"Thanks, sensei," Mikey murmured happily as his father handed him a steaming cup of hot chocolate. The smile that accompanied these words drew a matching one to the old rat's face as he reached out and gently ruffled his smallest son's new-found curls.

Leo watched the exchange from the corner of his eye as he took a sip from his own cup of hot chocolate and then curled closer to Donnie, doing his best to absorb the genius' warmth. He could feel the blankets that their sensei had tucked around them moving in a way that told him that Raph was probably cuddling up to Mikey on the far side of the couch. The thought made the leader smile and he found himself thinking back to when they were children. To a time when this wasn't an unusual position to find the four of them in. Cuddling together had been a daily occurrence back in the days before Donnie had figured out how to heat the lair up to a livable temperature. Heck, back then there hadn't even been a lair…

Leo's thoughts trailed off as Master Splinter stepped back from Mikey and began to survey the four of them, silently taking in their altered appearances for long minutes before he finally asked, "Now, my sons, what has happened to you?"

It was the second time that they'd heard that question in the past hour and Leo shuddered to think of the first time. Seeing their sensei, their father, rushing at them like that had been terrifying in a way that the blue-masked leader had never experienced before because this time? This time wasn't a training exercise. It wasn't even the influence of some evil psychic. This was their father defending his home from invaders and he wasn't going to hold back.

Instead of taking them out, though, Master Splinter had stopped mid-rush and stared at them with wide eyes as he whispered, "My sons? What has happened to you?"

Leo's memory got a bit fuzzy at this point because he'd been too overwhelmed with relief to actually pay that much attention as Donnie muttered something about olfactory senses and scent identification before launching into the beginning of what was sure to be a long-winded explanation about what had happened to them. Leo did remember their sensei cutting the genius off rather quickly and then rushing into his private chambers, returning moments later with some of his older robes. Robes that they were now wearing as they curled up on the couch and listened half-heartedly as Donnie began to once again explain what had happened.

And, once again, the genius was cut off mid-lecture.

"Woah, who are you guys?"

All eyes turned to the turnstiles that served at the lair's entrance where a wide-eyed Casey Jones was standing, staring at them and gripping tightly to a hockey stick, looking as if he wasn't sure if he should be getting ready for a fight or not.

When he saw his friend's uncertain stance and drawn weapon, Raph instantly took charge of the situation by calling out, "Casey, calm down, it's us!"

The sound of his best friend's voice coming out of the strange teen's mouth made the hockey-nut's eyes widen even further, but his stance became slightly more relaxed as he called back, "Raph?"

"Yeah, Case, it's me," the red-masked teen replied with a grin, motioning for Casey to join them.

After a moment's hesitation, the human jumped over the turnstiles and entered the lair, but he didn't come to join the family right away. Instead, he stood at the entrance and eyed them warily as he asked, "What happened to you guys?"

"We got remutated!" Mikey explained enthusiastically as he bounced up and down on the couch, drawing irritated looks from his brothers and a small smile from his father, but Casey didn't really notice any of that. He was too busy focusing on Mikey because, sure, the little goof looked different, but that grin was pure Michelangelo.

That realization took away the last of Casey's worries, replacing them with confusion as he holstered his hockey stick and asked, "Remutated?"

Donnie took that as his cue to once again explain what had happened and so he started the story for the third time as Casey finally joined them on the couch. This time, the genius actually managed to make it all the way to the point where the vat of mutagen had been shattered before he was interrupted by Casey's loud exclamation of, "So you guys got doused in that freaky, green ooze and turned into humans? That's crazy!"

"No, it actually makes perfect sense," Donnie replied calmly before taking another sip of hot chocolate and letting out a happy sigh as the liquid's warmth spread throughout his body, heating him up in a way that the blankets had so far failed to do.

When the scientist was done with his moment of contentedness, he looked up to see five pairs of eyes staring at him in surprise. Donnie blinked a few times and then stared back as he tried to figure out what he'd done this time. Was it the sigh? No, that didn't fit the reaction... Oh, his comment about the reasonable outcome of their secondary mutation. Right, they probably hadn't figured it out yet. He'd better explain. "The Kraang are trying to terraform the earth. For them to be able to do that effectively, they'd have to engineer the mutagen so that it only reacts to the first external influence and then ignores all others. Thus the nature of this remutation is, in fact, perfectly logical if we assume that we were exposed to a stable version of the mutagen and that my theory of its true nature is correct."

The purple-masked teen finished his explanation with a grin that quickly faded when he saw the looks that his father, his brothers, and Casey were giving him. Looks that meant that they'd understood absolutely nothing that he'd said. Honestly, one of these days he was going to start explaining things in German, just to see if they noticed that he wasn't speaking English.

Donnie's silent brooding drew to a halt when Mikey grinned at him and said, "Dude, six notches."

Six? Really? Donnie sighed and then tried again, saying, "The mutagen is supposed to make earth into a new version of the Kraang's planet, right?"

