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Getting into the Department of Mysteries had been easier than he had expected. It had been a rushed decision, but Sirius didn't know what else to do. He had stood amongst the wreckage of his best friend's home and seen his dead body, Lily's body, and seen Harry carted off to live with that horrid woman and her family.
Sirius had to do something. Anything. He had raced to the Ministry hoping to find something that could help him turn back the time. It was then he had remembered reading about Time Turners.
He had arrived in the middle of the night, thankfully, so it was practically empty. Sirius transformed into Padfoot and got inside with minor issue, escaping the occasional guard here and there. It was difficult to find his way round the Department but eventually; he found what he was looking for. A cabinet full of Time Turners. Sirius took one and clutched it in his hand. It was cold and shimmered gold. He thought about using it then and there, but he needed to get away first. He couldn't travel inside the Department of Mysteries.
He returned to Godric's Hollow and took another look at the house; it had been glamoured to look like it still stood, but Sirius remembered it as if it were yesterday. "Why, Peter?" he whispered. His heart hurt, and his eyes welled with unshed tears. Peter had always been… well, odd, but never had Sirius thought that he would betray them. He had to change that decision, to make Peter the secret keeper. He had to save them, all of them.
Sirius hung the Time Turner around his neck and began to turn it. He had to go back several months, and it took time to turn it. Just as he let the hourglass go, he saw a flash of an apparition; it was Kingsley Shacklebolt. "Stop!" he cried, but it was too late. Sirius was travelling back in time.
Time reversed and with it, he saw the scenes unwind. He saw himself leave to get the time turner, Hagrid arriving and taking Harry. Then he saw a scene that made his blood boil and his resolve strengthen. He saw the building slowly rebuild itself as if the explosion were in reverse, flashes of green light and then Voldemort leaving the house in reverse.
Time stopped, and before he knew it, Sirius was in front of the intact house. The sun was shining in the early hours of the morning and he could feel the heat on his skin. He glanced around to see if he could confirm the date, and luckily there was a discarded Daily Prophet on a nearby bench. It was the 15th of July, so Harry hadn't been born yet and they hadn't gone into hiding yet. But they would soon.
Taking a deep breath to calm his beating heart, Sirius approached the front door and knocked firmly. He had to stop them from changing their mind about the Secret Keeper, even though it was him who had convinced them to change. He had told them originally to change to Peter because he was not someone Lord Voldemort would expect to know the secret. Sirius himself was the obvious choice, and now that he knew the truth, he would do anything to keep his family alive, even if it meant dying himself.
James opened the door with a look of surprise. The clock behind him revealed it was six o'clock in the morning. "Padfoot?" he asked sleepily. "What are you doing here?" James stepped aside to let Sirius in and ran his hands through his messy hair.
Sirius pulled his best friend into a hug and tried to keep himself together. He had, just hours ago, seen his dead body and now, here he was alive and well. He would do anything to keep him that way.
"Prongs," Sirius said once the door was closed. "About the secret keeper thing… you need to choose me."
James looked at him, confused. "I thought that was what we agreed upon."
"In a few days, I will come here and try to convince you to switch to Peter… whatever I say to convince you, don't switch to him," Sirius pleaded. "Believe me when I say, Peter can't be trusted."
James nodded with a steely expression. "I won't ask how… what you know, but I believe you. I promise," he said, gripping Sirius' arm.
Sirius had excused himself and then walked some distance away to transform into Padfoot again. He had wanted to stop his past self from returning, even if James had promised. If he could stop himself from ever appearing to convince them, then he could be certain that the Potters would be safe.
A week later, he had appeared, just as Sirius had remembered. With a quick combo of confundus and obliviate, Sirius had visited the Potters with no memory of why he had chosen to visit. The week after Harry's birth, Sirius had watched as they performed the Secret Keeper charm and the house disappeared from view. His other self was the secret keeper, not him. He could not return to his own time after that. So, he had watched in hiding as Peter had appeared in front of the hidden house with Lord Voldemort that October night and had been unable to reveal the house to him. A green flash of light and Peter was gone.
Sirius' actions had prevented Voldemort from dying that night, but the Potters lived to fight another day. From the shadows, Sirius watched and protected his favourite family, never able to reveal himself nor his other self would surely go mad. It was worth it, however, to see his best friend live once more.