The only thing Donatello was sure of was that he was not the only one giving Master Splinter a perplexed stare. Master Splinter pretended not to notice as he handed Don's Shell Cell back. Don absently tucked the phone into his belt, trying to comprehend the situation.

Leonardo beat him to it. "Mikey's bringing government agents here?" he asked uncertainly, as if he hoped that he had interpreted Splinter's end of the conversation completely inaccurately.

"Two, if I understood correctly," Splinter replied. "Do not worry, my sons. They will be no threat to us."

Don really hated to be the one to vocalize the next question, but no one else was going to do it. "And… we're trusting Mikey's judgment on this?" he asked incredulously.

"Yes. And mine," Splinter said evenly, giving him a sharp look. No one raised any more arguments after that, though Raph heaved a frustrated sigh that spoke for them all. Don resigned himself to his fate and sat down on the couch. After a moment, Raph leaned against the arm of the couch beside him and crossed his arms, scowling. Leo tried not to pace with mixed results. Master Splinter turned on the television, seemingly oblivious.

A tense ten minutes later, they heard the door opening. The three turtles didn't jump at the noise, but they did shift almost imperceptibly to make their weapons easier to reach.

"Hey, guys!" Michelangelo called cheerily as he poked his head into the room. He received a few glares, but the tension lessened considerably.

Raph stood up. "'Bout time," he grumbled.

"Indeed," Splinter agreed.

Mikey chose to ignore that. "I brought home secret agents," he announced. He disappeared for a moment, and then returned, leading in two beings who were definitely not included in the general CIA stereotype. Don's eyes widened as he took in the odd pair before him. The smaller of the two, he observed, seemed to be a perfect blend of human and ichthyic DNA, much as he and his brothers appeared to be human-turtle hybrids. The other agent, though, was truly an enigma. Don had no idea what kind of genetic alteration could cause red skin or those strange bone structures on his forehead.

Engrossed in study, Don belatedly realized that Mikey had been introducing everyone, and Don was now being pointed at. "This is Donnie," Mikey told the agents.

Don smiled a little sheepishly. "Hi."

Mikey grinned. "And the moody emo turtle over there is Raph."

Hopefully, it was clear that Raph's heated glower was directed at Mikey, not at the guests. "So who're they?" Raph asked.

"Uh…"

The blue agent rescued him. "We are from the Bureau of Paranormal Research and Defense. My name is Abraham Sapien; call me Abe. Nice to meet you all."

"Hellboy," his colleague rumbled.

"Welcome to our home," Splinter said graciously, bowing. "Michelangelo. Ten flips."

Mikey had hoped that the appearance of his new friends would make Master Splinter forget about that. "Aw, man…." With a dramatic, long-suffering sigh, he trudged over to the nearest open space and began. Don didn't pay attention – he had seen Mikey punished with ten flips so many times in his life that he hardly noticed if it happened anymore – but the agents of the Bureau of Paranormal Research and Defense seemed to find it interesting.

"You wanted our assistance, agents?" Splinter prompted, effectively drawing their attention away from Michelangelo.

"Yeah," Hellboy confirmed. "You guys seen any other giant turtles down here?"

Don exchanged puzzled looks with his brothers. "Nope," Raph replied succinctly.

"Have there been any unusual disturbances?" Abe persisted.

"You mean besides Mikey's fascination with the toaster?" Don muttered. Raph snickered.

Leo took charge. "There have been some strange noises coming from farther down in the sewers. They started recently. We went down to check it out, but…"

Mikey, done with ten flips, jumped into the conversation – literally. "But we rushed down there, all geared up and ready to kick monster butt, and there was nothing there!"

"We didn't find anything," Leo concluded reluctantly, discreetly stepping on Mikey's foot. Mikey let out a very indiscreet, "Ow!"

Don perked up. "That's not exactly true," he argued.

"Well, we didn't find anything useful," Mikey conceded.

