He'd been stupid. He shouldn't have stunned the police officer, should have just stayed with his original story about being kidnapped by Professor Quirrell...
It might not be too late, whispered a voice inside him. You might still be able to fix your mistake. The Auror saw you, he remembers that you stunned him... but if he were dead, if Professor Quirrell were dead, if Bellatrix were dead, there would be no one to contradict your story.
Slowly, Harry's hand started to rise, pointing his wand at the police officer and -
Harry's hand halted.
He had a distant sense he was behaving uncharacteristically of himself, somehow. Like there was something he'd forgotten, something important, but he was having trouble remembering what it was, exactly.
Oh. That was right. He was someone who believed in the value of human life.
A sense of puzzlement accompanied the thought, he couldn't quite remember why other people's lives had seemed valuable...
All right, said the logical part of him, why has my mind changed between then and now?
Because he was in Azkaban...
And he'd forgotten to recast the Patronus Charm...
Doing anything at all, somehow, seemed like a tremendous effort, like the thought of action itself was a weight too heavy to lift; but it did seem like a good idea to recast the Patronus Charm, for he was still able to be afraid of Dementors. And though he couldn't remember what it was like to be happy, he knew that this wasn't it.
Harry's hand rose to hold his wand level before him, his fingers took the starting positions.
And then Harry paused.
He couldn't... quite remember... what he'd used as his happy thought.
That was odd, it had been something very important, he really ought to be able to remember it... something to do with death? But that wasn't happy...
His body was shivering, Azkaban hadn't seemed so cold before, and it seemed to be getting colder even as he thought. It was too late for him, he'd already sunk too far, he'd never be able to cast the Patronus Charm now -
That may be the Dementation talking rather than an accurate estimate, observed the logical part of himself, habits that had been encoded into sheer reflex, requiring no energy to activate. Think of the Dementors' fear as a cognitive bias, and try to overcome it the way you would overcome any other cognitive bias. Your hopeless feelings may not indicate that the situation is actually hopeless. It may only indicate that you are in the presence of Dementors. All negative emotions and pessimistic estimates must now be considered suspect, fallacious until proven valid.
(If you'd been watching the boy as he thought, you would have seen a distant, abstract, puzzled frown move across his face, below the glasses and the lightning-bolt scar. His hand stayed in the starting position for the Patronus Charm, and did not move.)
The presence of Dementors interferes with the part of you that processes happiness. If you cannot retrieve your happy thought by mnemonic association on the key of happiness, perhaps you can get at the memory some other way instead. When was the last time you talked to someone about the Patronus Charm?
Harry couldn't seem to remember that either.
A crushing wave of despair swept over him, and was dismissed by the logical part of himself as untrustworthy, external, not-Harry, the dull weight still pressed him down but his mind went on thinking, it didn't take much effort to think...
When was the last time you talked to someone about Dementors?
Professor Quirrell had said that he was already able to feel the presence of Dementors, and Harry had said to Professor Quirrell... he'd told Professor Quirrell...
...to hold to the memory of the stars, of falling bodilessly through space, like an Occlumency barrier across his entire mind.
His second Defense class of the year, on Friday, that was when Professor Quirrell had shown him the stars, and again on Christmas.
It didn't take much effort to remember them, the searing points of white against perfect blackness.
Harry remembered the great cloudy wash of the Milky Way.
Harry remembered the peace.
Some of the coldness at the fringes of his limbs seemed to retreat.
There were words he had spoken out loud on the day he'd first cast the Patronus Charm, his mind could remember the sounds and the speech even as the feelings seemed distant...
...I thought of my absolute rejection of death as the natural order.
You cast the True Patronus Charm by thinking about the value of human life.
...But there are other lives that are still alive to be fought for. Your life, and my life, and Hermione Granger's life, all the lives of Earth, and all the lives beyond, to be defended and protected.
Then the idea of killing everyone... that hadn't been his true self, that had been the Dementation talking...
Despair was the Dementors' influence.
Where there's life, there's hope. The Auror is still alive. Professor Quirrell is still alive. Bellatrix is still alive. I'm still alive. No one's actually died yet...
Harry could picture the Earth, now, in the midst of the starfield, the blue-white orb.
...and I won't let them!
"Expecto Patronum!"
The words came out a little halting, and when the human shape burst back into existence it was dim at first, moonlight instead of sunlight, white instead of silver.
But it strengthened, slowly, as Harry breathed in deliberate rhythm, recovering. Letting the light drive back the darkness from his mind. Remembering the things that he had almost forgotten, and channeling them back into the Patronus Charm.
Even when the light blazed full and silver once more, illuminating the corridor more brightly than the gas lamps, banishing fully the cold, Harry's limbs still shook. That had been too close.
Harry took a deep breath. All right. It was time to reconsider the situation now that his thoughts were no longer being artificially darkened by Dementors.
Harry reviewed the situation.
...still looked pretty hopeless, actually.
It wasn't the crushing despair of before, but Harry still felt wobbly, to put it mildly. He didn't dare go dark and it was his dark side that had the ability to take this level of problem in stride. It was his dark side that would have laughed scornfully at the very concept of giving up just because he'd lost Professor Quirrell and was marooned in the depths of Azkaban and had been seen by a police officer. The ordinary Harry was not able to take that sort of thing in stride.
But there wasn't any option except to keep moving forward anyway. You couldn't get any more pointless than giving up before you'd actually lost.
Harry looked around.
Dim gas lights lit a corridor of grey metal, whose sides and floor and ceiling were slashed in places, gouged and melted, telling anyone who cared to look that there had been battle here.
Professor Quirrell could have repaired it easily enough, if he'd...
The sense of betrayal struck Harry with full force, then.
Why... why did he... why...
Because he's evil, said Gryffindor and Hufflepuff, quietly and sadly. We told you so.
No! thought Harry desperately. No, it doesn't make sense, we were going to commit the perfect crime, the Auror could have been Obliviated, the corridor repaired, it wasn't too late but it would have BEEN too late if he'd died!
But Professor Quirrell was never really planning to commit the perfect crime, said the grim voice of Slytherin. He wanted the crime to be noticed. He wanted everyone to know that someone had killed an Auror and broken Bellatrix Black out of Azkaban. He would have prepared some kind of evidence, some proof he could reveal of your involvement, to use as blackmail against you; and you would have been bound to him forever.