His first year at Hogwarts had drained him of every ounce of – of energy, of composure, of joy. So he thought. He'd returned home grey, in threadbare grey trousers three inches too short at the ankles and a faded black jumper that smelled like it had spent the school year mouldering in a bottom corner of his truck (which it had).

The excused disappearances had piled up on him like tombstones, month after month after month. Hiding his inexplicable wounds and bruises, slipping out of the dorm room at the crack of dawn to shower in the cold deserted bathroom, avoiding his reflection – keeping his guard up had cost him dearly.

His dad took one look at him, waiting like a forlorn apparition at the station platform, and enfolded him in a hug. His side was clawed ragged, and it hurt, and the unfamiliar affection more so. He felt strung tight.

King's Cross was running at its usual madcap pace, sidewalks teeming with youths and suits and tour groups and shoppers. They cut away towards Regent's Canal, and the hubbub gradually faded as they followed the water east. Remus' trunk clunked along the uneven stones of the canal path.

"Good to have you home," said his dad.

"Good to be back – how's mum?"

"Mungo's running her into the ground, as always. A new pox outbreak in Lewisham, and some kind of invasive variegated ivy that's been hypnotising pensioners. She remains incapable of putting her feet up, that woman."

"And the Ministry?"

"Mmmm, still fatally blinded by its own self-importance."

Remus managed a sound he hoped gestured at the concept of amusement. "Business as usual, then."

"Pretty much."

They continued in silence, turning north onto a plane tree-lined side tree.

"Just – I know it's tomorrow night, and you m-might be worried," his dad stumbled over the word. "But we, uh, managed to sink the cellar a little deeper and open it up some."

"The space won't make any difference."

His face crumpled. "I'm – I'm so sorry, Remus. I know this isn't getting easier."

"As long as I'm not putting you in danger. Show me the doors and wards when we get in."

He spent the summer in parks and befriending stray cats and drinking endless cups of tea. He avoided human interaction. It helped that his parents were always at work. He'd wake up late, listen to Muggle radio, fry himself half a dozen eggs and some sausages and eat beans cold out the can, wash up methodically, and head out into the city. Remus perfected the knack of perpetual motion: keeping his hands, feet, and eyes steadily busy to escape the notice of others or his own thoughts. In the quiet of the early evening, he'd come home and shower and read while eating dinner and fall asleep. On the rare occasions his haggard mother took a day off, he'd try to gently convince her to teach him some simple medical magic.

And on two evenings, his dad had come home early to lock Remus behind a series of wards and ensorcelled doors. He couldn't met his dad's eyes the following mornings, as he'd emerged from the cellar hunched and blinking away pain and exhaustion.

The letter had arrived late in August, not long after his second transformation. He'd managed to break his arm this time, and the newly healed fractures were throbbing insistently. It was overcast and hot: Remus was secretly hoping for a thunderstorm, and couldn't bring himself to move much. He was sitting on the doorstep, watching the sleepy street while a black and white cat sidled back and forth past his shins and head-butted him when his petting lapsed.

The owl landed on the iron railings, scaring off the cat, and proffered a letter. It looked quite pleased with itself, which the envelope justified. It read tersely, Remus Lupin, London.

Remus found a knut inside and handed it to the bird in exchange for the letter. After establishing no biscuits or mice would be forthcoming, it took off.

Lupin, the letter went.

I'm going batshit, so thought I'd write you. Mum's still homicidal, dad's in Porth Mellin for the summer, house elves constantly snooping and very judgemental, and little brother is coming along nicely as a tiny pompous psycho. Would prefer a psychopomp. Can't take a dump without being told I'm besmirching the family name. Pigeon for dinner for the last ten days. Actually got beaten – with a black willow switch, no less – for nicking a Muggle paper from the local. Not because I nicked it, mind – because it was a, I quote, filthy Mudblood rag not fit for wiping the arseholes of the Noble Sons of Black. Tried sneaking into the British Museum for something to do, but they've got some proper magical security. Not a solo job. At least there's always virgin sacrifices if I can't find anything fun to do. Meaning suicide.

I miss you. Ten more bloody days.

Sirius

The second time they shared a bed Remus had slept fitfully, waking often to check Sirius was still breathing. He knew he wasn't likely to choke in his sleep, but the whiskey had soured in his belly and Sirius had drunk much, much more than he.

But the boy slept soundly on his side, mouth slack, snuffling occasionally. His breathing was slow: his sternum gently filling and releasing, the only site of motion in his still body. One arm cushioned his head, the pillow rucked in the crease of his elbow, which rested just above Remus' head. The fingertips of his other arm almost brushed Remus' hip. The sheer nearness of him was like a gravity well tugging at Remus' navel. It didn't help with the queasiness.

