( SHORT) You're Me:

Through a freak accident Fionna switches Bodies with the Ice queen.

Fionna and cake ran together through the valley chasing the Ice queen who was holding her latest 'husband'. Cake launched Fionna ahead, causing Fionna to tackle the Ice queen. They tumbled into the mountain crashing through into a room made of crystal. They crashed into a podium, knocking over a brilliant cut gem. The gem was black grey and shining pure white. The Ice queen groaned as Fionna kicked her in the gut. Cake went to tend to Slime prince, who was dropped outside in the canyon.
"Why are you always doing this on the most inconvenient days!" She sprang up and shot a few Ice blasts around the room, jarring the gem up into the air. Without thinking, Fionna caught the gem and quickly threw it to the side and dodged the blasts. The Ice queen laughed crazily as she blasted before Fionna punched her in the face. The Ice queen landed on the Gem and a magnificent light filled the room. Cake wandered in and Subdued the Ice queen.

"Got her Fionna!" Fionna just stood blank for a moment or two. The Ice queen struggled and seemed very confused.
"Cake what do you think you're doing?" She said. She grabbed her face and looked down on the ground at Fionna. "Cake! Cake stop!"
"What is going on here?" Fionna said looking at her hands. Cake put down the Ice queen and looked at the two of them.
"What?"
"Cake? I feel mad old." The Ice queen said touching her face.
"I am not old!" Fionna defended. Cake's tail frizzed.
"Oh. My glob! This reminds me of that movie me and Monochro went to see!"
"What are you talking about Cake!"
"THAT! You are the Ice queen or... yourself? No!" They looked at each other as Cake spotted the Gem on the ground. She rushed to it and picked it up "This looks like an artifact of the Monochromicorns. I be LM would know about it. Stay here." Cake stretched out of the room and outside taking Slime prince with her to go find help. Fionna and the Ice Queen looked at each other. Or themselves?
"So we're hanging out. Like BFF's and girly stuff."
"Yeaaah Ice queen I'm going to the tree house. You stay here and don't damage my body while I'm gone... How do I fly this thing?"
"You use your hair... or my hair? Anyway you fly!" Fionna looked down at the body that she was in and tried to fly, hitting everything on the way out of the cave.

Back at the tree house Marshall Lee had let himself in and was sucking the red out of a crayon. He spit on a the ground and groaned.
"Yuck, waxy." A crash came from the ceiling and the Ice queen's body crashed into the room. "Wait in line. I've been waiting for Fionna to get home all morning." HE picked up and apple from the table.
"No Marshall it's me!"
"... Yeeeeeah. The Ice queen. I know."
"No Marshall it's ME." Fionna was shrouded by the body and voice of the Ice queen and earned a confusing glare from Marshall Lee.
"Are you alright?" Their attention was caught when Fionna climbed up the ladder into the room.
"Uhh Hi." The Ice queen greeted with Fionna's voice. Marshall flew over and puckered up ready to kiss her.
"Stop Marshall!"
"What?"
"If you kiss the Ice queen I'll never forgive you!"
"Whoa what?" Marshall looked at Fionna's body and she puckered up her lips and closed her eyes. "Uhhh What's going on here?"
"The Ice queen and I switched bodies." Marshall floated over to the Ice queens body and stared into her eyes.
"Say something only the real Fionna would know."
"You used to be a hero."
"What I never... ooooooh... Okay you're Fionna." She clapped in triumph. "Either way though I'd be kissing the Ice queen."
"Don't kiss anyone!"
"That's unfair. You switch bodies and I pay for it."
"Ugh, Marshall I'm not going to deal with this today."
"So I don't get a kiss?" The Ice queen said using Fionna's innocent face.
"Yeah, No. So what are we supposed to do?"
"Nothing, Cake should return soon with-" Lord Monochromicorn came down through the ceiling with Cake right beside him.
"Fionna! Don't worry," She said squeezing the ice queens face. "We will get you out of there! LM What do we do?"

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"What did he say?" Marshall asked.
"Morse code for they both have to touch the gem at the same time." Fionna's body snatched the gem from cake.
"Well let's hurry this up. I have princes to kidnap!" The Ice queens body poked the Gem with a finger before the room filled with light again. Fionna looked around after it faded and punched Marshall's face playfully.
"Ow My cheek meat!"
"I'm me again!"
"Did you have to hit me?"
"Yes."
"Oh Yay," Ice queen sang. "You know I feel this experience has brought us closer together..."
"Yeah... You need to leave Ice Queen..." Ice queen sighed and fly out through the hole. Marshall looked at the gem as Fionna held it.
"All this trouble over that thing... can I see it?" Marshall reached for it.
"Wait Marshall no I'm touching It-" I bright light shined through the room again. "MARSHAAAALLLL!"
"Whoops"