his bitter muttering had subsided somewhat and he submitted to orders more docilely than usual, though once or twice Harry caught the house-elf staring at him avidly, but always looking quickly away whenever he saw that Harry had noticed.

Harry did not mention his vague suspicions to Sirius, whose cheerfulness was evaporating fast now that Christmas was over. As the date of their departure back to Hogwarts drew nearer, he became more and more prone to what Mrs Weasley called 'fits of the sullens', in which he would become taciturn and grumpy, often withdrawing to Buckbeak's room for hours at a time. His gloom seeped through the house, oozing under doorways like some noxious gas, so that all of them became infected by it.

Harry didn't want to leave Sirius again with only Kreacher for company; in fact, for the first time in his life, he was not looking forward to returning to Hogwarts. Going back to school would mean placing himself once again under the tyranny of Dolores Umbridge, who had no doubt managed to force through another dozen decrees in their absence; there was no Quidditch to look forward to now that he had been banned, there was every likelihood that their burden of homework would increase as the exams drew even nearer; and Dumbledore remained as remote as ever. In fact, if it hadn't been for the DA, Harry thought he might have begged Sirius to let him leave Hogwarts and remain in Grimmauld Place.

Then, on the very last day of the holidays, something happened that made Harry positively dread his return to school.

'Harry, dear,' said Mrs Weasley poking her head into his and Ron's bedroom, where the pair of them were playing wizard chess watched by Hermione, Ginny and Crookshanks, 'could you come down to the kitchen? Professor Snape would like a word with you.'

Harry did not immediately register what she had said; one of his castles was engaged in a violent tussle with a pawn of Ron's and he was egging it on enthusiastically.

'Squash him — squash him, he's only a pawn, you idiot. Sorry, Mrs Weasley, what did you say?'

'Professor Snape, dear. In the kitchen. He'd like a word.'

Harry's mouth fell open in horror. He looked around at Ron, Hermione and Ginny, all of whom were gaping back at him. Crookshanks, whom Hermione had been restraining with difficulty for the past quarter of an hour, leapt gleefully on to the board and set the pieces running for cover, squealing at the top of their voices.

'Snape?' said Harry blankly.

'Professor Snape, dear,' said Mrs Weasley reprovingly. 'Now come on, quickly, he says he can't stay long.'

'What's he want with you?' said Ron, looking unnerved as Mrs Weasley withdrew from the room. 'You haven't done anything, have you?'

'No!' said Harry indignantly, racking his brains to think what he could have done that would make Snape pursue him to Grimmauld Place. Had his last piece of homework perhaps earned a 'T'?

A minute or two later, he pushed open the kitchen door to find Sirius and Snape both seated at the long kitchen table, glaring in opposite directions. The silence between them was heavy with mutual dislike. A letter lay open on the table in front of Sirius.

'Er,' said Harry, to announce his presence.

Snape looked around at him, his face framed between curtains of greasy black hair.

'Sit down, Potter.'

'You know,' said Sirius loudly, leaning back on his rear chair legs and speaking to the ceiling, 'I think I'd prefer it if you didn't give orders here, Snape. It's my house, you see.'

An ugly flush suffused Snape's pallid face. Harry sat down in a chair beside Sirius, facing Snape across the table.

'I was supposed to see you alone, Potter,' said Snape, the familiar sneer curling his mouth, 'but Black — '

'I'm his godfather,' said Sirius, louder than ever.

'I am here on Dumbledore's orders.' said Snape, whose voice, by contrast, was becoming more and more quietly waspish, 'but by all means stay, Black, I know you like to feel . . . involved.'

'What's that supposed to mean?' said Sirius, letting his chair fall back on to all four legs with a loud bang.

'Merely that I am sure you must feel — ah — frustrated by the fact that you can do nothing useful, ' Snape laid a delicate stress on the word, 'for the Order.'

It was Sirius's turn to flush. Snape's lip curled in triumph as he turned to Harry.

'The Headmaster has sent me to tell you, Potter, that it is his wish for you to study Occlumency this term.'

'Study what?' said Harry blankly.

Snape's sneer became more pronounced.

'Occlumency, Potter. The magical defence of the mind against external penetration. An obscure branch of magic, but a highly useful one.'

Harry's heart began to pump very fast indeed. Defence against external penetration? But he was not being possessed, they had all agreed on that . . .

'Why do I have to study Occlu — thing?' he blurted out.

'Because the Headmaster thinks it a good idea,' said Snape smoothly. 'You will receive private lessons once a week, but you will not tell anybody what you are doing, least of all Dolores Umbridge. You understand?'

'Yes,' said Harry. 'Who's going to be teaching me?'

Snape raised an eyebrow.

'I am,' he said.

Harry had the horrible sensation that his insides were melting.

Extra lessons with Snape — what on earth had he done to deserve this? He looked quickly round at Sirius for support.

'Why can't Dumbledore teach Harry?' asked Sirius aggressively. 'Why you?'