Remus stared at the ceiling, and tried to close his eyes, and tried not to listen to Sirius' heartbeat, and tried to ignore the tickle of breath on his cheek.

They'd pilfered the whiskey from the Charms professor, a plot months in the making, initiated by a wholly unfounded but accurate assertion from Black that Professor MacInally had to be a lush. There'd been a broomstick heist and surreptitiously researched spells involved, but ultimately the plan had hinged around Remus keeping the professor distracted until the all-clear signal. Later, the boys had crammed round the dorm hearth, toasting bread and passing an amber bottle – then another and another – between them.

"To Moony!" Potter had raised the bottle. "The calmest and, ah, cunning-est of decoys!"

"To Moony!" the boys roared in response, drumming their feet and slapping his back.

"So," Sirius took a pull of whiskey and knocked Remus' knee with his, "how did you manage to keep him talking?"

Remus shrugged. "Fishing, mostly. A little religion."

"Huh. One day, Moony, you're gonna turn those verbal powers on me and – pfft."

"If you say so."

One-by-one, they had begun to slink to bed, Pettigrew stopping to noisily throw up in his trunk. Remus had tried a cleaning charm, but the smell lingered. Sirius opened the windows, and he, Potter, and Remus sat with their backs to the hearth, watching clouds gust past and the bats flitting from eave to eave. The fire spat and settled.

"It's not what I expected, really," remarked Potter.

"Whisky?" said Sirius, and drank some more.

"No, definitely more burnt sheep than I imagined. But I meant, I dunno, all this."

"Hogwarts?" Remus asked.

"Magic, really. I'm just – I'm not sure what it's bloody for. Why we're here. What we're supposed to bloody do with it all."

"Draughts to soothe, charms to cheer, goats to turn into rocking chairs," offered Sirius.

"That's it, right – 'cos why would I want to turn a goat into a rocking chair?"

"Ask me when you're eighty."

A planet, hazy and low to the horizon, had Remus transfixed. Mars, he thought. No, maybe Saturn.

"We'll get to the good stuff someday, I guess," said Potter. "The real magic. They probably just don't think we're ready."

"I am definitely not responsible enough," said Sirius, "for any of that real shit."

Potter laughed and stood unsteadily. "I'm to bed, good sirs. I will most definitely murder anyone who wakes me before midday."

Remus pushed himself over to lean on the fire-warmed wall, and stretched out his legs. His knees cracked. Sirius sprawled out on the hearthstone and prodded at the coals.

"You know, I'm not sure this shit gets any realer."

Sirius appraised his face. "Just petty incantations and goat-chairs for the rest of our days? What a thought." He swigged and passed Remus the bottle.

"Magic can't fix anything," he said, trying to keep the bitterness out of his voice. "And all the interesting stuff it can do, we have to unlock. Or dream up. Or make possible. Or something. But it's not the answer."

"Some people think it is," said Sirius. "And the question, the reason, the whole damn purpose."

"It's just a tool. A weirding one, at that."

"A powerful fucking tool passed down through generations of megalomaniacs," muttered Sirius.

"Speak for yourself."

"Believe me, I fucking do." Sirius glowered, knocked back some more whiskey, and added, "Still – damn sight more useful heirloom than preserved House Elf heads."

"I'm not gonna ask."

"Don't." Sirius tried to sit up and groaned. "Moony, I'm sleeping in your bed. Mine's too fucking far."

So there they were, Remus staring at the ceiling, aware of each millimetre of distance between Sirius' fingertips and his hip, Sirius' elbow and the crown of his head, Sirius' face and his shoulder. He was waiting for Sirius to move, paralysed at the thought. There was no right response. He couldn't think, couldn't breathe.

He fell asleep and woke to Sirius' arm across his chest. He woke to Sirius drooling on his chest. He woke to the other boys crowding around his bed, jeering at the pair of them. He woke curled around Sirius, with a leg thrown over him. He woke to an empty bed.

Sirius' breath hoarsened and hitched. His arm twitched under his head. Remus let out a sigh, and reminded himself, furiously: he trusts you. He's here. Fucking enjoy it. And so, for just a moment, he let himself feel the lines of curious, frenetic sub-atomic energy sparking between their bodies. Turning his head just a little, he allowed himself to admire the dark and dishevelled hair, the angry weal of a spot, the soft, crooked mouth, the eyelashes resting on gorgeous cheek, the fine hairs. So fucking what, he growled to himself. He's beautiful.

And he fell asleep and woke to the pre-dawn chatter of birds and to find Sirius grinning goofily at him with barely open eyes.

