Disclaimer: What can be found in Rowling's books are hers, the rest (that no one else has a claim to) is mine.
October 31, 1981
Thankfully, it was dark and deserted as Sirius Black landed on the small island of Lihou in the English Channel. Since he was a wizard, the Muggle repelling charms that had been placed around this empty area did not affect him, only the anti-apparation one and the Fidelius Charm; but Sirius had flown there, and since he was the Secret Keeper for this place, he only had to think the address to see a makeshift shack appear from what seemed like out of nowhere. Sirius removed the Disillusionment Charm he had placed on himself and his flying motorcycle (just incase some Muggle decided to look up at the night sky and saw a man on a motorbike flying across the waxing moon. Sirius, though an Auror of two years, had been in trouble with the Ministry of Magic before, and didn't feel like explaining to them why he had a motorbike that could fly.)
Cautiously, Sirius took his wand out of his back pocket and made his way to the shack. He had recently learned that the dark wizard he had spent the last five years of his life fighting against in the Order of the Phoenix (a group of witches and wizards brought together by Albus Dumbledore to stop the destruction Lord Voldemort, the Dark Lord, was spreading) was out to kill his two best friends and their year old son, his godson. That was the reason behind his being here tonight. The man who was at this shack was the Secret Keeper for the Potters. Sirius had derived a brilliant plan; when the Potters had to go into hiding, James Potter had insisted on Sirius being their Secret Keeper, Sirius believed that would be too obvious and convinced them to change Secret Keepers to Peter Pettigrew, another friend of the Potters and Sirius. James' wife, Lily, had told them that if they were going to switch Secret Keepers, she would feel safer with them switching to a third friend of theirs, Remus Lupin, but Sirius had reason to believe that Lupin had turned spy for Voldemort and was leaking information to him from the Order, including information on the Potters. See, Remus Lupin was a werewolf, and Voldemort was recruiting more and more of them to his cause everyday, even "good" ones like the werewolf Sirius and James had dubbed "Moony" during their time at Hogwarts School of Witchcraft and Wizardry where they had all met.
It was known that a spy from Dumbledore's Order was passing information to Lord Voldemort, and it was someone close to the Potters. Sirius wasn't willing to take any chances with his old pal Moony, and convinced James that Peter, a.k.a. Wormtail the Rat, would be the better choice for Secret Keeper of all three of them. But something tonight didn't feel right, something felt off, and Sirius couldn't place it. He was thinking that it may have something to do with Wormtail, and therefore James, Lily, and his godson Harry, but that was absurd. No one had known of the switch except for the Potters, Sirius, and Peter, and no one except Sirius and Peter knew where Peter was hiding. Even if Remus decided to show his true colors and attempt the murder of the Potters for his master, well, Sirius could only see that ending in the werewolf's death; werewolf or not, Remus Lupin was no match in a duel against James Potter, not when James was defending the people he loved the most.
Once he reached the door, Sirius took a shaky breath to try and calm his nerves. What was he so nervous about anyways? Sirius Black never got nervous, never got frightened. Wand still at hand, Sirius knocked on the door, firmly but not harshly. Thirty seconds later, (a long thirty seconds) Peter still hadn't answered, and Sirius was being to grow nerv-- impatient. He knocked again. "Wormtail! Hey, Wormtail, you there? It's me, Padfoot! Open the door!" Sirius referred to himself by the name his dog Animagus form had been given at school, and tried to hide the anxiety in his voice. Still, there was no answer. "Pete, it's me, open up!" Sirius knocked again, a little harder than he had intended, and the flimsy door swung open from the force.
"Lumos," Sirius whispered, holding his now lit wand in the air to get a better look around the darkened room. One glance around the room told Sirius that no one, in the form or rat or man, was there. Something must have happened to him... was the first thing that popped into the Auror's mind. But what? Peter Pettigrew was not one who enjoyed being out at night, especially not when he was alone, and especially not when he had so terrified of the danger he was in he didn't feel the least bit of comfort from being hid all the way out on Lihou, so it was out of the question that he went out for a nighttime stroll in the moonlight. Stepping inside for a better look at the place, Sirius saw that everything seemed just as it should, except the occupant of the place was missing. But the cot, chair, table, lamp, food, everything in that small shack was just where it should have been. Nothing was broken, toppled, or moved. No signs of anyone other than Peter, and Sirius of course, being in the place was evident. To Sirius, it appeared that Peter had, for whatever reason, gone out for a nighttime stroll in the moonlight, but that... that was impossible! Peter Pettigrew did not just walk around outside in the middle of the night when no one was there to urge him out the door! No, something had happened, something had definitely happened. Sirius just didn't know what.
