I don't own Adventure Time

The sound of footsteps echoed through the halls as two figures walked. It was rather quiet for they were in no ordinary place, but a place deep underground, unable to be found by rarely anybody except those who were accepted. Coming to a stop in front of a metal automatic door, the two figures stood on both sides, placing their hand simultaneously on separate electronic screens. A green grid appeared, flashing a line back and forth along the hands while creating a small buzzing noise. The scanning stopped and both machines flashed a light green, indicating clear identification. The two figures stepped back with their arms behind their backs and watched as the metal doors slowly moved to the sides allowing access to the other side. Once completely open, both forms walked in, not glancing back as the doors began to close.

Walking through a couple of more hallways, the two stopped in front of relatively large double doors with the label 'Main Desk' written on top of them.

"Are you sure, Finn? We could use a bit more training." One of the forms, a female in fact, spoke to the other known as 'Finn'.

"We've been training all of our lives, Fionna. I do believe we deserve a REAL mission by now." Finn, the male, spoke in a determined tone, not taking his eyes off once at the door. He reached for one of the doors, but was cut short when the female, Fionna, stepped in front of him.

"We have had real missions, Finn."

"You know what I mean. They treat us like we're kids and only give us the simple assignments. But now we deserve better." Finn pushed Fionna aside and knocked on the doors. She sighed as she took her position right back next to Finn.

Small footsteps were heard, held within the other side of the doors. One of the doorknobs jingled and turned causing the one door to open ever so slightly. A small man's eyes contrasted with the darkness of the room as he studied the two. Once he opened his mouth, a low voice vibrated as he spoke. "Team Agents Double F's. Wasn't expecting your presence today. State your business. Chief is rather busy." He gave a chuckle.

"Enough of the voice. We would like to speak with Chief. And tell that it's urgent." Finn crossed his arms as he stared down at the man.

The man grinned, his teeth shining a bright white. He closed the door, leaving to tell of the urgent unexpected presence.

"These high-heels freaking hurt!" Fionna sat down on the floor and pulled the fancy black footwear off of her feet. She groaned as her toes were freed from the squeezing straps of the shoes.

"It's just for now Fionna. They aren't going to take us seriously if you come here in flip-flops."

"You aren't the one wearing these feet killers." She looked over at the polished black shoes being worn by Finn. "Look at that. You get those super comfy shoes."

"Hey my feet are killing me as well! And it's super sweaty inside. You're lucky that yours are breezy."

"Why do we have to wear these clothes anyway…"

"It's our uniform. They wouldn't allow you in here if you didn't have your uniform remember? Also it's just for here. We can change when we get back."

"Why couldn't we have worn our combat uniform then… Those are way more comfy."

"Because we're here to talk with the boss and we have to look formal."

Fionna sighed as she looked at both of their attires. She was wearing a black dress that came down mid-thigh accompanied by a pair of high-heels. On her ears were some dangling diamond earrings and her hair was tied back into a nice high curled ponytail. Finn was wearing a black business suit accompanied by his shiny shoes. He had a blue tie and his hair was sprayed and combed back.

Fionna pouted. "You could have at least let me wear a bunny hairpin…"

"I would of, but then I'd have to see it and be mad that I couldn't wear my bear beanie."

Shuffling was heard once again, inside. Fionna gasped as she scurried to put back on her heels and stand back up formally. Luckily, just as the door opened, Fionna managed to stand back into position without a sign of emotion. The man opened the door wide this time. "Chief would like to see you." He said in a higher pitch of voice that wasn't close to how deep his other voice was before.

Finn motioned for Fionna to enter first, before entering himself. They both walked forward into the dark room, with the man following behind, stopping once in front of a large desk with a chair turned the opposite way in front of them. The man walked up behind the desk and whispered something to somebody on the chair. A light pink hand was held up, motioning for the man to stop talking. Then slowly, the chair turned, allowing Finn and Fionna to see the body of the person yet the head was still hidden in darkness.

