The night of February's full moon, Sirius was holed up with the rest of the Marauders on the bottom floor of the Shrieking Shack and gnawing on the end of a giant rawhide bone with Remus's wolf when stag-James began to strut back and forth past the exit door, rubbing against its wood with his antlers. He blinked at Sirius as he did, his message clear in his eyes: he wanted them to get out of there.

Ever since they'd become Animagi, the Marauders had been keeping Remus company during his transformations, and he'd remained as docile and playful as he'd been on their first night in the Shack. It was wonderful that they could help Remus so much just by being with him, but Sirius had to admit that their nights could get sort of boring. Of course, he would give anything to spend a night a month alone with Remus, but the wolf sharing his bone now wasn't him, not in any of the ways that mattered.

But Remus's wolf wasn't a vicious monster, either, at least not with the Animagi there. A sudden rush of daring swept over Sirius—why shouldn't they leave the Shack? They could handle whatever the wolf tried to get himself into; they'd always been able to handle him before. There was no reason to confine themselves to the Shack every moon.

James butted against the door again. Sirius barked at him, standing up, and Peter, having caught on to what the others were trying to do, gave a nervous squeak in response. Sirius cocked his head at the rat, trying his best to reassure him through his body language. With one last worried glance at the wolf, Peter dipped his head in resignation and climbed up onto Sirius's back, burrowing into his fur.

It was easy enough for them to get out once they put their minds to it—the werewolf-repelling charms placed upon the Shack did not work on Animagi, as the Marauders had learned from personal experience. Peter was able to get the door unlocked and unbolted with his nimble rat hands; James charged at it with his antlers lowered and busted clean through, the door wrenching back with a resounding bang. The wolf glanced up from his bone, startled by the sound, and padded cautiously over to peer out into the dark tunnel that ran between the Shack and Hogwarts. Sirius nuzzled his side and led the way into the tunnel, with the wolf breathing down his neck behind him.

When they reached the Willow at the tunnel's other end, Peter climbed off of Sirius to placate it by pressing the knot near its trunk before the other three emerged aboveground. As soon as he was out of the tunnel, Remus's wolf let out a delighted howl and took off across the moonlit fields, tumbling through the winter snow that covered them and nipping playfully at its coldness. It was the wolf's first time out of confinement, Sirius realized, his first time running free under the moon that had created him. Sirius and James watched him frolic around for a minute before taking off after him and chasing him playfully around the Willow and across the grounds, yipping and yowling and kicking up snow.

Suddenly the wolf skidded to a halt, going stiff and still. The fur rose along his hackles, and he gave a low, dangerous growl as his eyes fixed unblinkingly on something ahead of him. Sirius followed his gaze to a darkened corner just beneath the castle's West Tower, where a pair of students—seventh years Frank Longbottom and Alice Wren—were holding each other and kissing in the darkness.

Shit. Sirius's stomach churned, and he shared a glance with James as Peter buried himself deeper within his fur. It was three in the morning; he hadn't even considered that other students might've snuck out of the castle this late, especially considering it was the middle of February. Weren't there better, warmer places for people to snog?

With another growl, the wolf took off in their direction—James and Sirius leapt after him, Sirius grabbing his leg with his teeth and yanking him forcefully to the ground. The wolf snapped at them, snarling and writhing, and Sirius was hardly able to hold him down even with assistance from James and a frantic Peter. Remus had been right—the wolf was incredibly strong, even for a dog as large as Sirius's.

Sirius growled back at the wolf as threateningly as he could and grabbed his muzzle firmly between his teeth, feeling a strange sense of betrayal at the knowledge that he was hurting Remus along with the werewolf. But Remus would understand, he knew; he wouldn't be able to live with himself if his wolf managed to attack anyone at Hogwarts.

James and Sirius held the wolf down until Frank and Alice returned inside through the Marauders' favorite Rowena Ravenclaw portrait door. Once they were gone, their scents dulled by the stone walls of the castle, the wolf calmed down and went limp underneath Sirius. Relieved, Sirius released him and stepped away along with James, whimpering apologetically. The wolf's bloodlust wasn't really his fault, after all.

The Marauders steered the wolf away from the castle, James and Sirius flanking him tightly on either side to keep him from running off again. Maybe they could head for the Forbidden Forest, where the mysterious pack of wolves descended from werewolves lived. Would Remus's wolf get along with them even better than he did the Animagi?

But as they walked, Sirius noticed a strange scent coming from somewhere up ahead, a scent like stone and smoke and other things that were decidedly unnatural. Impulsively he left James to babysit the wolf alone and ran off in pursuit of it, stopping when he stood over the source of the scent and beginning to dig through the snow for it. The wolf quickly loped over to help dig, too, and soon Sirius felt his claws scratch against something wooden. His heart pounding, he continued to dig with the wolf until they had revealed a small wooden door set into the earth.

James approached the door with Peter perched atop his head and blinked questioningly at Sirius. They were in the middle of nowhere, halfway between Hogwarts and the Forest; what was the point of a trapdoor built out here?

Sirius gripped the door's handle with his teeth and drew it open, revealing a set of cracked stone steps leading into a narrow tunnel lit by enchanted flames built into its walls. It appeared to be heading in the direction of the castle.

The wolf charged down into the tunnel without a second thought. Sirius nipped at James's ear and took off after him, the stag following right behind. His antlers scraped against the roof of the tunnel, creating a harsh grinding sound that set Sirius's teeth on edge.

They followed the tunnel as it twisted and turned until the pads of Sirius's paws ached against the hard stone floor. He wondered where exactly they were going—eventually, the change of scent he could discern overhead made him guess they were somewhere directly beneath the castle.

Eventually they reached another set of stone steps, this time leading up to a little landing with a circular door carved into the wall above it. Sirius and James followed the wolf up the steps and studied the door for a moment before James made the bold executive decision to tease it open.

Sirius slipped past him and stuck his head through the opening, finding to his surprise that the circular door was actually the backside of a portrait. The room beyond it was filled with more portraits—their inhabitants fortunately sound asleep—and cozy yellow sofas and armchairs, with various plants strewn across the floor and hanging from the walls. Over a massive wooden fireplace was a rippling banner of a chittering badger.

Sirius had never seen this room before, but it was, without a doubt, Hufflepuff's common room. They'd found a secret tunnel not only into the castle, but into one of the notoriously difficult-to-access House dormitories.

Fur brushed against Sirius's as the wolf peered through the opening, sniffing with his lips drawn back. He let out a growl similar to the one he'd given when he spotted Frank and Alice. The wolf could smell the students in their dorms, Sirius realized, his insides twisting into knots—he could smell them too, faint odors of humanness wafting down from beyond the common room's little wooden doors. They had to get Remus out of there, fast.

Sirius pulled shut the portrait-door and helped James shove the wolf off the landing. He tumbled down the steps in a tangle of limbs and leapt back to his feet, baring his teeth and glaring at them with a distinctly animal savagery. James and Sirius would have their work cut out for them getting him away from the sleeping Hufflepuffs.

But as their eyes met over Remus's tensed, snarling body, Sirius could tell that they were thinking the same thing: next time there was a full moon, they most definitely had to do some more exploring.