Sorry, late again! I've been doing a lot of revision and finding time to write has been very hard. Thanks for reading!

Aziraphale went to ask Hermione is they could borrow the cat the next day, since they may as well be prepared for when Lupin returned to school. He found her in the Gryffindor common room with Harry and Ron. Fortunately, they had taken Crowley's advice and made up with her about the broomstick. Crowley had already gone to ask Professor McGonagall if he could help to check it for jinxes, in the hopes that he could get it back to Harry more quickly. Aziraphale thought this was very brave of him. Facing Professor McGonagall was not something he would do in a hurry.

Aziraphale opened the door of the common room, and was immediately faced by Harry, flanked by Ron and Hermione. Hermione was clutching a large and fluffy ginger cat, and looking slightly apprehensive, but Ron and Harry looked excited at the thought of being involved in another scheme like the Chamber of Secrets last year.

"We already asked her," Ron explained, "and she said yes."

"You're not going to do anything to him are you?" Hermione asked, hugging the cat closer to her chest.

"Of course not," Aziraphale wondered exactly what Ron and Harry had told Hermione, "I just came to make sure you were alright with us borrowing him in about a week's time. He won't be gone long."

"What are you going to do with him?" Ron asked. Harry stood on his toe.

Aziraphale cast about wildly for something to say. The part of his brain that spoke with Crowley's voice offered up some advice at once.

We have to stop lying to him angel. We don't need to tell him everything, but we shouldn't be treating him like a child.

"You know you saw him talking to the black dog?" Aziraphale said to Harry.

Hermione and Ron looked at Harry quickly. He clearly hadn't told them about seeing the dog with Hermione's cat. Aziraphale ploughed on before they could interrupt.

"We are going to try and communicate with the dog through…"

"Crookshanks," Hermione said.

"Crookshanks," Aziraphale repeated, "we are quite sure the dog is harmless, but we have reason to believe it isn't a dog at all."

"What is it then?" Harry asked.

A pause.

"I have no idea," Aziraphale told them. He just wasn't ready to enclose that information yet. Though Harry would find out soon enough. Crowley was not backing down on telling him about Sirius Black.

"So, it's not a Grimm?" Hermione asked.

"No."

Hermione shot a superior look at Ron, who scowled.

…

Lupin returned some days later and before he could do anything else, he was whisked off to the room behind the Defence Against the Dark Arts classroom and told the plan.

"Do you think you could write the note?" Crowley asked, "we don't really know Black, and well…"

"We thought he would be more likely to respond to a note from you," Aziraphale finished.

"Ok," Lupin said slowly. He still looked rather pale, "what do I need to say?"

"Well," Aziraphale pulled a handsome leather-bound notebook from his pocket. Crowley rolled his eyes as the angel flipped through it hastily, muttering under his breath. He found the correct page, complete with title and date, and began to read out the neat bullet points written there.

"One: Tell him we think he is innocent," Aziraphale looked up from the book, "this is important."

"Really?" Crowley feigned surprise, "I would never have guessed."

"Two," Aziraphale went on, ignoring him, "ask him to meet us in the shrieking shack. And three: tell him who we are, so he knows who he's meeting. We wouldn't want him to run off or something because he wasn't expecting Crowley and I to be there."

Lupin nodded, "Is that all?"

"Yep," answered Crowley, who had been looking over Aziraphale's shoulder, "did you want to take notes?"

Lupin grinned. It was surprising how much younger he looked, less grey and careworn. But the smile disappeared quickly, and he looked solemn again.

"Give me a few days," he said, "I need to get this right."

…

Finally, they were ready. Crookshanks sat purring loudly on Aziraphale's lap as they waited for Lupin to arrive with his note. It had taken him a long time to write it, but neither of them had thought it right to rush him.

"Maybe I should get a cat," Aziraphale mused, stroking Crookshanks' fluffy head.

Crowley was thinking the same thing. Crookshanks was quite fun to have around. The fur was a problem though, it got everywhere. Maybe he should invest in a pet without fur. A duck perhaps.

Lupin dashed in at that moment. Clutched in one hand was a small piece of paper, covered with black, spidery handwriting.

"Sorry," he gasped, leaning on the doorframe to catch his breath, "Harry and I had our first lesson today."

"How did it go?" Aziraphale asked. Lupin looked happy, which boded well.

"You know I think he's got a real talent," Lupin said, "he picked it up far faster than most adults. It can take years to master the Patronus charm."

Crowley's heart lifted a little. He had been worried that he couldn't cast a Patronus because he was a demon. Maybe, just maybe, it was something else. He made a mental note to ask Lupin later on.

"He reminded me of James," Lupin said quietly, "he just wouldn't give up till he had done it. James was exactly the same. He was like that with Lily, just wouldn't give up on her…" he trailed off into some distant memory before collecting himself and handing Aziraphale the note.

"It's in code," Aziraphale said, looking at it.

"I thought it would be safer," Lupin explained, "don't worry, he'll be able to read it. unless Azkaban has changed him beyond recognition."

