Disclaimer: I do not own Smallville, the Arrowverse, or any of the characters and elements pertaining to the mentioned franchises.

A/N: This fic is basically a rewrite of the Arrowverse, but with the inclusion of TV series Smallville. The point of divergence is during The Flash Season 2 (or Arrow Season 4), with the Tachyon device sending Barry to the Smallville universe instead of Supergirl's Earth. Anything before that still happened the same as in canon. As for Smallville, this is five years after the events of the Season 10 finale, so that the timelines of Smallville and Arrowverse could match. For example, if it's 2016 (the year this story takes place) in the Arrowverse, it's also 2016 in Smallville. Also, save for drawing occasional elements from them, most events from the Smallville Season 11 comics will be ignored here.

Chapter 1

(Keystone City)

"Okay, you guys, I'm here," Barry Allen announced into his comms. "You got my location?" He was standing in an open grassy area, while Cisco Ramon, Caitlin Snow, and Iris West were in the cortex of S.T.A.R. Labs back in Central City.

"You good? You don't need to, like, stretch or something?" Barry heard Cisco's voice ask.

"No, no, no, I'm good, I'm good," Barry answered.

"Let's take this Tachyon enhancer out for a test drive," Cisco said.

"All right," Barry said, the silver Tachyon device clutched in his hands. He attached it to the white chest emblem on his Flash suit. Almost immediately, the device glowed bright blue, sending a sudden surge of energy flowing through his body. "Ooh, yeah, I feel that."

"That is the Tachyon device powering up the Speed Force in your cells like a quick-charge battery," Caitlin explained. "How do you feel?"

"Different," Barry answered.

"Let's see how long it takes for you to get back here," Cisco said. "On my count. Three, two, one, go!"

Barry immediately super-sped off in a streak of yellow lightning, the powerful gust of wind left behind in his trail causing the 'Welcome to Keystone City' sign to fall down. Indeed, he was moving significantly faster than his usual speed limit, meaning the Tachyon prototype was working as intended so far. He would finally gain the advantage he needed to defeat Zoom, the villainous speedster from Earth-2 who had spent the last several months antagonizing Barry and his friends. To add insult to injury, about two weeks ago, they finally discovered Zoom's identity and, much to their immense shock and horror, found out that the man beneath the demonic black mask was none other than Jay Garrick, who, not dissimilar to Dr. Wells/Eobard Thawne a year earlier, had convincingly presented himself as a 'helpful ally' to Team Flash while secretly being the very threat they were trying to put an end to. Then, a few days ago, having grown more determined than ever to stop Zoom for good, Barry had traveled one year into the past to seek Thawne's assistance to increase his speed. While the time travel trip hadn't gone exactly according to plan, what with Thawne figuring out that Barry was from the future and the latter being pursued by a Time Wraith sent by the Speed Force itself, Barry managed to get what he needed, which led to him currently testing out his speed with the Tachyon prototype.

"Guys," Barry announced, "I can go faster!"

"Do it," Cisco said, encouragingly.

Barry increased his speed even more. Suddenly, a portal rapidly formed before him and he sped directly through it.

Completely unaware of what just happened, Barry emerged from the opposite end of the portal and continued moving across the country at speeds incredible even for him. He finally slowed down when he noticed an apartment building on fire. He sped into the burning building and started running in and out of there evacuating all the residents who were trapped inside.

(Elsewhere)

Clark Kent and Bart Allen were, for what had to be the thousandth time since they'd met almost twelve years ago, racing each other throughout the world. As always, Bart was ahead of Clark. The former was running on solid ground in a streak of yellow lightning, while the latter was flying in a red and blue blur.

"Always too slow, stretch, even with the head start I gave ya!" Bart teased.

"Maybe I'm just taking it easy on you, Bart!" Clark countered.

"Oh, yeah?" Bart said, skeptically. "So, does that mean you take it easy every time we race then? Because I cannot think of a time when you actually managed to beat me without me letting you."

