'Yes, it's on Valentines Day . . .'

'Right,' said Harry, wondering why she was telling him this. 'Well, I suppose you want to — ?'

'Only if you do,' she said eagerly.

Harry stared. He had been about to say, 'I suppose you want to know when the next DA meeting is?' but her response did not seem to fit.

'I — er — ' he said.

'Oh, it's OK if you don't,' she said, looking mortified. 'Don't worry. I — I'll see you around.'

She walked away. Harry stood staring after her, his brain working frantically. Then something clunked into place.

'Cho! Hey — CHO!'

He ran after her, catching her halfway up the marble staircase.

'Er — d'you want to come into Hogsmeade with me on Valentine's Day?'

'Oooh, yes!' she said, blushing crimson and beaming at him.

'Right . . . well . . . that's settled then,' said Harry, and feeling that the day was not going to be a complete loss after all, he virtually bounced off to the library to pick up Ron and Hermione before their afternoon lessons.

By six o'clock that evening, however, even the glow of having successfully asked out Cho Chang could not lighten the ominous feelings that intensified with every step Harry took towards Snape's office.

He paused outside the door when he reached it, wishing he were almost anywhere else, then, taking a deep breath, he knocked and entered.

The shadowy room was lined with shelves bearing hundreds of glass jars in which slimy bits of animals and plants were suspended in variously coloured potions. In one corner stood the cupboard full of ingredients that Snape had once accused Harry — not without reason — of robbing. Harry's attention was drawn towards the desk, however, where a shallow stone basin engraved with runes and symbols lay in a pool of candlelight. Harry recognised it at once — it was Dumbledore's Pensieve. Wondering what on earth it was doing there, he jumped when Snape's cold voice came out of the shadows.

'Shut the door behind you, Potter.'

Harry did as he was told, with the horrible feeling that he was imprisoning himself. When he turned back into the room, Snape had moved into the light and was pointing silently at the chair opposite his desk. Harry sat down and so did Snape, his cold black eyes fixed unblinkingly upon Harry, dislike etched in every line of his face.

'Well, Potter, you know why you are here,' he said. 'The Headmaster has asked me to teach you Occlumency. I can only hope that you prove more adept at it than at Potions.'

'Right,' said Harry tersely.

'This may not be an ordinary class, Potter,' said Snape, his eyes narrowed malevolently, 'but I am still your teacher and you will therefore call me "sir" or "Professor" at all times.'

'Yes . . . sir, ' said Harry.

Snape continued to survey him through narrowed eyes for a moment, then said, 'Now, Occlumency. As I told you back in your dear godfather's kitchen, this branch of magic seals the mind against magical intrusion and influence.'

'And why does Professor Dumbledore think I need it, sir?' said Harry looking directly into Snape's eyes and wondering whether Snape would answer.

Snape looked back at him for a moment and then said contemptuously, 'Surely even you could have worked that out by now, Potter? The Dark Lord is highly skilled at Legilimency — '

'What's that? Sir?'

'It is the ability to extract feelings and memories from another person's mind — '

'He can read minds?' said Harry quickly, his worst fears confirmed.

'You have no subtlety, Potter,' said Snape, his dark eyes glittering. 'You do not understand fine distinctions. It is one of the shortcomings that makes you such a lamentable potion-maker.'

Snape paused for a moment, apparently to savour the pleasure of insulting Harry, before continuing.

'Only Muggles talk of "mind-reading". The mind is not a book, to be opened at will and examined at leisure. Thoughts are not etched on the inside of skulls, to be perused by any invader, the mind is a complex and many-layered thing, Potter — or at least, most minds are.' He smirked. 'It is true, however, that those who have mastered Legilimency are able, under certain conditions, to delve into the minds of their victims and to interpret their findings correctly. The Dark Lord, for instance, almost always knows when somebody is lying to him. Only those skilled at Occlumency are able to shut down those feelings and memories that contradict the lie, and so can utter falsehoods in his presence without detection.'

Whatever Snape said, Legilimency sounded like mind-reading to Harry, and he didn't like the sound of it at all.

'So he could know what we're thinking right now? Sir?'

'The Dark Lord is at a considerable distance and the walls and grounds of Hogwarts are guarded by many ancient spells and charms to ensure the bodily and mental safety of those who dwell within them,' said Snape. 'Time and space matter in magic, Potter. Eye contact is often essential to Legilimency.'

'Well then, why do I have to learn Occlumency?'

Snape eyed Harry, tracing his mouth with one long, thin finger as he did so.

'The usual rules do not seem to apply with you, Potter. The curse that failed to kill you seems to have forged some kind of connection between you and the Dark Lord. The evidence suggests that at times, when your mind is most relaxed and vulnerable — when you are asleep, for instance — you are sharing the Dark Lord's thoughts and emotions. The Headmaster thinks it inadvisable for this to continue. He wishes me to teach you how to close your mind to the Dark Lord.'

