Five years after Harry graduated from Hogwarts.

Lupin arrived at the reunion dinner party and I hardly recognized him. He looked very young and radiant despite the fact that there was a full moon that very night. His hair was longer than I remembered and had gone completely gray, and his robes were the finest I had ever seen him wearing.
"Remus? I haven't seen you in ages," I exclaimed as I rose from my pre-party relaxation on the couch.
"Harry, wonderful to see you again. What has it been? Ten, fifteen years?"
"Not that long. I only graduated from Hogwarts five years ago."
"Ah! Where does the mind go?"
"You look great. You've obviously had some steady employment."
"Yes, I've been doing secretarial work for the school, away from students, of course, but it's work just the same."
"That's wonderful. "So, sorry to break the pleasantries, but how exactly is this going to work tonight? I was very surprised when you said you could come at all, you know, given that it's a full moon."
He suddenly went very pale and pinned himself into a dark corner of my entryway.
"What?" he gasped.
"The moon's just rising now, and it's full."
"Holy crap! I would have sworn upon my life that that was tomorrow night!" The enormity of his mistake had begun to make him tremble. Then an even worse thought hit him.
"Harry. I haven't taken my Wolfsbane yet. Severus has been under the weather this past week and I've been trying to brew the stuff on my own with no luck. I thought I had another day. How could I have been so stupid as to agree to coming tonight when it endangers everyone?"
"Calm down, Remus. Don't beat yourself up before you even start transforming. We still have some time if you stay in the dark, away from the moon, right?"
He nodded. "It slows the transformation, but it always happens eventually."
A thought struck me.
"Could we brew it now? I could help you a lot."
"There's no time. It's an immensely complicated potion. You've done Vampireswill, right? It's like that. Fifty-five ingredients, and even if it's in one of your books, it takes hours and hours to make."
Then he paused as an idea came to him.
"Wait a minute. Do you have 'Madam Moopsy's Household Remedies'?"
"Yes."
"Her 'Insomniac's Comatose Curer' will put me into a deep sleep even in wolf form. It has just three ingredients and takes only half an hour to make."
"Great! But we can't brew it in my main potions lab. There are millions of windows that would let in moonlight. The best place would be in the upstairs bathroom. No windows at all."
"Let's go."
"But, no! The only way to get to that bathroom is through a hall that will be flooded with moonlight be now. You know the one I'm talking about. Do you think I could cover you with something and run you across?"
"No, that much moonlight can only be stopped by solid walls."
"Do you apparate?"
"Sometimes. I haven't done it in a while, though, and never on a full moon. I might splinch myself."
"That's actually not a bad idea," I mused. "A splinched werewolf wouldn't be a danger to anyone."
"No way, Harry. Do you know how painful it is to be cut in half in odd ways? I've done it once before and I never want to repeat the experience. I'd rather rip myself apart as a werewolf."
"I'll picture the room in my mind for you and help you through the apparation."
"Okay."
He closed his eyes and I projected an image of him apparating into my empty upstairs bathroom. In an instant, he was gone.
I ran like a madman, gathering the recipe book and supplies needed. I took my portable cauldron, a small heat source, and the fairy dust, essence of prok dung, and milk of resinidula, along with various other prep tools.
Hermione stopped me as I ran thoughtlessly through the dining room where the party was gathering.
"Where are you going, Harry? You're the host."
"I'm not feeling well at all, doubt I'll come, except possibly at the tail end. Bye." I sped off.
"That doesn't explain the cauldron," Hermione shouted back.
"Headache remedy," I returned, as I raced up the recently renovated main staircase of the house at 12 Grimmauld Place.
Lupin's state had deteriorated by the time I had closed the bathroom door. Flashes of the old pain and exhaustion crept in and out of his features.
"Apparating took a lot out of me," he said, slumped against the back wall.
"I'm sorry," I said, and unconsciously ran my hand through wisps of his gray hair. I set up the equipment near him.
The potion was not difficult; it was designed for the average insomniac witch or wizard to stumble out of bed late at night and brew in a state of exhausted desperation.
Which was precisely the state Lupin was in at the moment. I heard a growl behind me and jumped a mile.
He sat on the floor with his arms around his knees, obviously fighting hard.
"Are you angry?" I asked, and immediately regretted saying such a random, idiotic thing. He nodded, however.