Don barely spared him a halfhearted glare before dashing down the stairs to his lab. He had known that saving those samples would be worthwhile. He sifted through sets of stored vials before finally withdrawing one. It was empty but for a few chips of rock huddled at the bottom. He held it up and tapped it, breaking the fragments loose, and set it in a slot at his work space. He sat in the worn chair and began assembling the necessary apparatus.

He didn't have to look behind him to know that everyone had followed him in. Hellboy certainly made a lot of noise, and so did Mikey's mouth. "What is that?" Abe inquired from behind him.

Don plugged up the Bunsen burner and selected a clean sample plate. "We didn't find a monster, but we found where he had been." He paused to inspect the label on a beaker.

"How did you know he had been there?"

"Big gash in the wall." He reached for his chemical cabinet, but before he could start rolling the chair, Abraham handed him three bottles – the exact bottles he had planned to grab. Don turned around, staring. "How…?"

Abe shrugged. "It comes with being a fellow scientist – and a psychic."

Don blinked turned back to his work station. "Um, thanks. Anyway, some of the cement showed up differently on initial scans, so I gathered some samples to analyze later. Of course," he added darkly, "I've been too busy with repairs to actually analyze them until now."

No one made any comment to that.

Don snapped on his goggles, separated out the sample, and began testing it. He scanned some in the computer, heated part of it on the Bunsen burner, and mixed the rest with selected chemicals. Half an hour later, he was adding all his findings to the results of the computer's scan. He set a few parameters and pressed 'enter.' A loading bar appeared and did not zoom straight to one hundred percent. Don blinked, removing his goggles. At the rate the blue bar was crawling across the screen, one would think it was working through a six-disc RPG rather than an everyday chemical analysis. Brow furrowed, he mentally reviewed the results. If they hadn't been so odd, he probably could have figured out what kind of chemical they were dealing with by now. He drummed his fingers on the desk in thought.

"Got any ideas?" Hellboy asked.

Don jumped; he had rather forgotten that there was anyone else in the room, much less six beings. He cleared his throat, reluctant to admit that he was lost. "Well…" Before he could come up with anything, the computer beeped, demanding his attention. He stared at the data covering the screen. It was full of question marks. That couldn't be good. He clicked frantically on the visual model, enlarging it. The collection of bonded atoms rotated around mockingly, giving him a full view of something far outside his knowledge. Steeling himself, he spun the chair around to face his expectant audience.

"I have never seen anything like that compound in my life," he admitted.

Leo and Raph gawked at him. Mikey looked a little as though he had been told that the Shredder was now writing children's books to raise money for charity and doing anti-drug speeches at elementary schools. "You mean… you don't know what it is?" he asked timidly, eyes round.

Don shook his head shamefully.

"I believe I could help you," Abe offered courteously. He stepped forward and examined the screen, cocking his head to one side. Don watched him. "We have encountered something like this before. I don't know what it is, either," he added apologetically. "But it has always appeared when certain demons are summoned."

"The alien ones," Hellboy added helpfully.

"Namely, demons from another dimension not directly connected to this one," Abe translated.

Raph sighed, annoyed. "What're we looking for?" he asked bluntly.

"Reiki, a turtle demon from Japan. It's mostly black and about…"

Something clicked. "Wait a minute," Don interrupted. "How can it be Japanese if it's from another dimension?"

Abe hesitated and looked at Hellboy. "It's not Reiki at all," he realized.

"So what is it?" Leo pressed, exasperated.

Abe shook his head. "I don't know. I would have to do more research."

Raph flipped out a sai, patience obviously expired. "Well, we'll never know unless we go find out, so let's go find it and do our thing, okay?!"

"Finally, someone who speaks my language!" Hellboy muttered appreciatively.

"Yes!" Mikey did a standing flip as if it hadn't just been used for punishment. "Monster-fighting teams joining forces! Totally awesome!" He bounced up and down. "Can we go now?"

Don tried and failed to restrain a grin at his brothers' antics. "It's probably the most effective approach."

Leo nodded. "Let's go." There were cheers all around. As he gathered a few basic scanners, Don had to admit that he had never seen an odder team of monster hunters.