"Raphael!" A voice echoed through the sewers. "Raphael!"

The red banded turtle heard the cries from the sofa and stared toward the entrance to their lair. At first he thought he was imagining this so he listened for the call again.

"Raphael!" It shouted again.

Raphael jumped off the couch and into the sewers. "Hello?" Raphael called back.

"Raphael!" The voice screamed again before loud footsteps rang through the sewers. Raphael could see the figure who was calling him, Mona Lisa. It had been a while since the salamander mutant had seen the turtles.

"Mona Lisa!" Raphael called back. He smiled at her and she grinned back brightly as she stopped in front of him.

"You're lair is impossible to find, by the way," Mona commented.

"Well, when you've only visited once," Raphael joked back. "What brings you by? Finally decided to admit your undying love for me?"

"Try again, Romeo," Mona laughed. "Actually, I need to talk to your brother, Donatello."

"What do you want that dork for?"

"I've been working some time on a retro mutagen so I can be a human again but I'm having some trouble with the chemistry," Mona admitted. "I thought he might be able to help me."

"Donatello's in his lab," Raphael told her. "Come on, I'll give you a tour." He lead her into the liar and back to Donatello's lab. The purple wearing turtle was busy on some gadget so he didn't notice either of the mutants enter.

"Donatello, you have a visitor," Raphael said.

"Huh?" Donatello asked, putting his welder down. He lifted his welding mask to stare at his brother and Mona. "Oh hey Mona." Donatello greeted. "What's up?"

"I was wondering if you could help me," Mona started. "I want to create a retro mutagen."

"Of course."

"Great," She pulled a notebook out of her bag and showed it to Donatello. "I have some research done here and I have the chemical formula for mutagen but reversing the mutation is what's stumping me."

"It's definitely a difficult nut to crack but I'm sure we can figure it out," Donatello added.

"Do you mind working on this on your own for a bit?" Mona questioned. "I've been walking for the last several hours and I'd like to sit down for a bit."

"Go ahead. I'll get started."

"Thanks." Mona and Raphael returned to the living room to relax and chat for a while.

"So what have you been up to lately?" Raphael asked.

"Working on a retro mutagen mostly," Mona Lisa replied. "I've started my own lab in the sewers in Brooklyn. It's not big but it works."

"Raphael, Donatello, we're back!" Leonardo called out as he and Michelangelo walked into the liar.

"Hey guys, what's up?" Mona Lisa said.

"Hey dudette, great to see you," Michelangelo waved.

"Yeah, it's been so long," Leonardo commented.

"I know," Mona replied. "I've just been busy."

"She's started to work on a retro mutagen," Raphael informed.

"It's a long process but Donatello said he would help me."

"How far have you gotten?" Leonardo asked.

"Not too far but I have the chemical equation of mutagen and the effect it has on DNA but reversing the process is an entirely different story."

"Sounds brutal," Michelangelo replied.

"It is but with both mine and Donatello's intelligence, I'm sure we can figure it out," She told them.

"Right, well, Michelangelo and I are going to go make some pizza," Leonardo said. "Do you want some?"

"Sure."

Leonardo and Michelangelo went to the kitchen, leaving Raphael and Mona Lisa alone in the living room again.

"So what have you and your brothers been doing?"

"Stopping Shredder and Kraang from destroying the city, defeating bad guys, you know," Raphael started. "The usual."

"Ha, yeah," Mona yawned. "Oh by the way, do you mind if I spend the night here?"

"We better ask Master Splinter but I'm sure he'll be fine with it," Raphael told her. "Let's go ask."

Mona nodded as she and Raphael headed toward the dojo. Master Splinter was meditating in the center of the dojo but opened his eyes once he realized one of his sons had entered the dojo.

"Yes, my son?"

"Hey, sensei, you remember Mona Lisa," Raphael started as Mona waved to the rat mutant.

"Of course, wonderful to stop by," Master Splinter replied.

"Do you mind if I spend the night here?" Mona questioned. "My lab is just a bit away and I don't want to have to travel that far again today."

"Stay as long as you would like."

"Really? Thank you, Master Splinter," Mona thanked.

"You can take my bed and I'll sleep on the couch," Raphael offered.

"Why thank you, Raphael."

"No problem, my lady," Raphael 'bowed' to Mona and the solomander mutant laughed. The two returned to the couch and started another conversation.

"So what are you going to with the retro mutagen?"

"I'm going to be human again," Mona answered. "I was hoping to return to college, maybe get a job."

"Oh, is being a mutant too good for you now?" Raphael joked and Mona smiled.

"No, it's just if my old friends and professors saw me like this, they'd be dissecting me," Mona told him.

"What major were you again?"

"Physics. It's a lot different from chemistry so that's why I need help with this."

"Hey guys! Pizza's done!" Leonardo called from the kitchen. The salamander and turtle mutant rushed to the kitchen where the other three terrapin mutants were. Raphael and Mona helped themselves to some pizza while Donatello started a rant.

"I've figured out how mutagen affects the body. It connects itself to the DNA strips then changes them. If we remove the mutagen from the DNA, it would morph back into its nature state," Donatello informed.

"Any ideas of how we're going to remove the mutagen from the DNA?" Mona asked.

"We could either create a solution to dissolve the mutagen or we could build a laser to extract the mutagen from the DNA," Donatello answered.

"I think we should go with the solvent," Mona told the purple wearing turtle. "I'm not much of an engineer plus a laser with the ability to do things like that are really hard to build. It would take us a lot longer to build than it would to create a solvent."

"Aw look, the nerds are bonding," Raphael smirked. Mona rolled her eyes but smiled a bit at the red banded terrapin.

"Shut up or else I'll use the retro mutagen to turn you back into a tiny pet turtle," Mona retorted.

"You can't do that!"

"Just watch me!"

"Calm down you two," Leonardo interjected. Both mutants crossed their arms and Raphael stuck his tongue out to the female mutant. "Donatello, isn't this a dangerous experiment? I mean, what if some of the retro mutagen gets on you?"

"It could happen but using normal mutagen could easily turn me back into a mutant," Donatello responded.

"What kinds of chemicals should we use to create the solution?" Mona started.

"Well, mutagen is made of these chemicals," Donatello paused to show Mona Lisa the list of chemicals that made up mutagen. "If we used these chemicals," He stopped once more to show her another list. "The solution would break down into a different substances that are harmless to the body and would disappear in a few days."

"That sounds like a really good plan!" Mona exclaimed. "Do you have these chemicals?"

"I have all of them actually," Donatello bragged. "All we have to do is figure out the right amounts of each chemical to put into the body. We can get started in the morning."

"Sounds good!" Mona said before she stood up again. "I think I'm going to head to bed. I've had a long day."