"Morning, Moony," he mumbled, and fisted his hand into Remus' shirt to pull himself closer, wedging himself between Remus' arm and torso. He nudged his head into the crook of Remus' neck and exhaled noisily through his nose. "I'll get up in a second, I swear," he muttered into Remus' shoulder, "just – one minute. Let me have this."

Dragging his trunk along Regent's Canal to the station, his body thrummed and coursed. It was unsettling, like a stomach bug. He had almost reached King's Cross when he realized what he was feeling was excitement: anticipation so intense it sparked across his skin. He felt like he was walking out of his shoes. His arms were strong in being taxed. He felt intact. His t-shirt was stuck to the small of his back.

He stumbled through the barrier and there they were, waiting on the platform: Sirius, James and Peter, respectively louche, stolid, and anxious, as was only right. They wrestled their luggage onto the train and found an empty compartment. Sirius sprawled out on the seats, while James hauled trucks up onto the racks and Peter murmured his fervent thanks. Remus knocked Sirius' feet out the way and took the window seat. And there they were. Him and his people. On their way back to magic school.

He permitted himself some hours of basking. They didn't expect him to talk much.

James had rescued Sirius in the last days of summer, it transpired. His mum had caught him with a dirty magazine – dirty because it featured naked women and also because those women were Muggles, nipples fixed firm and perky on the page – and he'd sent an owl to James before doing a runner. Apparently his plan had been to follow the Thames to the sea, and from thence? It hadn't been much of a plan.

"So my folks were just driving me round Barking and Dagenham in circles," James was explaining to a rapt Peter, "me with my head out the window and my mum yelling, encouraging me trust my instinct and sense his whereabouts, and me yelling back, 'GO LEFT!' We kept getting stuck in cul-de-sacs and, lest we forget, my dad can't follow directions for love nor money, so there was also a fair bit of my mum yelling at my dad.

"And then this enormous brute of a seagull dives at the car, and I'm certain it's after one or both of my eyes, but it narrowly misses my head and swoops straight in the car window. This bird sits all polite and menacing in the back seat next to me, evil glint in its eyes. My dad's smashed on the brakes and we're just gawping at it. It takes a foul, runny shit on the car seat, gags a little – us and the bird – and then coughs up this scrap of paper.

"I pick up this manky scrap while the bird watches me, and it says, "Trapped in power station, send help." Except partially digested. And then we speed on and my parents had a right old time talking security out of getting him nicked while I fetched him out and chucked him in the car."

What Remus especially appreciated about Peter, he decided, was how shamelessly he enjoyed it all – the everything of it, the self-absorbed delight with which they performed their friendship and wove it snug around their listeners.

"Can we please take a moment to appreciate," Sirius interjected, "the fact I convinced a Thames seagull to carry my note in the first place. Turns out they're all capable, but very few of them willing. I really had to amp up my natural charms."

"Can we also note," said James, "that Sirius was not the one tasked with cleaning up its skanky shit."

"I did have to sit next to it all the way back to Godric's Hollow, 'cos you were too scared to tell it to piss off."

"We didn't have any sardines! It's rude not to tip!"

"By Monday, me and Slicer were fast friends. I was thinking about bringing him with me to Hogwarts, 'cept we never really worked out the whole shitting business."

"You should just spend next summer at ours. You saw what my parents are like – they'd love it. Roasts for every dinner, bird-watching, you name it."

Sirius flashed a look at James, and then hooded his eyes. "Never know, Lilith might grow some vital organs. Lots can happen in a year. Not that I'm holding my breath for it."

The castle swallowed them back up. It was achingly familiar – the labyrinthine halls draped with sconce-shadow, the clamour of the Great Halls, the crimson drapes on the four-poster beds, the echoed bark of laughter from the stone walls, the vaulted windows framing a glittering night. As he contemplated the gibbous moon hanging above the forest, Remus felt his dread return and drape itself round his shoulders.

Sirius knocked him on the back of the head.

"Stop mooning, Moony. We've need of your brains. Plots don't scheme themselves, my friend. Answer me truthfully: in your infinite wisdom, can you predict where the stairs are gonna swing and when? This is of vital importance."

Remus kept growing, attenuating into a willowy strip of boy fletched with scar tissue and hard knots of muscle lumped on bone. He stood a head above most of his classmates, perhaps three above Peter, yet retained his knack of folding his limbs into small spaces and disappearing from notice. And the castle unfolded itself for him, as if it, too, was dedicated to unpicking his façade of disinterest – or, at the very least, in cultivating his nascent talent for skulking.

