Note: Some fans have already written similar things about Superhero Day, but this is my take on it (it's not my idea!)
"Oh, no, Tikki!" Marinette fretted. "It's a disaster!"
Tikki watched Marinette dash around her room, searching for objects that weren't there. "What is?" she queried.
"It's Superhero Day at school," Marinette explained, while rummaging through her sewing basket. "But I completely forgot about it, and I said I'd be doing it, and everyone in the entire school is doing it-"
"Doing what?"
"Well, dressing up as our favorite superheroes, of course!" Marinette extracted a swatch of black fabric from her bag, then shook her head and dropped it back in. "Most of the Miraculouses are dormant, and no one likes Papillon. So it'll probably be mostly Ladybug and Chat Noir."
"What's wrong with that?"
"I don't have a Chat Noir costume!"
"You have a Ladybug costume," Tikki pointed out.
"Yes, but imagine what'll happen if I come to school as Ladybug!" A vivid scene played through Marinette's head. "They'll wonder why Ladybug is at their school, or they'll put the pieces together and realize that Marinette isn't there, or-"
"You could go to school as Ladybug and say that you're flattered to be honored like this or something," Tikki suggested.
"Yes, but then I'll still have to go to classes, and- argh! I can't go as Ladybug." Marinette closed her sewing bag.
"Hey, I have an idea," Tikki piped up. "Switch kwamis with Chat Noir. You'll wear his costume, and he can wear yours."
"That's a brilliant idea, Tikki!" Marinette dialed Adrien's number and explained the plan. "Meet me behind the school, all right?"
Once they were behind the school, Tikki and Plagg flew out of their respective bags. The two humans swapped their Miraculous stones. Marinette carefully put on Adrien's ring, then looked suspiciously at Plagg.
"So... I just say 'Plagg, transforme moi'?"Marinette asked with uncertainty.
"Wh- whoa!" Plagg was sucked into the ring instantly. Marinette watched in awe as cat ears sprouted on her head, a mask swept across her face, and the bodysuit encased her.
"Hey, this is pretty cool!" Marinette said, twisting around to look at her tail.
"M-my ears aren't pierced," Adrien realized, holding Marinette's earrings.
"Oh, that won't matter," Tikki assured him. "Try putting them on."
Adrien cautiously held the earrings up to his ear and shoved them in. He felt nothing, but somehow the earrings were securely in his ears.
"Tikki, transforme moi," Adrien managed. He closed his eyes as the mask and bodysuit covered him and the yo-yo formed on his hip.
They looked each other up and down.
"Not bad, Ladybug," Marinette commented. Adrien blushed.
"We gotta go to class," Adrien realized, hearing the bell. They dashed through the halls into homeroom, passing many classmates dressed up as various heroes and some villains. Most classmates who had been Akumatized were dressed as their own villain. Aurora Boreale was dressed up as Climatika, unsurprisingly. Nathaniel was the Dessinateur, Alix was Chronogirl, Max was Le Gamer, and Rose was La Fleuriste. However, Alya was dressed as Ladybug, as was Chloe. Sabrina donned a Chat Noir outfit.
When Marinette and Adrien entered the classroom, Alya looked surprised to see Marinette in Chat Noir's costume. "Hey, Marinette! Nice costume. It looks really realistic!"
"Um, thanks!" If only you knew. "I would've thought you'd be Lady Wifi."
"Well, you're supposed to dress up as your favorite superhero," Alya reminded. "And I like Ladybug a lot more than Lady Wifi!" She looked at the ring Marinette was wearing. "You know, I thought that you'd be Ladybug, too. You have the same hair color and everything."
For an aspiring journalist, Alya didn't seem all too observant. "Oh, well, you know... Chat Noir's cool, too," Marinette said nervously.
Adrien turned around and winked at her. Alya didn't notice this, either.
Ms. Bustier, wearing a Ladybug costume, called for attention and began the class. Peppered through the day, people would stand up and yell, "Cataclysm!" or "Lucky Charm!" and throw their yo-yos at the nearby students. Some kids claimed to see Akumas fluttering by, while others spread rumors that Chat Noir and Ladybug really were in the school. It was mostly uneventful until one student screamed during the middle of class and pointed to the hallway.
