Author's note: Thanks once again for all of the support. I apologize for being a little trite, but hey, it's fun to write. And the possibilities for angst-ridden jealousy are just too delicious to pass up. I will also be the first to admit that the plot is mildly convoluted, but I like it that way. Enjoy.

Disclaimer: Well, if you've read the last chapters, you already know that I've given up any claim to the members of Mutant X. Seth, however, I will defend with the last breath in my body.

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NATURE OR NURTURE: Double Take

As Brennan's mind struggled to comprehend what Emma's words might mean, Seth tensed beneath him. "Is she right - are you Seth?"

In response, the man began to flail wildly. Brennan sent another shock through him. "Answer the question."

The man's eyes darted to Emma, who stiffened under his intense scrutiny. Her blue eyes grew to the size of saucers as something passed between them. She scooted closer and laid a hand on the man's forehead. "He's Seth," she whispered softly.

Brennan dragged Seth upright, breaking Emma's contact with the man. She didn't often touch people in a fight, usually choosing to use her powers at a distance. The sight of her actually laying her hand on Seth had disturbed Brennan in a way he didn't want to examine. All he knew was that they had to get Seth back to Sanctuary and get some answers.

Emma bit her full lower lip, her eyes still locked with Seth's. As though he weren't pinned to the floor by his doppelganger, stunned and immobilized, he spoke to her calmly. "My fight isn't with you, Red. You friend here - " his dark eyes flitted to Brennan " - took something that didn't belong to him."

"What, the letters? If you wanted them back so damned badly, you should have asked Lily for them before you slit her throat in that alley." That brought Seth's rage-filled glare back to Brennan. Good. The less time the sick bastard spent staring at Emma, the better.

With a growl, Seth tossed Brennan off of him. Brennan slammed into the wall and struggled to his feet. "Sensitive, isn't he?" he gasped.

"Now is not the time, Bren," warned Emma. She rose gracefully to her feet, never once taking her eyes from the enraged Feral. A brilliant blue ball of energy shot from her forehead and slammed into Seth. He collapsed in his tracks.

"Nice work." Brennan wiped at the blood trickling from the corner of his mouth. Seth had a nasty left hook.

"Yeah, well, it was either that or watch you two pound the crap out if each other for the next half hour. As we've got about thirty seconds before this room is swarming with guards, I thought I'd step in." She leaned over Seth, feeling for a pulse. "He should be out long enough for you and me to get him to the Double Helix."

Together, Brennan and Emma looped Seth's limp arms over their shoulders and dragged him out into the corridor. Sharp footsteps echoed just around the corner. With a glance backwards, Emma projected the image of an empty hall and a compulsion to check each and every room. Brennan thought he saw her flash an amused smile, but he couldn't be sure.

Brennan's battered body protested under Seth's dead weight, his ribs throbbing. He would have died before admitting it, but he was mildly irritated at the way Emma kept pace with him. She looked as fresh and clean as she had that morning at Sanctuary, not a spot of dirt on her cream- colored skirt. Hell, she hadn't even broken a sweat. He, on the other hand, felt jarred by every step they took. His clothing was torn and bloodstained and the entire right side of his face felt swollen.

Worse than his physical discomfort was the clawing uncertainty in his gut. Seeing Seth had sent his mind reeling. He knew that there were a finite number of explanations, but none of them were easy to accept. It was possible that, somehow, Genomex had had him cloned, but Seth had a long, complicated past of his own. Seth could have had plastic surgery, but that made even less sense, unless it was a very recent development. Seth could be transmorphic, but every other transmorph Brennan had known reverted to his or her true form went they were unconscious. Or Seth could be his twin.

That possibility had so many repercussions that Brennan refused to think about it. He could not, would not, did not have a twin brother.

They were outside of the compound's perimeter. Emma spoke a command into her ring, and the Double Helix reappeared. She and Brennan strapped Seth into one of the seats in the back of the plane. "Christ, am I that heavy?" Brennan muttered.

"Couldn't say. I've never been in a position to find out."

Emma would choose this moment to get flirty with him. Brennan saw the impish twinkle in her eyes and scowled. "I'm flying home."

"I wasn't that bad."

"Em, half of the bruises on my body are from the fight with Seth. Three guesses where the rest of them came from."

She slid unhappily into the co-pilot's seat. "Chicken."

"Ha! I let you fly here in the first place, didn't I?"

Emma mulled that over while Brennan took off and set the Double Helix on a course for Sanctuary. Brennan was glad she kept silent; the last thing he needed right now was for Emma to start picking at his emotions. The knowledge that she already knew every little thing he was feeling rankled.