Finn and Fionna stood still with their chins up and their hands behind their back, staring seriously at the person before them. Finn had decided to speak first, "Chief Bubblegum, you're looking rather fine today." He said politely yet sophisticated. "How has Captain Gumball been lately?" Finn started the small talk before getting into bigger conversations.

She waited a few before replying, "You're looking rather fine yourselves my two youngest agents. Captain Gumball is doing well since his recent injury." Her voice was unusually high-pitch yet was still very serious sounding.

"I see. That is very good news."

It became quiet as the ticking of the clock occurred every second. Finn and Fionna stared sincerely at Bubblegum who stared back. Then she crossed her legs and sighed. "I assume that was not all that you had come for."

"No ma'am." Both of them replied.

"Get to the point then. As you can see, I do not have all day."

Finn stepped forward. "We would like to receive a mission."

"Is that all? You would need to go to the agen-"

"Sorry to interrupt. But we mean a top-secret mission. The most dangerous of the dangerous."

Bubblegum studied him. She uncrossed her legs and stood up from her chair, with her arms folded behind her back. The chair squeaked as she moved, and the clacking of her heeled boots on the hard floor sounded with each step she took. Bubblegum walked over to a large portrait with her back to the two and looked over it.

Finn continued from her lack of reply. "Ever since we've arrived here at the ages of four and five all those years ago, we've been learning from the best and had done everything we could for the agency and for the outside people. We were paired together as copartners by our choice and have done even better working together. But the only missions we have ever obtained through our lives so far were so simple and easy. You know we're better than the majority of the agency and if not, all of it. We deserve a top secret mission. The ones that would take a life time probably to complete."

Bubblegum spoke. "Years ago, this agency was created for this large town that we all take residence in. There were many crimes and murders throughout the time before then. Since the agency was pulled together, the crime rates and murders have diminished. We work in secret, keeping everybody protected without them even knowing." She turned around. "Some villains are smarter and stronger than others. We balance our work out to each of our agents so we don't lose many reinforcements. You two are too young to be able to even think about taking on a top-secret mission."

"Age doesn't matter when it comes to fighting for the good. You know how skilled we are and age has nothing to do about it."

"It has to do plenty. You both have been only alive for sixteen and seventeen years, and yet you already ask for a top mission? Being young means that you are naïve and are unable to do things older agents can do. We have only taken you two under our care for both of your older adopted siblings are a part of this agency." She chuckled. "Agents older than you are still incapable of handling one of these missions. What makes you think you will be able to do what older more experienced cannot?"

"We're not like the rest. I believe that you have watched some of our smaller missions and know what we are capable of. Please, give us a chance."

Bubblegum sighed. "Denied. I'm sorry, but there is no way we'd let you out on a top mission. Now please excuse me."

Fionna cut in. "Wait!" She walked her way over to Bubblegum and spoke to her in private for Finn not to hear. "Look, Finn used to be a carefree guy. But ever since you sent his brother Jake on a top mission, he hasn't been the same and had become more serious. Who could blame him? Jake's been gone for five whole years on whatever mission you sent him on. So yeah… you kind of owe him. And you do know that we're better than most of your agency. Just give us a shot!" Fionna confidentially smiled.

"Your lack of respect in speaking is also a problem…" She sighed. "But. You are very persuasive in some ways. And I'll see what I can do for now. Then you'll see how hard the advanced agents have it."

"Deal." Fionna walked back to her place and watched as Bubblegum began talking to the little man. They both stood there, unmoving, as Bubblegum and the man spoke for a good fifteen minutes.

Bubblegum looked over at them. "Wait outside until further notice." She walked to her desk and took a seat back down on her chair.

Finn and Fionna turned with their heel and walked out of the door, being held by the man. The bright light from the outer hallways illuminated the dark room. Once taking their step back out, the man closed the doors behind their backs allowing them to let the breath they were holding go. Fionna groaned as she kicked her heels back off which hit the other wall and fell down onto the floor. She took a seat on the floor near the door and stretched her limbs, giving a relaxed sigh. Finn walked beside her, yet chose to stay standing instead. "So what'd you tell her?" He looked down at her casual form.