What he did know though, was that if it involved Peter, it, in all likelihood, involved James and Lily and, as much as Sirius dreaded this thought, Lupin. Knowing the answers he sought lied off of the island, Sirius ran as fast as he could out of the shack and back to his motorcycle, not caring to put the Disillusionment Charm back on, only to try and calm the feeling of worry bubbling in the pit of his stomach.
Somewhere over the English Channel
As Sirius raced to Godric's Hollow, the town that the Potters were hiding in, a terrible thought had crossed his mind. He had been thinking that Peter must have, somehow, become the victim of Voldemort's Death Eaters, that Voldemort knew Sirius wasn't the Potters' Secret Keeper. But what if... what if he had been wrong? What if Remus wasn't the spy? It wasn't exactly secret information that Albus Dumbledore, called the greatest wizard of all time, thought Sirius to be the traitor. If Dumbledore could make a mistake like that... it wasn't impossible that Sirius couldn't either... Sirius knew that the spy was neither James nor himself... but what if it was... Peter?
No, no. That was impossible.
But why? asked a sneaky voice in the back of Sirius's mind. Dumbledore obviously made that mistake. What if you did, too?
But that would mean... No. Sirius wouldn't believe that. He wouldn't believe that his mistrust in Remus might cost his best friends, his family, their lives. No. Remus Lupin was the traitor. He had to be.
But what if he isn't
Sirius couldn't take what was pulling him apart inside. He had to find Moony, sort this out now. Before it was too late. As Sirius saw England beneath him, he changed direction ever so slightly, heading towards the home of Remus Lupin.
November 1, 1981
It was well past midnight when Sirius landed his motorcycle in the outskirts of the town in Surrey County. Resolved in what he had to do, he knocked on the door of his fellow Marauder, Remus Lupin.
"Who is it?" Sirius heard Moony demand from the other side of the door.
"It's me," Sirius gulped. "It's-it's Padfoot. I've gotta talk to you, Moony."
Sirius heard nothing for a few moments. He knew Remus, like Dumbledore, thought him to be the spy, and had expected this. But that didn't make the wait any easier. That voice Sirius supposed was his conscience (he didn't remember hearing that voice before, and he couldn't recall if he had ever had a conscience either) was telling him that he had been wrong about Remus; but the stubborn portion of his brain kept thinking that when Remus finally did open the door, it would be to kill him, and then the Potters. Sirius was trying to shake that part of him into unconsciousness, but the wait Moony was putting him through didn't help.
Finally, Remus opened the door, but it was only to grab Sirius by the collar, yank him in, slam him against the now closed door, and jab his wand at Sirius's throat. "What do you want, Black?" he snarled.
To say Sirius had been expecting this as well would have been a lie, however, it didn't shock him. "Moony, you've got to trust me-"
"Why would I do that? For you to even ask that means you would have to trust me, and we all know you don't trust werewolves," he spat.
"Remus, listen, it's about Prongs-"
For a moment, Remus's eyes widened at Sirius's mention of James. "What did you do to them?! What happened to them?!" he demanded, pressing his wand harder against the throat of the man he had once considered a friend while his grip on the same man tightened.
"Voldemort might know where to find them-"
"You told him where they are?! How could you do this?!?!" Moony was shouting now, completely outraged by his former friend's actions.
"No! Never!" Sirius said, almost panicked. "Wormtail, it's Peter, Remus, it has to be. It's the only thing that makes sense. He would have been at his hideout if he wasn't."
"Why would Wormtail need a hideout, Sirius?!"
"Lily and James- He was their Secret Keeper, they switched. He went into hiding too-"
Remus slowly shook his head in disbelief. "No, they would have told me. James trusts me, he would have said something."