"I admit that you have the edge when it comes to speed, but I wonder how much of that cockiness of yours would remain if this was a battle rather than a race," Clark pointed out. "And kryptonite, magic, and red sunlight are not options."

"Okay, I guess you got me there, stretch, you win," Bart said, shuddering a bit at the thought of fighting Clark. "I mean, we both have the last name 'ever', but my first name is 'fastest' and yours, I guess, is 'strongest'."

"Fair enough," Clark agreed.

As the pair of heroes arrived back in the city limits of Metropolis, they noticed a burning apartment building and wasted no time rushing inside. Upon entering, Clark and Bart were surprised to discover that a figure – clad in a dark red leather suit that covered everything but his eyes, nose, and mouth and contained gold trimming – had beaten them to the scene and was rapidly spinning his arms around in an effort to extinguish the flames.

"Whoever you are, I'm sure you can use a couple of extra hands with this," Clark offered, as he and Bart appeared next to Barry.

"No, I'm good, I got this..." Barry trailed off as he noticed the smaller guy dressed in red also started spinning his arms around, while the bigger guy in red and blue started blowing out the flames with his super-breath.

Once the fire was dealt with, Barry got a better look at the other two heroes and studied them with a look of astonishment.

The smaller guy with light brown hair, whom Barry figured was a speedster, was wearing a red leather suit marked with white trimming. A large white lightning bolt symbol ran diagonally across his torso and, instead of a cowl, a red domino mask concealed his identity.

The bigger guy with dark hair, whose very appearance seemed to bristle with incredible power, was clad in a mostly blue suit containing a long red cape flowing behind his back and a pair of red boots, complete with a red diamond-shaped emblem centered by an 'S' symbol resting on a yellow background on his chest.

"You're... who are you guys?" Barry asked.

"I'm Superman," Clark answered, frowning a bit.

Barry also frowned. "You're who now?"

"Superman," Clark repeated, his frown deepening. "Surely, you've heard of me."

Barry shook his head. "No, I don't think so."

"Dude, are you joking right now or have you been living under a rock your whole life?" Bart chimed in. "How can you not know who Superman is? Everybody knows who he is! Just like everybody know me, the fastest man alive!"

"What?" Barry muttered at Bart's 'fastest man alive' comment.

"Now, the question is who are you?" Clark said to Barry. "I'm guessing you're a speedster like my friend here."

"That's right, I am a speedster," Barry nodded, his frown deepening. "I'm The Flash."

Bart burst into laughter. "Sure you are."

"What?" Barry said. "No, really, I'm The Flash, the fastest man alive."

"Hey, nobody but me is the fastest man alive!" Bart protested, defensively. "And I hate to break this to you, dude, but The Flash is my codename and it's already been trademarked. You gotta come up with your own. You can have Impulse if you like, because I pretty much kicked that name to the curb a few years back."

Barry was puzzled. Where had this new speedster come from? The only other speedster he knew of who was also known as 'The Flash' was Earth-2's Jay Garrick, or so he'd thought. The thought of Jay caused Barry's anger towards the man to suddenly flare up again. He still couldn't believe it. The same guy who taught him how to throw lightning was also the one who brutally broke his back, publicly humiliated him in front of all of Central City, and left him crippled for days. Was Barry really so blind and gullible that he couldn't see right through Jay's deception? Had he not learned his lesson at all after everything that Wells... Thawne had put him and his friends through the year before? Apparently not. If there was one person on the entire planet who deserved a medal for falling into the same trap twice in a row, it was Bartholomew Henry Allen.

Focusing his mind back on the matter at hand, Barry looked at Bart and said, "No, The Flash, that's me, uh, I'm The... wait, do you guys not know who I am?"

"Can't say that I do, amigo," Bart answered.

"Same here," Clark added. "Even if you're implying that we should know who you are, I'm pretty sure I would've recognized you from newspapers images or TV if I'd seen you before, but I don't. I'm sorry."