As students fled their first Transfiguration lesson and Professor Nadder surveyed the wreckage they'd left, Remus had packed his bag in the shadows of a moss-encrusted tapestry. His hands were tetchy with the pull of the impending full moon and he had to keep forcing his concentration back to slotting his textbook back among its kin, screwing the lid on his ink, putting away his quill – and then he saw an escapee rabbit dart under the tapestry and into the wall. Following it through, and into the belly of the castle, had been all too easy.

He'd be standing by a statue, waiting for the pre-lunch crowds to clear, and its wrist would slowly grate to point him towards another secret. Ghosts chanced upon him exploring subterranean passageways and sagely offered directions. His awareness of the castle's dusty workings intensified, itching like a rash between his shoulder blades whenever he chanced too near a forgotten room. Individual words would leap from the collective conversations of teeming corridors to suggest incantations to spell open tricked doors.

He had never gone looking, and never told anyone. He kept to the walls, and moved carefully and quietly. The hidden passages allowed him to escape the clamour – to lope, crawl, and hum, unobserved, and breathe the mildewed pungency of magic that pervaded every catwalk, tunnel, and crawlspace the castle opened to him.

Sirius had noticed, of course.

"The fuck did you go, Lupin?"

He'd cornered Remus by the bathroom sinks while he was brushing his teeth. "My mum was sick," he replied through a mouth of toothpaste.

"Not last week, you bellend – after dinner. You walked out the hall and disappeared into the thinnest of bloody air."

Remus looked at him, nonplussed.

"After dinner, today. You ate, you left, you flat-out disappeared, for hours, might I add, and now I find you here brushing your teeth. The fuck – did – you – go."

He had wandered dusty passages, singing folksongs in Irish Gaelic under his breath, until he found a pale stone arch that opened onto an elbow of the lake, lapping against hewn steps deep under the weight of the castle's body. He had taken off his socks and sat for interminable time with his feet in the cool black water, until the feelings had welled up in him and he'd had to lie back and focus on the harsh press of stone against each of the bones in his back.

Remus spat and rinsed his mouth. The furious attention Sirius had focused on him was bringing blood to his cheeks. He was intensely aware of their reflection in the mirror, but couldn't look. He hoped he didn't have toothpaste on his lips.

"I went to the library."

"Of course, the library," Sirius mimicked airily. His voice hardened. "No you fucking did not. Tell me – your – secret."

Remus did his best to stop his face revealing his internal broil. My secret, eh? He wiped his mouth with the back of his hand, and turned to leave.

"I'm much more boring than you think, you know."

Sirius trailed him back to the dormitory like an impetuous thundercloud. "I will dog your steps for the next six years. I will become your shadow. Lupin, Gods, just tell me. I need to know."

Remus paused by a window deep-set in the castle walls, running his fingers over carven vines in search of mettle, Sirius impatient at his shoulder. The waning moon hung lovely in the sky, spilling light across the school grounds. His chest ached.

"Alright. The massive fireplace in the entrance hall? The back of the hearth is enchanted to look solid, but you can walk right on down."

Sirius' expression was resplendent.

"Satisfied?" Remus' voice was brittle.

"Satisfied!?" Sirius clasped his shoulders, full of glee, like it was the most natural gesture in the world, his eyes all wicked and dark. "Lupin, I could fucking kiss you. I – I just – thank you. C'mon, we've got to tell James."

And so he had been shown the Potter's extraordinary heirloom, an Invisibility Cloak as soft and light as gossamer, and made accomplice to near-nightly expeditions where he was rarely more than a breath away from Sirius Black.

The castle continued to invite him ever deeper. Come second year, spells of unlocking and reverse-warding were assembling neatly in his mind if he so much as idled in a stairwell. Two dozen short-cuts to the kitchens revealed themselves to him, of which he freely availed himself, his hunger only sharpening.

He shared his discoveries strategically. Once the elation of their reunion had been scoured sufficiently by the realities of schoolwork and the impending full moon, Remus found himself struggling to stay afloat near James' mania, Peter's unctuousness, and the black hole that was Sirius. He slipped behind suits of armour and through mirrored halls, into stone shafts full of slow-turning gear-wheels wider than he was tall, moving lithe and tall.

Sleep came begrudgingly to him, but a little easier if he had new weird vistas and twisted stone warrens to walk his mind through until it curled up quiescent. His mind – his heart – might prove like a dog with behavioural issues. Tire it out.

He knew it was much worse than that, really: that he was burning up, fast, and every moment he spent around his friends was only stoking this tragedy and would likely bury them in pyroclastic effluvia. And his dreams, they terrified him. They were all fire. Sweaty foreheads, the hot pressure of hands on his skin, suction, breathlessness, fury, a winding heat in his belly, his body crumbling into pieces, black hair clenched in his fingers, dark eyes devouring him whole, until he snapped and slashed the world to pieces with his lethal claws.