A/N: Thank you to Twitter for inspiring me to write this. It wouldn't exist without you.

This is a marauders era AU with a teacher-student relationship (everyone is of age) so if that's not something you want to read, click away. If it is, enjoy and let me know what you think!

Dedicated to the one and only MrMxLemons. This wouldn't have made it so far without your enthusiasm. I hope it lives up to your wildest dreams.

Thank you to my alphas Renee561, Saffie, and InspiraPens for all of the amazing brainstorming, and to one of the coolest and most supportive friends i've made on here, augustlouise89, for reminding me to look for the humanity in the characters. The entirety of chapter 7 would not exist without you.

The title of this fic comes from the Lord Byron quote, "Between two worlds life hovers like a star, twixt night and morn, upon the horizon's verge." I think if anyone was caught between two worlds, between here and the other side of the veil, May-December, free and imprisoned, and every other star-crossed image we could think of, it would be an age gap version of Sirius and Remus.

As for a posting schedule - I don't have one per se, but the majority of this fic is written (i.e. the first 7 chapters, and I believe there are about 3 more to go). I was going to hang onto this till it was done, but I'm impatient and it's Valentine's Day, so what better time to share the love. I'll probably finish the rest of it within the next week and then post it either a chapter a day or every few days till they're up. I don't have the patience to hang onto this one for ten weeks lol.

That's all my notes for now; enjoy and let me know what you think!

Chapter 1

"Sirius, can you stay after class, please?"

The head boy looked up from his desk, wavy black locks falling across his forehead as his eyes opened wide with genuine surprise. The DADA instructor with sandy blond hair and iridescent green eyes stood next to the solid oak desk at the front of the classroom, those same eyes locked on Sirius with a stern, unfamiliar expression.

Sirius swallowed. Was it possible that the man was just in a bad mood? Sure, he and James were wont to cause trouble in class now and then, but Professor Lupin had never taken umbrage with it before. In fact, he usually seemed amused by it, if only in that way teachers smiled when they weren't supposed to approve of something naughty but actually found it quite funny. He had certainly never kept them after class for a prank before, and today's little experiment with exploding quills was tame by comparison.

But the longer he stared at the man in front of him, the less Sirius could work it out, so he nodded and watched as Lupin's lips pulled into a thin smile. The lump in his throat pressed uncomfortably as he waited for the bell to ring, anxiety in the pit of his stomach.

~0~0~

"You wanted to see me, professor?"

Lupin looked up from where he sat at his desk as the rest of the class filed out. As the last student left, he finally met Sirius' eyes. The younger boy stood motionless as he waited for whatever storm was coming.

But Lupin's anger seemed to dissipate immediately, and he sighed instead. "You're getting reckless, Sirius."

Sirius felt the whiplash hit him like a hex he hadn't seen coming. "What—"

"I am always one for a good prank, as you know, but I fear I may have encouraged you too much by looking the other way in class." He paused and lowered his voice, but did not break eye contact with Sirius. "I heard about the near miss with Severus."

Sirius' face reddened and he shifted his bag uncomfortably. "Was nothing," he mumbled.

"If he didn't have such a good nose, he wouldn't have smelled the aconite in the babbling potion you slipped into his drink. Seeing as he's deathly allergic to aconite, it was not nothing."

Sirius shoved his hands into his pockets and hunched his shoulders. He had already heard a lecture from Dumbledore, and McGonagall, and Mrs Potter—but for reasons he couldn't understand, Lupin's censure hurt worse than any of theirs had. He hated feeling like he was disappointing his favorite professor.

He sighed and looked at the ground. "I know."

"That is why," Lupin began, "I feel it is necessary to insist you serve detention with me every night this week. And every Friday for a month." Sirius must have looked like he was about to protest, because Lupin continued, "To make up for all the other times I should have given you detention and did not. Also, though we can't prove it was you, this will be for the missing aconite in Professor Slughorn's stores as well."

Sirius pressed his lips together so hard his teeth hurt. It was completely unfair, especially since he had never acted alone, but they both seemed to know he would not contest Lupin's theory.

Lupin tilted his head, and after several moments had passed, he smiled. "It is admirable, Sirius, that you would rather accept punishment on your own than insist James share the blame."

"James is like my brother."