"I convinced him not to, it would have been the prefect bluff! Moony, please, you've got to believe me!"
"And why should I?!" Remus's voice then turned dangerously soft. "If you've done anything, hurt them in any way, I swear I'll kill you."
"Remus, please, I would never-"
"Go crawl back to your master."
Sirius's entire disposition changed when Lupin said that. "Don't. You. Ever. Accuse. Me. Of. That." His wand was out and facing Remus now.
Moony's only response was "Get out."
"Take that back," Sirius snarled, raising his wand and acting as though he didn't hear Lupin tell him to leave.
Whatever it was that Remus hit him with, Sirius didn't know, but it was a good one. The force of it sent Sirius crashing out the door and he landed in a heap next to his motorbike. He heard the door lock behind him.
Sirius ignored the throbbing pain where he hit the ground, he had more important things to worry about. Again, Sirius flew off on his flying motorcycle, heading for Godric's Hollow.
Sirius had nearly worn his bike out he was flying it so fast. By the time he had reached his destination, there was not half an hour left before dawn. Thinking the Potters' address as he jumped off his motorcycle, he ran towards the house that didn't need to materialize before him, praying to Merlin that it wasn't too late; he was certain now, after that little display at Remus's, that Wormtail was the spy. Sirius was afraid.
The sight that greeted him at the door only enhanced his fear, and Sirius was sure that, for a moment, it had caused his heart to stop.
Sirius felt the tears build up in his eyes. His best friend, his brother, dead. Because of him. Because he hadn't trusted Remus, because he said to switch to Peter instead. Sirius choked back a sob. Now wasn't the time for this. James wouldn't want him to just stand there and cry, he'd want him to find his wife and son. He ran through the house, searching every room for Lily and Harry. Until at last, he came to his godson's bedroom.
There she was. Lily. She was dead too.
"Oh no." If Lily was dead, then Harry... "No!" Sirius didn't even want to look for him, he had just seen his best friends dead. He didn't noticed the tears freely flowing down his face, but he knew that he wouldn't be able to endure the sight of his godson's dead body. Don't think about that. Don't think about Harry... dead. But he had to, he just had to. James would want him to find Harry, not some mindless Auror who didn't give crap about the people who had been killed tonight. How this could even be happening though... Sirius hadn't even seen the Dark Mark...
He hadn't seen the Dark Mark.
Voldemort, the minute he had finished killing the person he wanted dead the most from a certain attack, would always cast the Dark Mark. He wanted Harry dead more than James... That was why they were in hiding, to protect Harry...
A small ray of hope filled his chest, but Sirius wouldn't allow it to consume him, not until he found his godson. It was then he noticed some slight movement and whimpering in Harry's crib. Not daring to breath, Sirius carefully stepped around Lily and reached into it.
He was alive. Sirius couldn't begin to describe the relief he felt. His godson was alive! He held the small child close. Harry had stopped making the sad sound that broke Sirius's heart to listen to once he heard his godfather's voice. He must have been crying for a while, Sirius noticed where tears had been streaming down his innocent face, and his eyes, emerald and exactly like his mother's, were still wet. Sirius also noticed a lightning-shaped cut on his forehead. "Oh, Harry." He kissed the top of his godson's small head and hugged him tightly but gently.
"Sirwus," Harry said happily, now that he was being held by someone who, like his mummy and daddy, could always make anything all better. He didn't understand that no one could make this all right again.
"Yeah, kiddo, I'm here." Sirius was resting his cheek on Harry's head as he held him, letting his tears fall as they would. What did it matter if he cried? Harry was alive. After seeing James and Lily dead, nothing could have brought him the relief that holding his living godson alive did.
Sirius heard something. He was sure of it. There was someone downstairs. He looked down at the child in his arms. He couldn't leave Lily and James here, but he had to do what was best for Harry. Thinking fast, Sirius came to the conclusion that if the Fidelius Charm had been broken, then, chances were, the anti-apparation charm had been too. Harry probably wouldn't like apparating, he had never done it before. But now was the time for (Sirius never thought he'd be thinking along these lines) safety, not comfort. With a crack, Sirius apparated away with Harry held securely in his arms.