"And you're known as The Flash too?" Barry asked Bart.

"Yep," Bart answered. "Fastest man alive!"

"But I thought I was..." Barry began, then paused for a few moments. "I didn't know there was another speedster out there with the same name as me. I mean, I've been The Flash in Central City for almost two years now, ever since I was struck by lightning the night the Particle Accelerator exploded and I got my powers."

"Two years?" Bart repeated. "Dude, I've been living in Central City for a while now and I assure you, I never heard a thing about another Flash running around and doing the whole superhero thing like me."

Barry took in Bart's words for a few moments. "How long have you been in Central City?"

"It's going on about, um, seven years now," Bart answered.

"Seven years? So you got your speed the same way I did from the Particle Accelerator two years ago?" Barry asked.

"Dude, I don't have the slightest clue about this Particle Accelerator thing you're talking about," Bart said. "Besides, I've had my speed a lot longer than just two years."

"Okay, but if you're from Central City and you're a speedster like me, how come you don't know anything about the Particle Accelerator or its explosion?" Barry asked. "What about S.T.A.R. Labs?"

"Of course I heard of S.T.A.R. Labs," Bart answered. "But it's not even in Central City. It's actually right here in Metropolis."

"Metropolis?" Barry questioned, having never heard of a city by that name before. "Where's that located?"

"You're in Metropolis right now," Clark said. "Metropolis, Kansas, to be exact. It's your first time here I assume?"

"Yeah, it must be," Barry muttered. He pondered for several moments, trying to figure out what was going on. He'd never heard a thing about the red and blue caped hero named 'Superman' or a second scarlet speedster also running around in his city protecting people. And what had the latter meant by S.T.A.R. Labs not being in Central City, but located in some city called 'Metropolis'? "Oh, boy," Barry said with a sigh, a realization hitting him like a flash of lightning, pun intended. He only hoped he would be wrong. Looking firmly at the other two heroes, he asked, "Hey, do you guys mind waiting here for a minute?"

"What for?" Clark asked.

"I think I'm starting to figure out what's going on here, but I need to know for sure if it's the case," Barry explained. "If it is, I think I'm gonna need you guys' help."

"Sure, we'll wait for you," Clark nodded.

"I'll be right back here in, well, a flash," Barry promised, then sped off in a steak of yellow lightning.

"Well, based on that lightning trail, his powers definitely come from the Speed Force just like yours," Clark noted.

"So are we really gonna just stick around here and wait for a guy that we just met?" Bart asked. "I mean, as you know, staying in one spot all day long doesn't really sit well with this super-fast stud and there's no telling if he'll keep his word and show up again."

"He did say he might need our help with something," Clark pointed out. "Not that we know what that 'something' is, but I think we should at least give him a chance."

"How can we be sure we can even trust him?" Bart asked. "I'm all for meeting someone who is exactly like me, but for all we know, he might not even be on our side."

"Considering that he helped us put out the fire in this building, we'll give him the benefit of the doubt," Clark said. "If he does something that proves otherwise, that's when we'll cross that bridge."

Suddenly, there was a whooshing sound as Barry reappeared in front of Clark and Bart. While he was gone, he had checked out Central City and noticed that, while it was located in the exact same spot and had, more or less, the same layout, his city looked quite different than the way he remembered it, complete with the total absence of the highly recognizable S.T.A.R. Labs building. That confirmed exactly what he had started thinking.

"Okay, guys, I'm back," Barry announced.

"Where did you even go?" Bart asked.

"Central City," Barry answered, "and confirmed exactly what I was thinking." He pulled back his cowl, revealing his face to Clark and Bart. "My name is Barry Allen, and I am the fastest man alive. Or at least the fastest man alive where I come from. I also think that I am on the wrong Earth. I'm definitely gonna need your help."

Clark and Bart exchanged stunned glances, as they allowed their minds to process what Barry had just told them.

"Barry Allen?" Clark said, before turning to Bart. "Wasn't that one of the names you saw in your head when you first discovered your speed?"

