They made sure no one was watching them, then they pushed open the door to the bathroom. Harry slipped into the cubicle at the end of the aisle and Hermione locked the stall door after them.

'Look,' said Hermione, leaning against the door of the stall, as Harry perched on the toilet lid, facing her, 'I think I've figured out roughly when we are. The fall of You-Know-Who was – well, will be – in 1981, right? And your birth was a year before. If we say that your parents were about twenty-one when they were killed, then that would make this around the year 1974 – which means that we've gone twenty years back in time.'

Harry thought about this. 'Blimey, we're in it deep,' he said finally. 'Is there a way to travel forwards in time? Or does the Time-Turner only work backwards?'

'I don't know.' Hermione sighed. She pulled out the Time-Turner from around her neck and looked at it sombrely. 'Even if it does, we can't use this one. It's malfunctioning. We might be sent further back in time, or even too far into the future. If we ever go back using this, we'll have to get it fixed.'

'How?'

Hermione shook her head. 'We can't fix it without a teacher's help. And a teacher will want to know who we are and why we have a Time-Turner. We could end up altering the future! And we can't let that happen!' She looked stricken, then she glanced at her watch. 'It's seven o'clock. We've got to find somewhere to stay the night. We can't go to the dorms or even the common room, and we'll get in trouble if we stay out in the corridors beyond eight-thirty. Any ideas?'

'No,' Harry said immediately, and unhelpfully.

'Big help you are.'

'Sorry.'

She smiled. 'It's all right, I'm just worried.'

'Don't be.'

'What do you mean, don't be? We're stuck twenty years away from home and you tell me not to worry?'

Harry laughed. Hermione shot him a fierce look.

'Fine,' he said. 'I've got an idea. Let's look for a secret passage or something. Hogwarts is a thousand years old, right? There are loads of secret passages and rooms; I saw some on the Marauder's Map.'

Hermione gasped. 'The Marauder's Map! Why didn't I think of it? Won't Sirius and Lupin and the rest of them see us on it? I mean, they might see your name and wonder why there's another Potter on the map.'

'No, they wouldn't,' Harry corrected her. 'They only created the Marauder's Map after they became Animagi, Lupin said – and they only became Animagi in fifth year.'

'Still … I think it's best if we don't talk to anyone we know in the future. It could get awkward. They shouldn't know anything about their future selves.' She paused and then said quietly, 'The Time-Turner never sends people to a particular time without a reason – even if there's a side effect. For instance, when it used to send me back a few hours too early in time, I found it actually helped, because it would give me more time to study. The side effect was that I was always too tired to really pay attention.

'Look, Harry, we must be here for a reason. Maybe it's meant to be that we don't save Sirius and Buckbeak and instead go back twenty years to change something else for the better. Maybe –'

'– you think we won't save Sirius? That he's meant to get Kissed? Maybe you think that, Hermione, but I don't. It's just … cruel.'

'It's cold in here. Let's go,' Hermione said suddenly, ignoring him.

Harry stood up and edged out of the way as Hermione opened the door with a loud squeak. At the same time, a similar squeak issued from the door of the stall next to them, which had opened to reveal the same sallow, stringy boy that had stood next to Harry in the Great Hall earlier. He moved swiftly, blocking their way out.

They had heard no sounds before. It was clear that he had been in the bathroom since they arrived and had unintentionally eavesdropped on their conversation. Hermione gasped.

'Who are you?' he said, pulling a wand out of his robes and pointing it at them.

Harry pulled out his own wand. The boy was about the same height as he, but he felt unnerved as he looked into the deep black eyes. Ignoring the question, he asked defensively, 'Why were you spying on us?'

'I wasn't spying!'

Harry snorted. 'Maybe you don't call hiding in the toilet stall next to us, being extremely quiet while listening to us spying, but I do.'

'I don't care what you call it. It was an accident. Now tell me who you are.'

Harry gripped his wand tightly. 'And why should we do that? You heard what we said. We can't tell anybody.'

The boy raised his eyebrows. 'Have it your own way. All I have to do is take you two to Professor Dumbledore. I'm sure he'd be curious to find out why two unidentified students wearing Hogwarts robes are sneaking around his castle.'

'He won't believe you,' Hemione retorted, speaking for the first time to the boy. His eyes moved from Harry's face to hers.

'Oh, no? And how do you know? Dumbledore's a lot smarter than a lot of the other teachers. Even Slughorn's no match for him.'

'I've met Dumbledore, and I don't care who Slughorn is. Let us past and we'll leave you in peace.' Harry tried to move sideways, edge around the boy, but he blocked their way. 'Let us out!'

'He's the Potions master … And why should I?' The boy had a maddeningly superior tone of voice.

'Expelliarmus!' Harry yelled suddenly, but the boy retaliated with a swift Shield Charm, forcing them apart. Now two things were clear; Harry and Hermione could not get out of the bathroom, due to the Shield Charm, and neither could jinx the other, at least until the protective shield was removed.

They were stuck.

'I might tell you why we're here,' Harry said carefully, 'if you tell us things about you. Like who you are. And the exact date. And why you were spying. And only if you don't go to Dumbledore.'

The boy sighed deeply, then said, his black eyes watching them from the other side of the Shield Charm, 'My name is Severus Snape, half-blood.'