Next to him, Emma turned in her seat and gazed at Seth. Brennan's fingers went white as he gripped the controls. "Is he waking up?"

"No. I was just thinking ..." She faced Brennan. "Don't you think it's a little weird that he just fell into our hands?"

Some of Brennan's frustration boiled over. "Not as 'weird' as the fact that the son of a bitch looks exactly like me. And don't say, 'There's a reasonable explanation for all of this,' because there's nothing reasonable about any of this."

"I'm sure that Adam can - "

"Save it, Emma." The second the words left his mouth, Brennan regretted them. He had no right to take his anger out on Emma. They sat in moody silence the rest of the way back to Sanctuary.

********

Emma rubbed her gritty eyes. She had been staring at a computer screen for the better part of two hours. After taking a blood sample from the still unconscious Seth, she'd all but ordered Brennan to take the Feral to a secure room.

Emma hadn't seen Bren since - and, frankly, she didn't want to. From the moment he'd first seen Seth, his emotions had hit her like a maelstrom. Confusion, anger, fear, betrayal. The flight home in the Double Helix had been almost unbearable. Emma knew that Brennan was upset with her, with her abilities. She'd felt his discomfort around her before, but nothing like the constant resentment that pounded her in the cabin of the Double Helix.

And, to make matters worse, Adam was still nowhere to be found. Overwhelmed by the need to resolve at least some of the questions surrounding Lily Cross's murder and Seth, Emma had decided to run every test she could think of on Seth's blood.

As she thought, it was a match to one of the samples found in Lily's apartment. More disturbing was the fact that Seth's DNA information already existed in Adam's records. Adam had been lying to them from the beginning.

Emma ran the last test with shaking hands. Seth's DNA was an exact match for Brennan's, which meant that he was either a clone or a twin. Neither possibility boded well. Brennan was not going to be happy with this bit of information.

Desperate to distract herself, Emma tried to contact Shalimar and Jesse. Perhaps they'd found something in Vermont that could help explain what the hell was going on. Pressing the tiny button on her ring, Emma said, "Are you there, Shal? It's Emma." Nothing. Emma tried Jesse, only to get the same result. Not good.

Shoving away from the desk, Emma went to look for Brennan. She found him sitting on the floor outside Seth's cell, still radiating anger and resentment. "Bren?" she ventured.

He glared up at her, his black eyes burning. "What?"

"I can't get a hold of Jesse or Shal. We've got to go after them." At her words, the sulkiness faded from Brennan's handsome face. "There's more, but I'll tell you on the way."

Brennan rose to his feet. "Should one of us stay with him?" He gestured at the sealed door.

"No, I can keep him under with a sedative while we're gone. But I think we should see what he knows about the farm in Randolph before we head up there."

Emma reached out with her mind, feeling for any sign of consciousness in the room. She felt only the buzz of long-suppressed rage that marked Seth's presence. Nothing indicated that he was awake. Just to be sure, she opened the door and slipped into the room. Brennan had strapped him to an examination table - and none to gently, if the marks on Seth's arms were any indication.

As she reached Seth's side, Emma felt Brennan step into the room. She reached out to touch Seth's forehead, marveling at his resemblance to Brennan. The instant her skin brushed against his, Seth's eyes snapped open. He gave her a weak smile.

"You pack a hell of a punch, Red." She felt his amusement break through a swirl of darker emotions.

Mustering all of her coaxing abilities, Emma blanketed the Feral with feelings of trust and comfort. "We need to know about the farm, Seth."

Seth angled his head away from her hand. "Save it, sweetheart. Psionic manipulation doesn't work worth a damn on me."

Brennan moved to stand directly behind Emma. Every nerve in her body tingled at the sudden warmth along her back. "Tell us about Vermont," he snarled.

Seth's face was stony. "You'll find out on your own." His eyes met Emma's, and she would have sworn that his expression softened. "Watch your back. That place is Tyler's personal playground. And as good as you are, Red, you're no match for Tyler."

"Thanks. We'll be back in a bit." Without warning, she injected a strong sedative into Seth's arm. His eyes remained locked with hers until he lost consciousness.

"I'm really starting to hate that bastard," spat Brennan. He grabbed Emma's arm roughly and pulled her out of the room. "Let's go find Shal and Jesse."

Through all of Bren's emotions, Emma caught an odd impression of ... protectiveness? Emma had sensed Brennan's protective instinct before - usually when Shalimar was in danger. This, however, felt darker. It danced at the edges of her mind, then slipped away as they lifted off in the Double Helix.