Harry felt like he couldn't tear his eyes away from the scene, but all too quickly, the Patronus faded as the Dementors scattered. Snape turned around and Hermione pulled Harry back into the shadows as he passed within a few feet of them and disappeared into the darkness – presumably to the other side of the lake to conjure stretchers for Harry, Ron and Hermione, who by now would be all unconscious.

The images in Harry's mind blurred and disappeared; he put his hands to his forehead, leaning against a tree, and sank to the ground, breathing hard. He had never felt so tired in his life.

Hermione felt him slide down next to her. 'What's the matter? What's wrong?' she asked, kneeling beside him.

Harry squeezed his eyes shut again, trying to quench the dizziness. It dissolved slowly, leaving him drained and exhausted. It didn't help that he hadn't eaten anything since he'd had breakfast before entering the Chamber of Secrets. 'I'm fine,' he said hesitantly, though he didn't feel it. 'I –'

'What is it? Was it the Dementors? I didn't see –'

'I'm fine,' Harry repeated, even though he was feeling weak and shivery. With an enormous effort, he pushed himself to his feet, leaning on the tree trunk for support. The warmth of the Patronus had gone, leaving him cold and, ironically, drenched in sweat.

'If you really are all right, we've got to get back to Buckbeak and rescue Sirius,' Hermione said practically, partly to get Harry's attention and partly because they really did need to. She took Harry's arm and guided him away from the lake. 'Remember?'

'Yeah …'

They untied Buckbeak from his tree and were leading him out of the Forest when Hermione said, 'Why do you think he did it?'

Harry hadn't thought about it, but as soon as he opened his mouth he knew the answer. 'He was repaying me because he betrayed us. Lily said that he wanted to in the letter.'

'But why did he take so long? Twenty years …'

'He's taken nearly as long before,' Harry reminded her. 'Dumbledore told me once that Snape saved my life in first year so that he would be on even footing with my father, because my father saved his life once – that time when Sirius played a trick on him at the Shrieking Shack. I guess he really did know who I was after all – or Lily told him. Maybe he did it for her, because he loved her, I know he did.'

'So maybe that was the reason we went back in time. So that Snape would save your life, because he owed you something.'

Harry nodded wordlessly. There was a lump in his throat that prevented him from speaking even if he'd wanted to. It was Snape, he thought. Snape – and all this time I never knew.

'How do you know? It's like you're – you're reading his mind or something …'

They mounted Buckbeak, Hermione holding a bit too tightly for comfort onto Harry's waist as they left the ground. 'Maybe I can,' Harry said over the beating of the Hippogriff's wings. 'Where's Sirius?'

'I can't remember exactly, but I think one of the windows to the right of the West Tower. What makes you think so?'

'Because,' Harry yelled back, as Buckbeak gained speed and the wind whipped through their hair, 'When we were standing near him, at the lake, I could tell what he was feeling – feel what he was feeling – like I've got a sixth sense or something.'

'What was he thinking of, to cast such a bright Patronus?'

Harry knew, but he had to swallow several times before he answered. 'My mother.'

'Maybe you've got an empathy link. I've read about them, and …' The rest of Hermione's sentence was lost in the wind as a chill swept through them.

'Maybe.'

'So, when you thought it was your dad casting the Patronus, it was really –?'

'Snape … The person I saw was me, but I thought it was my father, see? Where's the West Tower?'

'Just here … tell Buckbeak to slow down …'

'Whoa, Buckbeak, whoa!' Harry yelled, leaning down to make sure Buckbeak could hear him. Buckbeak slowed down obediently and they counted the windows, looking in each one just to make sure.

'Stop – stop! This one!' Hermione shrieked in Harry's ear when they came to the thirteenth window. Sirius was inside, in Professor Flitwick's office. His eyes widened when he caught sight of them.

'Wha––' he gasped.

'Sirius, come on,' Harry called, as Sirius hurried over to the window. Hermione opened it with her wand and, but Sirius stood at the sill, still gaping.

'Hurry up!' Hermione moaned, glancing at her watch before remembering it was hopelessly inaccurate. She edged closer to Harry to allow room on the Hippogriff. Sirius clambered out of the window and behind Hermione as Harry whispered to Buckbeak, 'OK, go!'

'Harry, take him up to the tower,' Hermione called, gripping his waist again. Harry jerked Buckbeak around, back the way they came and towards the West Tower. Buckbeak landed with a flash of his talons in the bright full moon, and Hermione let go of Harry's waist at last as they slid off.

'Go, Sirius, go!' Harry said, reaching up to pat Buckbeak. Sirius, however, did not move.

'Where's the other boy – Ron?'

'He – he's fine,' Harry answered, hoping it was the truth for Ron's sake. Come to think of it, he hadn't seen Ron for weeks. 'Just – go!'

Buckbeak shifted his wings impatiently. Then Sirius grinned suddenly. 'You two made enough noise on that Hippogriff to wake the dead.'

Harry cracked a smile. 'Sirius, please –'

Sirius sat up on the Hippogriff carefully, but still looked down at Harry. 'How can I thank you – you – truly are your father's son, Harry –' Before Harry could reply, Sirius nudged Buckbeak and Buckbeak kicked off from the tower, his enormous wings beating furiously. Sirius's final words came floating back to Harry on the wind: 'We'll see each other again someday …'