'I suppose because it is a headmaster's privilege to delegate less enjoyable tasks,' said Snape silkily. 'I assure you I did not beg for the job.' He got to his feet. 'I will expect you at six o'clock on Monday evening, Potter. My office. If anybody asks, you are taking remedial Potions. Nobody who has seen you in my classes could deny you need them.'

He turned to leave, his black travelling cloak billowing behind him.

'Wait a moment,' said Sirius, sitting up straighter in his chair.

Snape turned back to face them, sneering.

'I am in rather a hurry, Black. Unlike you, I do not have unlimited leisure time.'

'I'll get to the point, then,' said Sirius, standing up. He was rather taller than Snape who, Harry noticed, balled his fist in the pocket of his cloak over what Harry was sure was the handle of his wand. 'If I hear you're using these Occlumency lessons to give Harry a hard time, you'll have me to answer to.'

'How touching,' Snape sneered. 'But surely you have noticed that Potter is very like his father?'

'Yes, I have,' said Sirius proudly.

'Well then, you'll know he's so arrogant that criticism simply bounces off him,' Snape said sleekly.

Sirius pushed his chair roughly aside and strode around the table towards Snape, pulling out his wand as he went. Snape whipped out his own. They were squaring up to each other, Sirius looking livid, Snape calculating, his eyes darting from Sirius's wand-tip to his face.

'Sirius!' said Harry loudly, but Sirius appeared not to hear him.

'I've warned you, Snivelus,' said Sirius, his face barely a foot from Snape's, 'I don't care if Dumbledore thinks you've reformed, I know better — '

'Oh, but why don't you tell him so?' whispered Snape. 'Or are you afraid he might not take very seriously the advice of a man who has been hiding inside his mother's house for six months?'

'Tell me, how is Lucius Malfoy these days? I expect he's delighted his lapdog's working at Hogwarts, isn't he?'

'Speaking of dogs,' said Snape softly, 'did you know that Lucius Malfoy recognised you last time you risked a little jaunt outside? Clever idea, Black, getting yourself seen on a safe station platform . . . gave you a cast-iron excuse not to leave your hidey-hole in future, didn't it?'

Sirius raised his wand.

'NO!' Harry yelled, vaulting over the table and trying to get in between them. 'Sirius, don't!'

'Are you calling me a coward?' roared Sirius, trying to push Harry out of the way, but Harry would not budge.

'Why, yes, I suppose I am,' said Snape.

'Harry — get — out — of — it!' snarled Sirius, pushing him aside with his free hand.

The kitchen door opened and the entire Weasley family, plus Hermione, came inside, all looking very happy, with Mr Weasley walking proudly in their midst dressed in a pair of striped pyjamas covered by a mackintosh.

'Cured!' he announced brightly to the kitchen at large. 'Completely cured!'

He and all the other Weasleys froze on the threshold, gazing at the scene in front of them, which was also suspended in mid-action, both Sirius and Snape looking towards the door with their wands pointing into each other's faces and Harry immobile between them, a hand stretched out to each, trying to force them apart.

'Merlin's beard,' said Mr Weasley, the smile sliding off his face, 'what's going on here?'

Both Sirius and Snape lowered their wands. Harry looked from one to the other. Each wore an expression of utmost contempt, yet the unexpected entrance of so many witnesses seemed to have brought them to their senses. Snape pocketed his wand, turned on his heel and swept back across the kitchen, passing the Weasleys without comment. At the door he looked back.

'Six o'clock, Monday evening, Potter.'

And he was gone. Sirius glared after him, his wand at his side.

'What's been going on?' asked Mr Weasley again.

'Nothing, Arthur,' said Sirius, who was breathing heavily as though he had just run a long distance. 'Just a friendly little chat between two old school friends.' With what looked like an enormous effort, he smiled. 'So . . . you're cured? That's great news, really great.'

'Yes, isn't it?' said Mrs Weasley, leading her husband forward to a chair. 'Healer Smethwyck worked his magic in the end, found an antidote to whatever that snake's got in its fangs, and Arthur's learned his lesson about dabbling in Muggle medicine, haven 't you, dear?' she added, rather menacingly.

'Yes, Molly dear,' said Mr Weasley meekly.

'That night's meal should have been a cheerful one, with Mr Weasley back amongst them. Harry could tell Sirius was trying to make it so, yet when his godfather was not forcing himself to laugh loudly at Fred and George's jokes or offering everyone more food, his face fell back into a moody, brooding expression. Harry was separated from him by Mundungus and Mad-Eye, who had dropped in to offer Mr Weasley their congratulations. He wanted to talk to Sirius, to tell him he shouldn't listen to a word Snape said, that Snape was goading him deliberately and that the rest of them didn't think Sirius was a coward for doing as Dumbledore told him and remaining in Grimmauld Place. But he had no opportunity to do so, and, eyeing the ugly look on Sirius's face, Harry wondered occasionally whether he would have dared to mention it even if he had the chance. Instead, he told Ron and Hermione under his voice about having to take Occlumency lessons with Snape.