Fionna shrugged, not wanting to bring up the subject of Jake to Finn. "I'm just very persuasive."

Finn frowned at the half-true response, but let it slide nevertheless. "We make a pretty good team."

"We do. Ever since I was seven and you were eight. We've only met because we were the youngest with everybody being eighteen or older. So they tried to train us together to save time." Fionna closed her eyes and put her hands behind her head, remembering once again about the time before.

Finn gave a short laugh. "And we've been best buddies ever since. I wouldn't have chosen a better copartner than you."

"Same here." Fionna laughed. She began stretching again, trying to ease her sore legs. They waited in silence, watching very few people pass by. The people passing by gave them weird looks as if wondering what they were even doing in the advanced floor. Finn sighed as another one passed by. "So what's Cake been up to?"

Fionna tensed up, not wanting to say anything wrong since both Jake and Cake have kind of the same lifestyle. They both had one adoptive younger sibling and were quite close to being the same age. They were also evenly skilled in the arts of the agency and had been sent on countless missions. But unlike Finn and Fionna, Jake and Cake didn't get along and barely knew, or even tried to know, each other. "Um. She's been fine." Fionna somewhat lied. She hasn't heard in Cake for quite a while, but that didn't even compare to the five years of Jake. Nobody knows Jake's whereabouts and hasn't heard from him since. They aren't even sure if Jake is even still alive.

Finn put his hands in his pockets and leaned back. "That's cool." He replied.

Fionna sighed with that being done with.

After another hour, there was still no sign of the doors being reopened. Finn began pacing back and forth, having his suspicions that the doors wouldn't even open again at all. The loud steps of his shoes echoed through the empty halls. Fionna sighed as she watched him.

A small clicking sound was heard from the doorway.

Both Finn and Fionna snapped their heads to the doors and watched as the doorknob began to turn. Fionna jumped up from the floor and quickly ran over to the wall, obtaining her high-heels and putting them on. Finn fixed his tie and stood in front of the doors with his arms now repositioned behind his back. Fionna hopped over on one foot while putting on the other high-heel and stomping it on the floor a few times for it to fit, and then hastily fixing her now messed up hair and finally putting her arms behind her back as well.

The doors opened with the small man opening it wide and motioning for them to come in. Since both doors were open this time, both Finn and Fionna walked in together. Bubblegum sighed as she watched them make their way to her desk once again. She was holding a yellow folder on her lap as she sat on her chair. Once they positioned themselves in place and stared straight into her eyes, she spoke. "I actually can't believe I'm going to allow you to do this." They didn't reply as she placed the yellow folder on her desk. "As you know, we have very few top missions."

"Yes."

"It has seemed that all of them were already or are still in progress."

"What is the folder may I ask?" Finn answered.

Bubblegum sighed again, continuing where she had left off. "Well. Except for one. This very folder has one of the most difficult missions contained. It may be even ranked one as most difficult. We have sent many agents on this very mission and they would either return from years of work empty-handed, or they wouldn't return at all. Even returning seriously wounded."

Finn tensed up and his eyes grew rather cross, yet he still managed to stay stern.

She continued. "In the past, some of our agents were even killed. May it be on purpose or may it be on accident. Fortunately, the deaths during this mission seemed to have mellowed out for some reason." Slowly she reached for the front of the folder, and opened it, revealing the data held within. "This one mission is known for capturing 'The Duo of Thieves' also can be known as 'The Abadeer Twins'." A few pictures fell out from within the folder, allowing both Finn and Fionna to pick one up each.

Finn frowned. "Is this some kind of joke? Thieves? We've dealt with numerous thieves in our lifetime. You're trying to trick us into thinking this is a top mission, aren't you."

"These aren't any normal thieves. If I wanted to trick you then I wouldn't be able to. I don't have time for tricks. Read through the few journals and articles that the past agents have written about them. Of course you don't have to do this mission."

