A/N: In the last author's note, I meant to say thank you to Silverthreads, who used the phrase "ping-ponging" to describe the team's situation in a chapter 4 review. I loved it so much I put it in chapter 5. (To become Sam Carter's phrase, no less!)

Again, thank you to everyone for all the wonderful feedback. I'm absolutely thrilled that you're enjoying the story. Hope Ch. 6 lives up to expectations. Let me know what you think.

Oh, and one more thing: Carson's back. :-)

The pain receded and the spinning room slowed to a stop. In some recess of his brain, McKay noted it all. But that blissful absence of pain — and the dimensional shift it signaled — wasn't the all-out attention getter it once was. McKay stared at the empty box.

"Did we shift?" Ronon asked, his voice drifting from somewhere behind McKay.

McKay didn't turn. He stared at the empty box.

"Hey, Rodney," Sheppard said, his voice closer than Ronon's and moving closer. "Are you all right?"

A hand grasped his shoulder. Sheppard, McKay figured idly. He knew he should say something. Do something.

He stared at the empty box.

Suddenly, his vision filled with a waving hand and McKay lurched backward with an undignified "Gah!" Sheppard grabbed his shoulder to keep him from stumbling back and falling on his butt.

"Rodney." Sheppard shook him lightly. "Hey."

Numbly, McKay turned to look at him. He blinked, drawing the world into focus. In front of him, Sheppard frowned, concerned.

McKay slowly smiled.

"That was. . . so. . . cool!" he exclaimed, shrugging Sheppard's hand off his shoulder and tearing around the table to grab a tablet PC from a hook on the wall. "That was the first time I've kept my eyes open. I know what we look like when we shift now, what we go through."

"Jesus, McKay!" Sheppard said, blowing out a breath in frustration. "I thought you were having a seizure or something."

"No, fine," McKay mumbled, waving the concern away with one hand while he scribbled frantically on the tablet with the other.

After a few moments, he became dimly aware that Sheppard and Ronon were having a conversation. He glanced up to see Ronon looking at the Dangerous things that almost made Dr. McKay go boom sign, which was now printed in swirly blue letters rather than blocky red ones. At the table, Sheppard was cautiously tipping the steel storage box on its edge and peering inside.

"It's empty," McKay told him.

Sheppard let the box fall back to the table with a thud. "So I see. That good or bad?"

"Bad. Good." McKay scribbled a new thought on the tablet. "Bad."

"Well, that's clear," Ronon said.

McKay looked up with a huff. Why did everyone always want an explanation right when he was inspired? "Bad because the device stayed in our universe when we shifted. It's there and we're here, which means I'll have to wait to examine it until we return. That is, assuming things continue as they have been and we do return." He jotted down another thought. "Good because if I'd gotten my hands on that thing and brought it with us, it would have been, well, let's just say 'catastrophic.' Bring a thing that jumps you to a parallel universe with you to a parallel universe and you'll just keep shifting forward, not back. Not unless you know how to work it. Right now I think it's acting as a sort of homing beacon for us in our universe. Without it there, we never would have gotten home."

Sheppard blanched. "Then why the hell did you reach in there to take it?"

McKay shrugged a little, embarrassed to admit: "I didn't think about it until after we'd already shifted."

Sheppard heaved a long-suffering sigh. "And what's the second 'bad' for?"

"My hands were inches from the device when we shifted. If we're crossing into this universe at specific access points — dimensional doorways — it would have gone with us. We're definitely shifting randomly."

"Getting jerked here and there no matter what we're doing or where we are at the moment," Sheppard said.

McKay rolled his eyes. "Yes, thank you for defining 'random.'"

"Then shouldn't Radek start shifting soon, too?" Sheppard asked.

Without waiting for McKay to answer, Ronon cupped his hands around his mouth, tipped his head back and shouted, "Zelenka!"

McKay fumbled the tablet and dropped it on the table with a clatter. "Stop doing that!"

Frowning into his coffee mug, Zelenka came around the partition that separated the front of the lab from the back.

"What? It works," Ronon said.

Zelenka looked up. "Did you find the device?"

Sheppard opened his mouth to answer, but Ronon interrupted. "Is he ours or theirs?"

Looking at Zelenka, McKay felt a mixture of pity and relief. "Pretty sure he's ours."

Last year, Zelenka went with Lorne's team to check out P2X-555, a planet that was supposed to be inhabited by friendly traders who specialized in odd pieces of tech bought from the farthest reaches of the galaxy. Turned out they were friendly and they were traders, but they were also members of an alien society that had evolved from the genetic equivalent of cockroaches. Giant, talking cockroaches.

McKay was in the gateroom when the team returned. Zelenka's expression then was only slightly more horrified than his expression now.

"I put down my coffee because I was feeling sick. The feeling passed." Zelenka said. He held out the mug, his hand shaking a little. "My coffee is now tea."

-------------------------------------

This universe's Zelenka had left his radio on his workbench and McKay used it to call Beckett. He didn't get quite the heartwarming welcome he'd expected.

"Och, Rodney, I was sleeping. And I distinctly recall telling you to do the same. We've got — " McKay heard a clatter, a crash, then a muffled, tired curse "— almost eight hours until you shift again, according to my clock, which is now in several pieces on the floor. Someone better be bleeding profusely or in imminent danger of —"

"Wrong Rodney," McKay announced. Who said he couldn't get to the point when he wanted to?

Silence.