A student had been Akumatized and was prowling the halls. Marinette heard the monster announce herself, but she couldn't make out what the student said, and she couldn't discern the true identity through the costume she was wearing: a shockingly blue thing that nearly hurt to look at, which covered her head and body.
Marinette almost raised her hand, then cursed under her breath. There wouldn't be time to sneak out and transform into Ladybug, especially since Adrien had her Miraculous stone and kwami. Adrien looked back at her, clearly panicked. Marinette squirmed in her seat.
"Stay calm, everyone!" the teacher tried to say, but the students were already screaming and hiding under desks.
Marinette leaned forward, unnoticed by her classmates, to talk to Adrien. "We don't have time to swap kwamis!" she hissed.
"What can we do, then?"
"Slip out unnoticed and make sure no one sees us battle the Akumatized one."
"What, Forget-Me-Not?"
"Is that her name?"
"I think so. But- we'll have to battle her like this?" Adrien slid his right hand over his left, touching a ring that wasn't there.
"We have no choice."
Adrien resignedly followed Marinette out of the classroom. No one noticed. They were all either curled into balls or covering their eyes in fear.
The door locked behind them with a click. Adrien muttered something unintelligible.
Forget-Me-Not noticed them immediately. "Ah, L-" She stared at them in confusion. "Ladybug and Chat Noir?"
"Um... kind of," Marinette said, embarrassedly. She noticed the classroom that Forget-Me-Not had exited from. No one seemed to remember anything. They all sat slumped, in a daze, as if they had even forgotten basic motor controls.
"What did you to them?" Adrien shouted. He reached for his staff, intending to point it at Forget-Me-Not, but instead only found a yo-yo. He sighed and resigned himself to spinning it menacingly.
"Oh, those guys? They don't remember anything anymore. Except for me. But that's not important. It's time to give me your Miraculous, or you'll meet the same fate!"
"Looks like Papillon's still at it," Adrien muttered to Marinette, who nodded and extended the staff she was holding.
"Nice try," she said to Forget-Me-Not, and then whispered to Adrien, "Where's the Akuma?"
He shrugged.
"Well, if I can't have your Miraculous, I'll settle for the burning memory of me in these people's minds forever." She laughed evilly and held the flower-shaped keychain attached to her suit's zipper. It began to glow, and as she pointed it to Adrien and Marinette's classroom, it blasted the entire room with energy.
Marinette gasped and looked through the window. Her classmates looked glassy-eyed and collapsed.
"The Akuma must be in her keychain, Ch-" She turned around as she stifled the rest of the name. Was she Chat Noir now? The thought was incredibly weird to process.
"Right, My Lady." He swung the yo-yo at Forget-Me-Not and tried to grab the keychain, but the yo-yo wrapped around her forearm instead.
"Uh, what do I do?" he yelled, as Forget-Me-Not shook her arm violently. He was dragged back and forth, but didn't let go.
"Drag her to me!" she commanded, turning her attention away from the window. She thrust her arm in the air and cried, "Cataclysm!" Her fist crackled with destroying energy. To wield so much power made her... uncomfortable, almost itchy. As if she might wipe her hand on her suit by accident and destroy it.
Ladybug/Adrien hauled Forget-Me-Not, with some effort, towards himself and Chat Noir/Marinette, who swiped at the keychain. Forget-Me-Not leaned and jumped away from her hand. Marinette, unbalanced, twisted onto her back and fell onto the floor, hand in air.
"La- Chat Noir, no!" Adrien cried, and released Forget-Me-Not instantly to check on Marinette.
"You stupid cat," Marinette muttered, rubbing her head with her non-ringed hand. "Or bug, I suppose. She's getting away!"
"Sorry, minou. Are you hurt?"
"I'll be fine. Let's go." Marinette jumped onto her feet and raced after Forget-Me-Not. Adrien followed, hoping that she really was okay.
They ran at full speed behind the villain. The neon blue made her hard to miss.