Harry's heart was pumping fast again. None of this added up.

'But why does Professor Dumbledore want to stop it?' he asked abruptly. 'I don't like it much, but it's been useful, hasn't it? I mean . . . I saw that snake attack Mr Weasley and if I hadn't, Professor Dumbledore wouldn't have been able to save him, would he? Sir?'

Snape stared at Harry for a few moments, still tracing his mouth with his finger. When he spoke again, it was slowly and deliberately, as though he weighed every word.

'It appears that the Dark Lord has been unaware of the connection between you and himself until very recently. Up till now it seems that you have been experiencing his emotions, and sharing his thoughts, without his being any the wiser. However, the vision you had shortly before Christmas — '

'The one with the snake and Mr Weasley?'

'Do not interrupt me, Potter,' said Snape in a dangerous voice. 'As I was saying, the vision you had shortly before Christmas represented such a powerful incursion upon the Dark Lord's thoughts — '

'I saw inside the snake's head, not his!'

'I thought I just told you not to interrupt me, Potter?'

But Harry did not care if Snape was angry; at last he seemed to be getting to the bottom of this business; he had moved forwards in his chair so that, without realising it, he was perched on the very edge, tense as though poised for flight.

'How come I saw through the snake's eyes if it's Voldemort's thoughts I'm sharing?'

'Do not say the Dark Lord 's name! ' spat Snape.

There was a nasty silence. They glared at each other across the Pensieve.

'Professor Dumbledore says his name.' said Harry quietly.

'Dumbledore is an extremely powerful wizard,' Snape muttered. 'While he may feel secure enough to use the name . . . the rest of us . . .' He rubbed his left forearm, apparently unconsciously, on the spot where Harry knew the Dark Mark was burned into his skin.

'I just wanted to know,' Harry began again, forcing his voice back to politeness, 'why — '

'You seem to have visited the snake's mind because that was where the Dark Lord was at that particular moment,' snarled Snape. 'He was possessing the snake at the time and so you dreamed you were inside it, too.'

'And Vol — he — realised I was there?'

'It seems so,' said Snape coolly.

'How do you know?' said Harry urgently. 'Is this just Professor Dumbledore guessing, or — ?'

'I told you,' said Snape, rigid in his chair, his eyes slits, 'to call me "sir".

'Yes, sir,' said Harry impatiently, 'but how do you know — '?

'It is enough that we know,' said Snape repressively. 'The important point is that the Dark Lord is now aware that you are gaining access to his thoughts and feelings. He has also deduced that the process is likely to work in reverse; that is to say, he has realised that he might be able to access your thoughts and feelings in return — '

'And he might try and make me do things?' asked Harry. 'Sir? ' he added hurriedly.

'He might,' said Snape, sounding cold and unconcerned. 'Which brings us back to Occlumency.'

Snape pulled out his wand from an inside pocket of his robes and Harry tensed in his chair, but Snape merely raised the wand to his temple and placed its tip into the greasy roots of his hair. When he withdrew it, some silvery substance came away, stretching from temple to wand like a thick gossamer strand, which broke as he pulled the wand away from it and fell gracefully into the Pensieve, where it swirled silvery-white, neither gas nor liquid. Twice more, Snape raised the wand to his temple and deposited the silvery substance into the stone basin, then, without offering any explanation of his behaviour, he picked up the Pensieve carefully, removed it to a shelf out of their way and returned to face Harry with his wand held at the ready.

'Stand up and take out your wand, Potter.'

Harry got to his feet, feeling nervous. They faced each other with the desk between them.

'You may use your wand to attempt to disarm me, or defend yourself in any other way you can think of,' said Snape.

'And what are you going to do?' Harry asked, eyeing Snape's wand apprehensively.

'I am about to attempt to break into your mind,' said Snape softly. 'We are going to see how well you resist. I have been told that you have already shown aptitude at resisting the Imperius Curse. You will find that similar powers are needed for this . . . brace yourself, now. Legilimens! '

Snape had struck before Harry was ready, before he had even begun to summon any force of resistance. The office swam in front of his eyes and vanished; image after image was racing through his mind like a flickering film so vivid it blinded him to his surroundings.

He was five, watching Dudley riding a new red bicycle, and his heart was bursting with jealousy . . . he was nine, and Ripper the bulldog was chasing him up a tree and the Dursleys were laughing below on the lawn . . . he was sitting under the Sorting Hat, and it was telling him he would do well in Slytherin . . . Hermione was lying in the hospital wing, her face covered with thick black hair . . . a hundred Dementors were closing in on him beside the dark lake . . . Cho Chang was drawing nearer to him under the mistletoe . . .