After a second, McKay wondered whether Beckett had disconnected. After a few more seconds, he was pretty sure he had. McKay was reaching up to the earpiece to call again when Beckett spoke, this time awake and alert.

"I thought you . . . he . . . you shifted every twelve hours."

"So did I," McKay said unhappily.

"It's only been four," Beckett pointed out.

"And yet, here we are."

Over the radio, McKay heard movement, rustling. The doctor getting dressed, he guessed. "You, the colonel and Ronon?" Beckett asked.

McKay glanced at Zelenka. He was sitting on a stool, staring at the now-blue Dangerous things that almost made Dr. McKay go boom sign. "And Radek, on his inaugural voyage across dimensions."

"Where are you?"

"Lab."

"I'll be right there. I have something for you. Don't go anywhere."

McKay smiled ruefully. "Like we can help it."

While they waited, McKay enlisted the team's help in searching the lab for this universe's version of the device. It had to be similar to theirs and if they were going to figure out how to stop shifting, he wanted to examine it ASAP.

They'd covered McKay's dangerous things unit and the storage area reserved for untested tech, and were in the middle of searching the workstations at the front of the lab, when Beckett hurried in out of breath. He'd traded his pajamas, robe and slippers for his standard uniform, minus the lab coat. He carried a med kit.

Zelenka paused in his search of Dr. Lee's station, clutching the scanner and tablet PC that he'd found there. He stared at Beckett and swallowed visibly. "I knew. . . I mean, Rodney said. . . but seeing . . . . my god."

Beckett smiled. "I've been getting that a lot lately." His gaze slid to Ronon, who was ransacking a desk, and his smile turned sad. "Trust me, the feeling's mutual."

"No injuries here, Carson," McKay said, gesturing distractedly at the med kit as he rummaged through his counterpart's desk. "What's up?"

"Ah!" Beckett patted the case. "Something you'll be happy about. Unless you're unusually fond of headaches and dizzy spells."

McKay's head snapped up. "You can fix that?"

"Aye, temporarily." Beckett thumped the case onto a clear table and flipped it open. "When I stopped being a bloody fool and believed you, both of you, I realized the headaches were instant migraines caused by the sudden expansion of blood vessels in your brain just before and during the shift." McKay watched him unpack syringes and swabs, four bottles of pills and packages of . . . were those seasickness patches? "The vertigo is just that, vertigo. You're getting motion sick."

To McKay's astonishment, Sheppard acknowledged, "Everything did tilt a bit."

Translation: The world flipped upside down and I nearly lost my lunch.

"I would imagine so, lad," Carson said, moving around the room to dole out the patches. "I was there when you lot went on your way last time, and let's just say the human body isn't supposed to do what yours did."

"Lots of wavy. Kind of shimmering. Like a mirage gone wrong?" McKay asked.

Carson nodded, handing him a patch. "Put it behind your ear and then don't touch it. That's what you see when you shift, then?"

"When he looks," Ronon said, slapping a patch behind his ear.

"Oh, like you kept your eyes open through all that spinning and tilting and whirling and. . . ." McKay pressed his palms against the cool top of the desk and hunched forward. "Never mind. I think I'm making myself seasick."

Beckett chuckled and patted him placatingly on the back before continuing on to his med pack. "Well, the patch will help that."

McKay shook his head, wincing as the aching muscles at the back of his neck strained and pulled. He straightened up and tiredly ran a hand over his face, glad Beckett had his back turned. "Finding that device is what would help. I need to get a look at it sooner rather than later."

He turned around and surveyed the lab. Sheppard, Ronon and Zelenka had moved to sit around the worktable that belonged to Zelenka's counterpart. Sheppard, arms crossed, leaned back on his stool, balancing it precariously, effortlessly, on two legs. They wereobviously out of places to search. . . er, ransack. In their effort to find it, find it now, they (McKay too, truth be told) hadn't been exactly careful about putting things back where they found them. Or at all. Drawers were pulled open, papers strewn over desktops, tech storage boxes scattered everywhere. If it had been his lab, he would have been livid.

But it wasn't his lab.

McKay turned around. "Carson, do you know where it is?"

Syringe in hand, Beckett gestured him over. "Radek put it in the dangerous things storage unit."

McKay sat down on a stool next to Beckett and rolled up his sleeve. "It's not there. We looked."

Beckett frowned as he swabbed the crook of McKay's arm. "Rodney, Colonel Sheppard and Teyla called me a couple of hours ago, said they'd checked on Radek and he was fine. Rodney was going to take a quick look at the device and then get a few hours sleep. Here we go now, wee pinch."

McKay didn't even notice the needle go in. He was staring at Sheppard, who was staring back at him. Sheppard set his stool on all three legs with a thud.

"Uh, doc," he said, standing and coming over. "Your Rodney was in the lab here — messing with the device — when they shifted?"

Beckett put a band-aid on McKay's arm and looked up, unconcerned. "I wouldn't be surprised. He — " He glanced at McKay with mock ferocity "— and you, too, I'm sure, never listen to me when I order rest. Not when there's some shiny new Ancient device that needs figuring out."

McKay stood up, absently rubbing at the band-aid with his thumb. "He had it when he shifted."

Sheppard glanced at Beckett, who was preparing another syringe. If this universe's Rodney carried the device with him, with his team, the doctor likely just lost his closest friends for good. "Are you sure this Rodney'd bring it with him?"

McKay nodded, and for the second time in as many hours his heart sank. "I would have."