"Well," we'd better get on with it," Aziraphale stood up hastily. He had been pleased to see Lupin in such a good mood and wanted him to stay that way.

They went to the grounds, creeping through the darkened corridors. Aziraphale, who was bringing up the rear, had a feeling of doing something wrong, and kept peering over his shoulder warily, as if there were spies hiding in the shadows behind the suits of armour. Lupin led the way, as he knew the school surprisingly well. He led them through secret passageways that neither Aziraphale nor Crowley had ever been through before.

Finally the passed through a trap-door which opened up into the grounds. The cool air was refreshing on their faces, and the sky sparkled above, the waxing moon illuminating the grounds with a silvery light. Crookshanks purred, making them all jump.

Feeling slightly stupid, Aziraphale bent down to talk to the cat.

"Alright Crookshanks," he whispered, "we need you to find the black dog. Sirius Black. You need to give him this note."

Lupin carefully tied the note around Crookshanks' neck, loosely so he wouldn't try and bite it off.

"Off you go," Aziraphale breathed, shooing the cat gently with one hand.

Crookshanks walked a few paces, stopped and licked himself while they all watched in a tense silence, then saunted off down the lawn, the note around his neck twitching in the night breeze like a dry leaf.

"This is ridiculous," Aziraphale muttered, half-laughing. Crowley and Lupin joined in. it was completely ludicrous, relying on a cat to help them prove the innocence of one of the most wanted people in the wizarding world.

But it might just work.

…

After they had come in from the grounds, Crowley manged to get Lupin alone by saying he wanted to borrow a book about mermaids. But it wasn't until they were halfway to Lupin's office that Crowley managed to get up the courage to ask Lupin what he wanted to.

"So, what makes a Patronus so hard to form?" he tried to keep his tone light, staring pointedly at the corridor ahead.

"Well," Lupin didn't seem to notice Crowley's strange behaviour. Or maybe he chose not to. it must have seemed suspicious to him that. Defence Against the Dark Arts teacher was asking such a thing, "it's often hard to find a happy enough memory. Especially when the dementor is right in front of you, making you think of all the bad things that ever happened to you," he glanced at Crowley, taking a deep breath, then said, "I struggled with it for a long time. But then…"

He didn't need to continue. He had found his friends. Happiness born of love like what he must have had is stronger than any dementor. He suddenly felt a surge of pity for Lupin, who had been so happy, and was now so alone.

"You also need to believe that you can cast one," Lupin added, "it sounds ridiculous, but it makes a big difference. That's why a lot of people can suddenly manage them when under pressure. The necessity drives out the self-doubt."

"Oh," Crowley nodded, as if Lupin had just told him something vaguely interesting.

They had reached Lupin's office.

"Thanks," Crowley said, more sincerely than he meant to.

"You're welcome," Lupin gave him a look which told him that he understood, and wasn't going to judge him, "Goodnight."

Crowley didn't go to sleep that night. Lupin had given him a lot to think about,

…

The days passed. Aziraphale almost forgot about the message, busy as he was with teaching. However, every now and then he found himself wondering. Had the note arrived? And had Sirius Black read it, been able to understand the code he had used with his friends as a boy?

Crowley did not forget about the note, and spent breaktimes deep in thought, glancing about him for any sign of Crookshanks. He actually believed that the cat would complete its task and bring Black's reply to them. Harry had seemed to think it would work, and he had faith in him.

Most anxious of all was Lupin. For him, this was more than just a job. This was his only remaining childhood friend, and a truth he had accepted for most of his adult life was likely to be shattered when they finally saw Black.

At first, they met in the staff room every day, finding times when they knew no other teachers would be present. They didn't know who Crookshanks would seek out with the note, so it was important they consulted each other regularly.

But after a week, their meetings stopped. Aziraphale and Crowley started hunting the grounds for Crookshanks instead, often accompanied by an anxious Hermione. Aziraphale felt horribly guilty for losing her cat, though Hermione assured him that Crookshanks wandered off a lot, and she as only worried because of the cold weather. A north wind had struck the castle, and the air was sharp and icy.

They walked around the lake for what felt like the hundredth time that day, calling out as discreetly as they could.

"Crookshanks," Aziraphale called, his teeth chattering.

"Just miracle yourself warm angel," Crowley said. He was sick of the angel shivering.

"I'm perfectly fine Crowley can you stop-" Aziraphale stopped abruptly, staring hard at a clump of bushes nearby.

The bushes rustled, and a ginger head carefully disentangled itself from the winding stalks. Crookshanks wriggled out from the bush, walked serenely over to them, and sat down, starting to lick his fur back into place.

With fingers trembling only partly from the cold, Aziraphale untied the note about his neck. It was written on the back of one of the Ministry of Magic posters, which warned all residents of Hogwarts not to come out at night, until Sirius Black was caught. The prisoner of Azkaban had written back.

I'm very excited about the next chapter. I think I've started to deviate from the plot of the book a bit more now, but hopefully it will all come together. I'm worried that the chapters are getting a bit repetitive, so if you agree please PM me or leave a review, I wouldn't like it if people were getting bored.