Finally, they had cornered her. Marinette twirled the staff in the fingers of her non-ringed hand and set the end on the ground, leaning on it like a walking stick. Adrien whipped the yo-yo out and gyrated it in front of him like a shield.
"N-no!" Forget-Me-Not desperately pointed her keychain at Ladybug and Chat Noir, but the shield Adrien was maintaining blocked the attack. They advanced slowly.
Forget-Me-Not quickly glanced from one to the other fearfully. She widened her eyes in apparent realization and shot at a spot somewhere between Ladybug and Chat Noir. The yo-yo deflected the attack at Marinette, who fell to the floor immediately, mouth agape and limbs slack.
Time seemed to slow as Adrien realized that he had to keep Marinette's hand away from any surfaces until she could come in contact with the keychain. "NO!" he cried, desperately wanting to drop everything to help her, but instead grabbing the staff and propping up her forearm with it in the 2 seconds before it hit the floor. All while spinning the yo-yo like a shield.
He glanced upwards at Forget-Me-Not, hatred blazing in his eyes. "No one messes with Ladybug. And by messing with Chat Noir, you've messed with Ladybug." He swung the yo-yo with unbelievable force at Forget-Me-Not, who dodged quickly.
"Lucky Charm!" he shouted. A stream of red and black illuminated the fish hook that fell into his hands.
"Okay, that's just weird," he murmured, and studied it closely. "What would Ladybug do in this situation...?"
His eyes darted around the room, falling on the round part of the keychain. He gasped and tied the fish hook on the yo-yo about one inch down from the case. Hurling it at Forget-Me-Not, the hook snagged the keychain and dragged her closer to Marinette's open hand.
"No! Please!" Forget-Me-Not struggled against Adrien, to no avail. With a sharp tug, the keychain fell onto Marinette's outstretched hand and disintegrated. The Akuma danced away, and Adrien flicked the yo-yo open and caught it. So this is what it feels like. He then threw the fish hook in the air and said Ladybug's signature phrase.
A flash of red and black swirled around Marinette/Chat Noir, who blinked and ran to Adrien/Ladybug immediately. "Adri- erm, Ladybug! You did it!"
"I did it!" Adrien agreed, pleased with himself.
Forget-Me-Not transformed back into... a student Marinette still didn't recognize, but who had jet-black hair and wore a red jumper with galaxy leggings.
"We have to go before she recognizes us and we de-transform," Marinette urged. As sneakily as possible, she backed into the nearest bathroom.
"Wait! Plagg will only eat Camembert. Here, I have some, I think."
Marinette wrinkled her nose. "You carry Camembert around with you?"
"Hey, I have to feed Plagg! Does Tikki need anything?"
"She likes cookies. I think I have some in my backpack, in the classroom." Marinette accepted the cheese and dashed into the stall.
"Plagg, I can detransform now," she whispered urgently. She still had three more minutes, but the more energized Plagg was, the better. No need to waste all his energy.
Plagg zipped out of the ring, and Marinette reverted to her usual self. "Here, have some cheese," she urged.
Plagg yawned. "You're not too bad with that staff, you know. You caught on quickly."
"Well, you know, it's not so different from my yo-yo. Are you ready to transform back?"
"Cheese takes patience." Plagg sniffed the cheese like it was the most precious thing in the world, then shoved it in his mouth whole. "Mmm."
"Plagg, transforme moi!" Marinette commanded, and Plagg flew right back into the ring. She dashed back to the classroom, slipping in under all the commotion and confusion. No one noticed. Adrien followed only moments after.
"Bien joue!" they said in unison, fist-bumping. Marinette flashed Adrien a fanged smile.
While the classroom sorted themselves out, Marinette couldn't help but to feel a little impressed with Adrien. He had gotten the hang of her powers almost instantly, freed the akuma without any guidance, and somehow snuck a cookie out of her bag without anyone noticing.
Alya leaned over and plucked another cookie from Marinette's bag. "You know, I still think it's weird that you chose to be Chat Noir when you could have pulled off Ladybug so easily. I mean, you really do have the exact same hair color. And hair style. ...And eye color. And... you have the... same... voice." Alya's voice faltered, and a strange look came over her face.