1975, the Gryffindor Dormitory...

After stuffing themselves with cake, Moony, Wormtail, Padfoot and Prongs collapsed on their four-poster beds in their room. After ribbing James about that afternoon and his obsession with Lily Evans for a bit, James got tired of being teased and changed the topic. "So, Padfoot, what do we say we start working on that Map of yours?"

Sirius smiled almost evilly. "Don't think that you're free of us mocking you about Evans, Potter. However, I agree, this gets priority. Wait, I had a map of Hogwarts somewhere in here..." Saying so, he dug into his trunk for a while and pulled out an old faded parchment. Remus looked at it and frowned. "Sirius, you do realize that none of the secret passages are drawn onto this map of yours? It isn't going to be very useful in its current state."

Sirius stretched himself out next to Peter and yawned. "Well then, draw them Moony! And they're secret passages! They're hardly going to be present on any standard map now are they? Has the moon addled your brains?" Peter and James burst into laughter at that, and Remus frowned anew. "Just because I'm a werewolf-"

Sirius interrupted. "Where-wolf?" James and Peter simultaneously pointed at Lupin and cried "There-wolf!" and the three of them howled with glee. Remus cracked a reluctant smile and shook his head. His friends had every right to joke about his transformation, given what they'd done for his sake, just so he wasn't alone when he became a wolf. He hadn't thought in his wildest dreams that he'd actually have a friend, much less belong. With a warm feeling in his heart and a smile still etched on his face, he began drawing the missing passages onto the map.

Sometime later, the four stared at Lupin's efforts and Sirius smacked his head. Remus glared at him. "What in bloody hell was that about?" Sirius glared right back and pointing at the fourth floor, said "You forgot about the one behind the mirror, you idiot. Did you get hit in the head by the Whomping Willow the last time we sneaked out?"

The four of them spent the next week mapping out the castle during day and its grounds at night under James's invisibility cloak. They met up a few days later at half past twelve, and after some more friendly squabbling about various locations and Remus making the appropriate changes, all of them finally agreed that they had added all the knowledge they possessed about secret passageways in the castle.

James spread the map out on the bed and stated, "Now, for the next step. I did my research on the charm, and the incantation has to be cast silently. Not to boast, but I am the best at wordless spells-"

Sirius cut in, "Yes, yes, and the worst at talking to Evans. Get on with it, Prongs." James gave Sirius the evil eye, before closing his eyes and sweeping his wand in an intricate motion over the paper. Nothing happened. Peter looked at the parchment and observed, "I don't think you did it right, James. Maybe you should try again?" Remus grinned. "Wormtail, we have to write down the names of the people in the castle. Let's see if it worked. Want to write our names first?" Wormtail grasped the quill and immediately handed it back. "No, Moony, you go on. You have the best handwriting." Sirius and James chortled.

Remus wrote the first name. James Potter. The dot attached to it immediately moved to the Gryffindor Dormitory. The four of them whooped and patted each other on the back. "This calls for a celebration, boys!" Before Sirius could get on his feet, James grabbed his shoulder and pushed him down again. "Let's finish adding all the names first."

Hours later, Remus yawned and stretched, dropping the quill with a slight clatter. James and Sirius, who had stayed awake with him, prodded Peter in his shoulder, which made him jump awake and look wildly around. "Relax Peter, you're safe.", said James as he leaned over the Map. "This is beautiful Remus. Absolutely beautiful. All that's left is a title that states we were the ones who made it. I have no doubt that this will be used by future generations of Hogwarts students to cause all kinds of mayhem. And we'll be helping them the whole while. Ah, this is a proud day for wizardkind", he continued, wiping an imaginary tear.

Peter stared at the Map in awe but then his face fell. "Say, isn't this against the rules? I wouldn't put it past our professors to confiscate it if they saw it, and-" James and Sirius started chuckling. "Like we haven't broken enough rules already. For heaven's sake Peter, we're unregistered Animagi. It doesn't get more rebellious than that!" Remus however, thoughtfully spoke, "Peter does have a point. I have an idea. Let's give the Map a catch-phrase, one that will cause it to work, and another to make it appear blank after we're done using it. That should keep it safe and sound."

Peter cried eagerly. "And for further security, we can use our nicknames for being called the makers of the map!" Sirius sighed. "You never change, Peter. Live a little, won't you? So the activating phrase can be..."

James flashed his teeth. "I solemnly swear, I am up to no good." Sirius nodded his agreement. "Should keep any goody two-shoes from accidentally activating the thing. As for the blank-phrase, I vote for 'Mischief Managed'."

The others offered no resistance to the proposal and after enchanting the map to do as mentioned, Remus added their nicknames to the map:

"Messrs Moony, Wormtail, Padfoot, and Prongs
Purveyors of Aids to Magical Mischief-Makers
are proud to present:
THE MARAUDER'S MAP "

The four of them stared in admiration at it before Sirius spoke. "There's just one thing left... What if someone like Snivellus gets their hands on this thing and tries to use it? There ought to be some kind of snarky reply, right? An insult or two or a thousand?" James snickered. "I have some intriguing ideas on that front..."

The four put their heads together and managed to Charm the Map to be able to reply to people based on their intentions, and specially added a few lines for Snape, should he ever try and use it. Finally, at the break of dawn, James stood up and held the map stretched out before proclaiming, "Here's to a new era of detention-free pranks! "

The other three slapped his back enthusiastically and finally, before they sunk into bed, exhausted by the night's labor, Remus tapped the Map and whispered, "Mischief managed."