The others nodded and Donnie allowed himself a small smile before pressing on. "So, the mutagen takes a non-mutated life-form, in this case us, and transforms that life-form into a mutant. In cases such as ours where the mutation blends together the DNA of two life-forms into a hybrid creature instead of blending together a life-form and non-living matter such as what happened with my spy-roach, the form that the mutant takes is based on the predominate foreign DNA that the life-form has been recently exposed to. In our case, a human. This results in a creature that is some mix between whatever the life-form originally was and whatever the foreign DNA was. In our case, a human-turtle hybrid. Still with me?"

Mikey and Casey began to shake their heads, but Leo and Raph seemed to be following along for the most part, so Donnie continued with his explanation. "In order for the Kraang to be able to use the mutagen effectively, the mutagen would have to be able to take a life-form and change it into another one, or at least create a life-form that is genetically closer to the foreign DNA than the original DNA. This mutation also has to be a singular occurrence, meaning that the mutagen will only turn you into the first foreign life-form that you're exposed to, otherwise spreading the mutagen would just result in an amalgam of every type of creature on the planet. An amalgam that would be far more earth-like than Dimension X-like. Given that requirement, it logically follows that, after one's initial exposure to the mutagen, further exposure would simply make the mutant go further along the path to the creature that they initially hybridized with. Get it?"

The blank looks were back and Donnie was left in despair as Mikey frowned and said, "Dude, that was so not six notches. That was, like, one notch. Maybe even half a notch."

The genius sighed and then fell silent as he tried to figure out how he could dumb this down further. How could he put this so that even Mikey got it? Oh, that might work… After taking a moment to consider what words to use, Donnie looked up at his father and the other four teens as he said, "Rahzar is more dog-like than Dogpound and Dogpound is more dog-like than Chris Bradford."

This statement was met with confused frowns as Raph stared at his smartest brother and asked, "So?"

Donnie smiled sadly and then said, "Timothy became a… mutagen-blob-thing and further exposure to mutagen only made Timothy more… blob like."

The genius could see that Leo and Master Splinter got it then, but the others were still floundering and so he added, "And we're more human-like now than we were before."

Now Raph got it, too, but Casey and Mikey were still looking at him like he was nuts, so Donnie gave it one last shot. "That means that, if the mutagen works like it's supposed to, once you've been mutated once, any further mutation will just make you more like what you initially mutated into."

Realization dawned on the final two faces and Donnie allowed himself a small grin of satisfaction that was instantly wiped away when Mikey asked, "Oh… why didn't you just say that in the first place?"

Before the scientist could reply, though, Leo interjected, "But we're not actually human, right?"

"Right," Donnie agreed. "We're more human, but we're not fully human. In fact, now that we're all warmed up, we should head to my lab so that I can run some tests to…"

The purple-masked ninja's words were cut off by a loud groan from Mikey followed by a command of, "Donnie, dude, chillax. We're good, bro. Your tests can wait until after sleeping."

The scientist frowned and opened his mouth to protest, but Leo cut in before Donnie could utter a single sound. "Mikey's right, Donnie, we're exhausted. Our first priority should be rest and then we'll need to deal with the whole clothing thing before we even think about medical exams."

"Clothing thing?" Casey interjected curiously as his eyes swept over the pile of blankets that the four brothers were currently hidden under. "You're not wearin' clothes?"

As Raph muttered, "Depends on if you count robes as clothes." Donnie leaned forward and said, "We're… we were mutant turtles, remember? Clothing wasn't exactly a concern before this."

Casey blinked in surprise, clearly never having thought about that particular issue, but then he grinned that gap-toothed grin of his and said, "I can get ya some of my clothes."

The offer made Donnie tense as he was suddenly treated to a mental image of himself wearing Casey's clothing. An image that made him shudder because, even though the scientist knew nothing of fashion, he was instantly certain that he and Casey's styles didn't match in the slightest.

Then again, beggars can't be choosers.

As Donnie sighed and resigned himself to his fate of wearing Casey's cast-offs, Leo frowned and said, "I don't think I'd fit in your clothes."

Donnie glanced at his blue-masked brother and realized that Leo's statement was absolutely correct. In fact, now that he thought about it, none of them would fit into one of Casey's outfits. They were too broad shouldered for Casey's shirt and too muscular for his pants, not to mention the fact that that Casey was probably a good half-foot taller than Leo, Raph, and Mikey.

Casey seemed to realize the problem, too because his only reply was to frown and then sink back into the couch cushions with a sigh, crossing his arms across his chest as he did so.

While the hockey-nut sulked, Mikey looked over at Donnie and asked, "So how are we gonna get clothes?"

The question made the purple-masked genius scowl as he tried to think up a solution and found himself drawing a blank. There was no way that they could go shopping like this. Sensei's old robes would draw far too much attention and their lack of undergarments would definitely be a problem when it came to trying on clothing in a shop. Even if the shop owners didn't notice, there was no way that Donnie was going to let his siblings do something so unsanitary.

Which meant that they'd need someone to get them the basics and that brought them back to Casey.

Donnie glanced over at the human as his mouth twisted itself into something between a frown and a pout. Shell, this was this going to be embarrassing.

Still, it had to be done and so Donnie took a deep breath, closed his eyes, and then blurted out, "Casey, would you go and buy us underwear?"