"I can feel the wolf pounding on me to let him out. I hate him, I hate him."
"Think of something positive. He's feeding off that hate." I was improvising now, jumbling facts in my mind from the night my third year at Hogwarts when Lupin turned into a wolf and the Dementors appeared.
I was losing him. The cauldron still had twenty minutes to simmer, and Lupin was getting furry. I was overcome with such adoration for him that I abandoned my brewing to focus on him entirely.
"Remus, do this for me, for Harry. I love you so much. You came to Hogwarts when I was a lonely boy and gave me more kindness than anyone had before. Now that I'm a man, I have no reservations about telling you that my love has grown along with me."
"You should get out of here and save yourself before I'm dangerous," he whispered, but I saw the wolf had been held at bay a little longer.
"No. I'm staying as long as it takes to get you through this. You spent too many nights alone in your youth. I'll stay with you even if it means getting bitten."
"You're too much like your father, but you have to get out of here just the same." He pushed me to my feet, but just then, we heard the unmistakable sound of a certain man's footsteps in the bedroom outside. A soft heel followed by a hard tap. A gentle placement and a menacing resolution in each step. I heard a voice.
"Marcus, come in here, now!"
"No, Father. I won't," I heard a small boy say. I whipped around.
"Who invited Lucius Malfoy to my dinner party? Into my house?" I raged as quietly as I could.
"Must have been Snape," Lupin grunted, "the bastard."
I wanted to rush out there and throw Malfoy out of my house, into the Muggle street, and save Draco's slimy, snot-nosed little brother from what happened next.
"Father, please let me go and play."
But I couldn't, because I had my dear werewolf in the bathroom with me whose secret would be betrayed to that singularly horrible man, the worst man who could ever find out.
"Let me go, Father!" I heard Marcus Malfoy cry pitifully.
"This won't take long," came the cool reply. "You know it never does."
Lupin could not contain himself. It was the teacher, the nurturer in him now, not the wolf, who tore across the bathroom and towards the door. Who was I to stop him? Me, a selfish weakling not willing to risk my own skin for an innocent boy. I deserved any laugh Snape would have about me if he heard a report of this.
Lupin burst out of the bathroom door, too much a wolf physically to be recognized, and ran between Lucius and Marcus, ripping apart father and unfortunate son. At that moment, the potion began to boil over and I deftly decanted some of it into a glass. Lupin began to bolt towards me now, and he was all werewolf.
"Stupefy," I hissed, and he slowed enough for me to grab the scruff of his neck and toss the potion down his throat. He dropped to the floor in a furry, slumbering heap.
"Out!" I screamed at Lucius Malfoy, bursting out of the bathroom. "How dare you defile the noble and most ancient house of Potter with your horrible, uncouth activities? I never invited you and I don't care who did. You're never welcome in this house, not you, not Draco, not your wife even though she's of the Black family. Not even him!" I pointed at Marcus, who was cowering by the bed. He looked exactly as Draco must have looked at his age.
"I have no love for any Malfoys, but you are the worst, Lucius. And your son will never come back here because he won't BE your son by the time I'm through with you and the Wizard Children's Protection Board. I don't know how long you've been doing this but I don't have to stretch my imagination too far to be you did it to Draco as well."
He raised his wand and opened his mouth.
"Expelliarmus! And shut up! Get out and take your poor son with you- for now. Go ahead and report that I'm an unregistered animagus like my father. It's petty crime after what you've done."

The morning after, Lupin was on the windowsill where I had finally placed him the night before. He was naked and still somewhat furry, and he had a tail. I handed him his cloak as I woke him up.
"Remus," I said, "we have work to do."
He nodded groggily, comprehending.
"I've already talked to the Child Protection Board, and they've arrested Malfoy. They've set the trial date for a week from now.
"Do we both have to testify?" he asked, standing up and swinging his cloak on.
"Just me," I answered, "since I'm the unregistered wolf animagus, the only other witness besides Marcus Malfoy himself."
"Now wait just a minute, Harry. You're not taking all the credit for rescuing the little twerp." I knew he was right, and immediately felt ashamed.
"But. you'd have to publicly announce that you're a werewolf."
"To tell you the truth, most of the world already knows. It's hard to hide. Come on. Like you said, we have work to do."