I'd take book canon over movie canon any day, but in the book Remus learns the truth when surrounded by people and would have had to shut his more emotional reaction away for a more convenient time. The movie canon allows for something a little different.

Remus Lupin clutched the blank Marauder's Map tightly as he made his way up the stairs to his office. He had not expected his old creation (or rather their old creation), to turn up in the hands of James' son, but in retrospect he knew that he probably should have.

Although it had been difficult, when he had first seen it, to resist the urge to immediately open the parchment and speak the words so that he could relive the joy of using the map at least one more time, for nostalgia's sake, he had waited until Harry was gone and he was sure he wasn't going to be disturbed. Sitting down at his desk and spreading it out in front of him, his hands smoothed over the blank surface of the pages, welcoming it like an old and bittersweet friend.

"I solemnly swear that I am up to no good," he said, and just saying the words put a lump in his throat the size of a hippogriff.

The first blotch of ink appeared in the centre of the parchment, and it instantly spread out in spidery tendrils, mapping out every hallway and room with the practiced precision Remus and his friends had spent so long researching and perfecting. Within ten seconds the entire school was visible, with the small labelled dots moving around it, clueless as to the fact that they were now being watched.

Memories flooded through Remus, of bending over the map with James and Peter and Sirius, laughing and plotting and thinking they ruled the world, or at least, the school. And they had.

The dot labelled Harry Potter caught his eye, safe in the Gryffindor dormitory and hopefully asleep. The boy was a true mix of his parents, painfully so, but it was a joy to see them living on in him, albeit a slightly bittersweet one.

All that was left now was making sure that Sirius didn't get to Harry. It was the least Remus could do for James, and for Peter. He could keep Harry safe and make sure they hadn't died for nothing. I promise you, Prongs, I'll do everything in my power to keep him safe for you and Lily.

He had almost stopped paying attention to the map, running his eyes over the various parts of the castle with a lazy ease. He had no true purpose or plan for the map, after all, at least not right now. But then something caught his eye. A name. The most impossible name.

Remus wiped at his eyes blearily, sure he had imagined it, but there it was, moving slowly along out in the grounds of the castle. He sat up in his chair, his entire body rigid.

Peter Pettigrew.

It went against everything he knew. It should have been a mistake. An impossibility. But the map never lied, and as one of its creators Remus Lupin knew that for a fact.

But if Peter was alive…

The room faded away around him as the name on the map became the only thing his mind could process. A strange noise escaped his lips and he covered his mouth from the sheer shock, staring at the map with horror and alarm and grief and everything in between.

If Peter was alive, it meant Sirius hadn't killed him, but Peter had wanted everyone to believe he had. If he wanted everyone to believe that Sirius had killed him, it was because he was the one who had betrayed Lily and James, not Sirius. And if Peter had betrayed Lily and James...Sirius was innocent. Of the betrayal and the mass murder.

It had been Peter. It had all been Peter. And in twelve years, Sirius had been the only one to know the truth, rotting in Azkaban with the Dementors eating away at his sanity day after day.

"Padfoot," Remus whispered, his voice hoarse, an ache in his chest for his old friend, "Oh, Padfoot." He became aware of a few tears rolling down his cheeks, for James and Lily but mostly for Sirius, and did his best to blink them away. He didn't quite succeed.

Peter Pettigrew was alive. Sirius Black was innocent. And Remus was now only the third person to know it.

Sirius' betrayal, paired with the death of the Potters, had been easily the most painful thing he had ever experienced. As someone who underwent an agonising transformation into a wolf every month, it was not a claim he made lightly. But he had believed all his friends to be dead with the exception of the one who had killed them, directly or not. He remembered hearing about it, about how Sirius had blasted Peter to bits after the small man had asked how he could have betrayed Lily and James. Remus had fallen to his knees and wept, almost as much for the friend he had lost to betrayal as the ones he had lost more permanently.

To learn of how wrong he had been was staggering but also somehow a tremendous weight off his chest. Peter's betrayal was still painful, of course, but unlike with Sirius it made sense. Seeing Sirius as the type to betray his friends to Voldemort had been what Remus had struggled with the longest. It was a relief to know he was right to believe Sirius was a better man than that. Deep down he had known. But at the time it had been impossible to ignore the facts.

Not anymore. Sirius was innocent. Which meant after years of living in poverty, alone (so painfully alone), Remus had one of his best friends back.

A few more tears leaked from his eyes at the same time that a strange and small laugh of joy fell from his lips.

He wasn't alone anymore. And neither was Sirius. They had each other now.

Sirius was obviously here, not to kill Harry as everyone thought, but to kill Peter for his treachery. And now he would not have to do it alone. Remus was going to watch the map carefully, and when the time came he was going to find his old friend and help him put down the little rat once and for all.

And then, perhaps things could almost be like old times. Remus thought of how now he could be in his wolf form and retain his mind, and imagined running with Sirius in his dog form for miles and miles through woods or fields. It was such a simple dream yet such a beautiful one, and not actually at all out of the realms of impossibility.

"Soon, Padfoot," he murmured, "Soon I'll find you."

Just a little look into what might have happened there. Feedback is greatly appreciated!