Lupin nodded with quiet contemplation. "Very well. Seven o'clock tonight. Met me here."

"Yes, sir." Though he should have felt the weight of Lupin's judgment bearing down on him, the praise won out, and Sirius felt lighter again as he shifted his bag and turned for the door.

And for the life of him, couldn't explain why he felt himself smile.

That evening, during the detention Professor Lupin had set him, Sirius found himself repeatedly looking up from his task to stare at his professor. The detention itself was rather dull; Lupin had instructed him to copy out defensive spells onto flashcards for the younger students, though Sirius was beginning to wonder if the man thought he needed remedial lessons himself. But it was fairly mindless work, which left Sirius plenty of time to steal glances at the older man, who had not seemed to notice at all.

Ironic really, for a Defense professor , he thought. But Sirius couldn't help it; Lupin was working on a complex series of spells that were weaving together in thin air before binding themselves to the piece of jewelry before him. Sirius recognized the spells. Lupin was enchanting jewelry to protect the wearer.

As he watched the ribbons of different colored light bind together, Sirius' eyes grew wide with fascination. He knew such objects existed—and his family had plenty of heirlooms laced with Dark magic—but he had never seen someone work with them firsthand before. And since Sirius was a dab hand at charmswork (if he did say so himself) he was even further captivated by it. Maybe it was something he could do someday. Maybe he didn't need to be nothing more than a disappointment (if you asked his mother) or a troublemaking marauder (if you asked Filch) or James Potter's best friend but still a Black at the end of the day (if you asked anyone else).

He watched Lupin's careful wandwork, marveling in the way this powerful wizard used his magic gently; it was so different from what he was used to—the harsh, blunt edges and sharp corners that cut through the air with visceral and unforgiving precision when his family wielded magic. His stomach did a flip as his eyes traced Lupin's wand hand, up his arm, to his face—screwed up in concentration—and his sandy blond hair that fell just so on his forehead.

Sirius didn't want to interrupt at a critical moment, so he waited until Lupin sat back and wiped his brow before saying, "Could you show me that?"

The professor looked up suddenly with his mouth slightly agape, and Sirius had the impression that he'd completely forgotten anyone else was there. Recovering quickly, he resumed his usual expression with nothing more than a slight furrow to his brow giving away his confusion.

"Why?" he asked.

Sirius felt his cheeks heat up, flustered. "Well, because it looks seriously cool." He smirked.

Lupin clearly tried and failed not to roll his eyes. "Do you always do that?"

Sirius shrugged. "You would too if your mum named you something stupid."

"Well it was this or Romulus and she thought founding a city might go to my head."

Sirius laughed and a peaceful sort of camaraderie hung in the air between them.

"Why did you ask about the enchantments?" Lupin finally enquired.

He bit his lip nervously. "I like charms," Sirius almost whispered. "I'm good at them, too. And I've never seen someone do practical defensive magic with objects like that before. Why jewelry?"

The professor steepled his fingers together as he leaned his elbows on his desk, looking at Sirius with a searching expression. Finding whatever he was looking for, he answered, "It's accessible. Anyone can war it, anyone can give it. It's easy to hide protective charms in because it's so ubiquitous. The more common something is, the more easily it's overlooked, which in this case is to the wearer's benefit."

"Could you teach me?" Sirius was leaning forward in his seat, expression serious.

To his surprise, Lupin smiled. "Perhaps. But not tonight." It didn't sound like a hard no, and Sirius felt instinctively that he should let it go for the time being. "Did you finish the cards?"

Sirius nodded, pulling out his wand to tie the stack of cards neatly with a bow before sending it zooming onto Lupin's desk.

Lupin raised one eyebrow as he looked from the neatly packaged stack to Sirius. "Impressive."

Sirius shrugged again, but there was a faltering quality to it and he was sure the back of his neck was turning red. "Thanks," he mumbled, quickly pulling his bag on his back.

"I'll see you here tomorrow, Sirius. Seven o'clock."

Sirius just nodded, not trusting himself to speak. He turned and headed for the door.

He was out in the hallway before he would let himself think about why his heart had started racing at Lupin's praise. Sirius had never cared whether a professor was impressed with him before. But something about Professor Lupin was different… The man had captured his attention and every encounter left him feeling something alight in his stomach.

He didn't understand it yet, but he knew he wanted more of that feeling.