Finn crossed his eyebrows as he looked down at the picture in his hand. The picture was rather dark and was hard to make out the two figures held within. The only way you were able to see them at all was because of the bright moon that was located in the back of the picture. One of them was holding up a peace sign and the other was merely sitting on the roof of a house. Both faces were completely dark and unable to see except for the extremely white, sharp teeth of the grin coming from the one holding up the peace sign. Finn still didn't believe that this mission was actually true because of this. Why would two thieves pose for a picture taken by an agent? Unless they were stupid and actually wanted to be caught of course.

Finn looked up and listened as Bubblegum began speaking again. "These two are major bandits and have been on our top wanted list for more than a decade. Beware, they are rarely seen by anybody. Our agent who we have sent on his mission has just returned with nothing. So this position has just reopened to your convenience."

"If they're so great and smart, then why are they posing in this picture?" Finn flipped the picture over, showing Bubblegum the picture.

She sighed. "The Abadeer Twins have been known to be tricksters and probably posed for that picture just for the fun of it, knowing that there is no way they would be caught."

"Then what makes them so special…"

She smiled. "It's the fact that they're only the same age as you two."

"You're telling me… That you've been after these two, ever since they were seven?! And you still haven't caught them yet?"

"The Abadeer Twins have been rather slippery."

"You're making this even less believable than it sounds already."

"And that's why you're still too young to be taking on a top mission. If you don't believe me, fine. You don't have to accept the mission and now you are allowed to leave my presence. I knew that this would be a waste of my time."

Fionna cut in once again. "Now hold on just a second." Fionna leaned in to Finn and whispered. "Finn? What are you doing?! This is a top mission that we've always dreamed about!"

He whispered back. "Yeah but just look at this picture! No 'master thief' poses in a picture! Besides, they're only seventeen too! How could the agency not capture them yet?"

"Are you saying that they can't be master thieves because of their AGE? Just like how we shouldn't have a top mission because of our AGE?"

Finn sighed as he looked back and forth between the picture and Fionna's glare. "Fine."

Fionna smiled triumphantly and repositioned herself. She cleared her throat, signaling that they had finished talking. "We accept the mission."

Bubblegum sighed as she handed them the entire folder. "You can't be too naïve in this mission. It will be hard just to track these two down. And if you do manage to track them down, be prepared to have to do it again. The rest of the information about them is held within. Of course we don't know everything about them and only know a few scraps about them in all. Do you think you can do better?"

"Yes ma'am."

She smiled. "Then be off now, I have business to take care of."

The man opened the doors once again, allowing them out. Closing the doors behind their back, he walked back over to the desk where the Chief was focusing on her thoughts. "Do you think this is a good idea?"

She continued staring fixedly at the door.

"Ahh! Pajamas!" Finishing changing, Fionna stepped out of her room and flopped down on the couch inside of the living room. "Yo Finn? You gonna change yet?" She inside the kitchen and watched as Finn read through a few of the articles of the mission. He didn't reply as she began to turn on the television and flip through the channels.

The agency funded a large apartment for their few agents. Both Finn and Fionna, being copartners, shared their own apartment room. Of course there were separated bedrooms for the each of them, so they didn't have to worry about that. Since the agency had few people they had to pay for, they had decided to give their agents the best of the best. That being said, all of the apartment rooms were the size of a regular house, even being well furnished with the theme of the agents choice. Finn and Fionna had chosen a sort of a teenage hangout theme. Their furniture was a mix of light colors all from the rainbow. Fionna was currently laying on a light blue couch while Finn was sitting on a red barstool held within the kitchen. The entire apartment room consisted of a large living room, a kitchen, a few bedrooms, and two bathrooms.

