Hello, everyone! Welcome to a new installment of my fanfic series Batgirl of the Future! Been a while since I've written one, huh?

This time, I'm going to be doing a crossover between Batman Beyond and Teenage Mutant Ninja Turtles. With TMNT, the 'verse will be a mix between the 2003, 2012 and IDW Batman/TMNT crossover series, but may be heavier on the 2003 side.

At this point, the title does not refer to an enemy within a group. Rather, it alludes to an enemy within oneself, and is a nod to a song that helped inspire this story: "The Enemy Inside" by Dream Theater. If you haven't heard the song, I highly recommend that you do so, as it's a magnificent work and has to do with an underlying theme of this story.

What's the underlying theme, WNR?

Post-traumatic stress disorder, also known as PTSD.

Anyway, this story also has a specific two-part episode from Batman Beyond wrapped in it: "The Curse of the Kobra." However, instead of taking place during the episode, it takes place a week after the end of the episode, with the episode's events being brought up in flash backs.

With that said, please read and leave a review!

The wind blew gently in the ice-capped Japanese Alps as a muscular green form stood on one of the peaks. In spite of his three-toed feet being bare, he was not fazed by the cold. His brown trench coat, brown fedora and the ends of his dark blue mask rippled as the breeze picked up, blowing snow three yards in front of him, obscuring his vision. Within a few moments, the snow cleared up, revealing two more figures. Unlike the masked figure, their forms were slightly transparent, and did not move with the wind.

The first figure bowed respectfully to the two newcomers. "Kon'nichiwa, masutā Yoshi, Splinter," he greeted. It had been over a month since he was able to reach them, as traveling had made him unable to reach them.

The other two, one a human man and one a gray man-sized rat, bowed as well. "Greetings, my son," the rat, Splinter, replied.

As the three stood straight again, the human looked at Splinter's "son." "We bring grave news, Leonardo-san," Hamato Yoshi said grimly. "Your friend, Kairi Tanaga—"

Before he could go on, another gust of snowy wind spun around them. Beside the rat appeared an old woman in black garb. Leonardo was surprised to see her after so many years. "Tanaga-chan?" he gasped. Looking closer, he realized her body was transparent like his adoptive father and Yoshi. "Oh, no..." A look of sadness plastered his face. He bowed respectfully to the fallen warrior, closing his eyes.

The woman bowed grimly. "Do not grieve for me, Leonardo. My students already do. One week of time has done little to help."

Leonardo opened his eyes once more, and was shocked by what he saw. Gone were his fedora and trench coat. Instead, a martial arts master's robe covered him, the top of it awkwardly peaking over his shell. He realized what Kairi was asking of him. "I'm honored, Kairi," he said as he straightened, his voice catching. "But I can't take over your dojo, let alone teach all of your students. You know what happened last time I taught."

"You fear the past will repeat itself, yet you have no proof that it will. Much like one of my students." Tanaga lifted out of her bow. "But, I wasn't asking you to teach all of my students or take over my dojo."

"Then, why the robe?"

"I want you to complete the training of two of my students, both allies of a mutual friend in Gotham."

The mutant turtle raised an eye ridge. "Two children allied with Bruce Wayne? I thought he stopped taking children in after what happened to his son."

Kairi paused. "You haven't watched the news for the past three years?"

Leonardo shook his head. "Not since what happened." It hurts to see what I had done, what my own ambitions for my students led to, he thought grimly.

She looked at him for a moment, then shrugged. "You can catch up with Bruce when you get there," she told him.

"I didn't say I was going."

"You didn't have to. I saw it in your eyes."

Splinter looked at the turtle. "I'm glad you've considered Tanaga-chan's words, my son."

"I'm still considering them, Father," Leonardo stated. "I'll have to speak with Bruce and Kairi's students before coming to a final decision." He looked at Kairi. "Can you at least tell me what to expect of them?"

He swore he saw amusement glimmer in Kairi's eyes. "You'll have to see for yourself, Leonardo-san."

Before he could question further, another flurry whipped by, taking the spirits of Splinter, Hamato Yoshi and Kairi with it. He closed his eyes, then blinked them open, waking from his meditation to see he was back in the lair located in the sewers of New York City, where he and his brothers had lived when they were children. Only one of his brothers, Michelangelo, still lived there. Leonardo himself was visiting from Japan after living alone there for years. In a few hours, their brothers Raphael and Donatello were due to arrive.

But now I have to leave before they get here, he thought with a sigh. Leonardo stood up before exiting the room. He found Michelangelo in the living room, sitting on a couch while reading a new comic book about a speedster named Bolt. "Mike?"

The turtle in question looked up and grinned. "Hey, bro. What's up?"

Leonardo hesitated. I didn't realize I missed him that much. Do I really have to leave? He closed his eyes. Yes, I do. I owe that much to Kairi. "I have to leave for Gotham."

Mike's smile evaporated into a look of shock. "What?! But you just got here, Leo!"

"I know," the blue-masked turtle said grimly. "But I just had a vision... of Kairi. She's gone."

The younger turtle blinked his light blue eyes sadly at the news. "And, you want to pay respects to her?"

"Yes, and... see to matters she left unsolved, as well as visit Bruce."

"Shouldn't we wait for Don and Raph, then? They'll want to go. Kairi was their friend, too. And Raph'll be glad to see Bruce again after so long."

"I know," Leo said again. A thought occurred to him. "Why don't you wait for them, and come after they arrive? That would give me time to discuss things with Bruce."

"What kind of things?" Mike asked curiously with a raised eye ridge.

The older turtle hesitated. I can't tell him about teaching again. At least not yet. "I'll tell you later," he finally answered.

The orange-masked mutant sighed. "Fine, I'll let you be mysterious for a while. I know Raph won't let you be."

"Thanks, I think. See you in a while." Leo exited the lair without waiting for his brother to reply. Upon entering the garage, he selected a motorcycle. Better use this before Raph won't let me use it, either, he thought. He put on old riding gear before sitting upon the bike, then opened the garage door to reveal the road he would travel to Gotham.

Oh, WNR, you're so cruel...

How?

You've given the reader even more questions to ponder!

So? What's the point in reading if you're not allowed to let your imagination go wild?

... True...

Okay, so here are some questions you may be wondering that will be answered later: How do the Turtles know Kairi? What happened that caused Leo to stop teaching and go into exile? Which student of hers constantly fears that the future will repeat? (Hint: the character has a major role in this story and all of the previous stories in this series.)

Questions I can answer now: What do the turtles look like? Who's Bolt?
More like they did in the 2003 series, but with the eyes of their 2012 selves. Bolt is a character made by my dear friend TRON0602 as an experiment CADMUS did with the DNA of Bart Allen (Impulse) and Coba Brooklyn (Batgirl Beyond). He asked that I mention Bolt somehow in a story, so for now I'm going with fictional superhero.

Japanese words and translations:

Kon'nichiwa: Hello, good day

Masutā: Masters

If you have any questions I haven't answered, please ask! I don't bite. :)