Fionna rested her legs out on the glass coffee table as she continued to find something to watch. Upon discovering that there was nothing, she jumped off of the couch and made her way to Finn, for whatever he was doing seemed more interesting than doing nothing. Peering over his shoulder, Fionna looked over what was currently in his hand. It was the same picture of the two thieves posing. She then took her own barstool and sat near him, beginning to look through the folder as well. The chief was right, there was very little written about the two. In the back of the folder were two 'Most Wanted' posters. One each for one of the thieves and a reward for whoever was able to catch them. Of course the pictures on the posters still didn't allow the viewer to see who exactly they were. "Hey can I see that?" Fionna pointed at the picture Finn was holding to pair it and the most wanted posters together. Finn handed it to her and began looking through the folder for any other helpful information.

Fionna held the three pictures together, attempting to figure out at least something useful. Then she had it. It wasn't all important, but it was enough to get a start somewhere. "Ones a girl and ones a guy." She studied the pictures a bit more as she spoke.

"How could you tell?" Finn looked up from a letter and looked over the three pictures with her.

She pointed at one of the most wanted photos. "Look. You could barely make out a ponytail."

"That could just be a guy with long hair?"

"Could be. But I doubt it." She pointed at the picture that Finn studied for quite some time. "See, her face structure is a lot more feminine than a guy's structure." She pointed at the other most wanted picture. "See, his face is a lot more… what's the right word… guy…ish? Kind of the same between you and me. We look almost identical yet we aren't even related and you have the guy face and I have the girl's."

"That's a pretty good observation. I believe you. But we should make sure by actually seeing them. Hey do they even have any names?" Finn flipped through the folder once more.

"They probably do, but the agency never got a hold of it." Watching as Finn flipped through the folder, her eyes widened as she snatched one of the papers that Finn nearly skipped over. The paper was ripped in some areas, so it was hard to make out what some of the sentences said. Skimming through it, Fionna came to a stop once in the middle of the paper. "M. Two M's. The names were ripped out except for the beginning letters."

Finn stood up from his chair and looked over her shoulder. "So whoever wrote this article knows their names then."

"Yes. We should find out whoever wrote this as our first step."

"Agreed. It's pretty late. We'll start bright and early tomorrow."

"Alright. See ya tomorrow." Fionna stood up from her chair, carefully placing everything back into the folder, and then making her way back to her room.

"Night." Finn said as he stepped into his own room, the automatic door swishing closed behind him.

Why am I writing another story instead of updating my others? I have no idea.

As you can probably see, I'm going to take this story rather seriously and the updates for it might not come as quickly as my other stories. This is to help me improve in my writing skills by being more descriptive.

This is my first story ever going to involve the gender swapped characters. The Abadeer twins are indeed Marceline and Marshall Lee. This story is an alternate universe so Marceline and Marshall aren't vampires which is why they are able to age. Finn and Fionna are secret agents along with Cake and Jake. Everybody is probably human in this story. Finn and Fionna are the YOUNGEST out of the entire agency yet they stated that they are one of the best. They are also copartners due to being the youngest and basically growing up together.

So Finn is the serious one in the partnership due to Jake being sent on a mission five years prior and never returning. And I have a question to ask you guys before the next chapter comes out. Okay, so Marceline and Marshall are supposed to be all trickster like, but whose going to be the serious one in their team? You are able to choose either Marceline or Marshall. I could make it work both ways but it would change a few facts and events in the story.

Princess Bubblegum an d Prince Gumball's ages are altered. Actually all of their ages are altered. Finn is seventeen, Fionna is sixteen, and Marceline and Marshall are seventeen.

Finn and Fionna won't actually meet Marceline and Marshall until a later on.

Finn and Fionna are not related. But Marceline and Marshall are twins. (Just if you're confused)

I'm not really sure if I should include pairings in this story. But I probably will. So send me pairing ships. Although I'd probably just go with my choices anyway. Tell me if I should make it a pairing story.

And I will repeat: Who do you want to be the more serious one in Marceline and Marshall's team? They will both be tricksters and awesome still, but one of them will just be a bit more serious and harsher than the other.

Also I have no idea what this story should be rated.

There aren't going to be skyscrapers and stuff in this story. They basically live in like a large village or town