"Yeah, it was," Bart answered, "and ever since I found out I was from the future, well, I always assumed that this Barry Allen guy was related to me somehow. Like, maybe he's my ancestor or something."

"Given that I'm from another Earth, most likely I'm not the Barry Allen you think I am," Barry said.

"You came here from another Earth?" Bart asked, intrigued.

"Oh, yeah," Barry answered. "Believe it or not, multiple Earths are a thing, believe it or not."

"We know," Clark said, thinking back to his experiences on the Earth where his doppelganger had been found and raised by Lionel Luthor instead of Jonathan and Martha Kent. "This wouldn't be the first time we've encountered visitors from another universe."

"So you guys do have experiences with more than one Earth then," Barry said with relief. "That's good."

Bart took off his domino mask. "I'm Bart, by the way," he said to Barry. "Bart Allen."

Barry blinked. "Bart? As in Bartholomew Allen?"

Bart nodded. "Yeah, how did you know?"

"Because Bartholomew happens to be my full name too," Barry answered. "But I've always gone by Barry."

"We should get to Watchtower and you can tell us more about what happened and how you to end up in our world," Clark suggested.

(Watchtower)

"So this is you guys' base of operations," Barry said, looking around as he followed Clark and Bart into a large high-tech room located within a high-rise building in the center of Metropolis. "Nice place."

He noticed a man with spiky blonde hair and a woman with neck-length blonde hair sitting together at a desk. The man was reading a newspaper, while the woman was using the computer.

The two of them looked up as they noticed Clark, Bart, and an unfamiliar young man with dark brown hair clad in a dark red suit.

"Cl... Superman," the blonde man called out. He then turned to the stranger with wary expression. "Who is this you're bringing into our hero clubhouse, huh? Last time I checked, we didn't sell tickets to this place to just anybody."

"It's okay," Clark said, reassuringly. "He's a superhero like us. And he has an interesting story to tell."

"Well, you're our leader, Boy Scout," the blonde guy said. "If you say it's okay, I guess it is." Turning to the stranger, he asked, "So who are you supposed to be?"

"Uh, I'm Barry," the stranger introduced himself, "Barry Allen."

"Barry Allen?" the blonde woman questioned in surprise. She turned to Bart. "Bart, is this guy related to you? His name sounds a lot like yours."

Barry shook his head. "No, I'm not related to Bart. My story is quite complicated actually."

"What story?" the blonde woman asked, curious.

"Before you get into that, Barry, I'd like you to meet Oliver Queen and his wife Chloe Sullivan," Clark said.

"Chloe Sullivan-Queen," Chloe corrected, with a smile.

Barry's eyes widened in surprise as he looked at the blonde guy. "Wait, your name is Oliver Queen?"

"Just as it says on my birth certificate and ID," Oliver answered.

"As in the Green Arrow Oliver Queen?" Barry asked, still looking surprised.

"The only and only," Oliver said, proudly. "Considering that I'm something of a local celebrity, I'd react the same way if I was meeting me."

"No, no, it's not that," Barry said, shaking his head. "It's just, um, you look nothing like the Oliver Queen I know from my Earth."

Oliver looked confused. "Wait, the Oliver Queen from your Earth?"

"What Barry is saying is that he's from a parallel Earth," Clark clarified.

"Wait, a parallel Earth?" Oliver asked, stunned. He then laughed. "That's something we haven't dealt with in a while. Visitors from another universe."

Chloe turned to Barry with an intrigued expression. "You're from another universe?"

"Yeah," Barry nodded. "And like Bart, my full name is Bartholomew Allen, and I'm a speedster superhero known as The Flash."

"At least you seem to be a lot friendlier than the last inter-dimensional travelers who paid us a few not-so-peaceful visits to this world were," Chloe said, thinking of Clark's murderous doppelganger Ultraman and the Lionel Luthor from what she, Clark, Oliver, and the rest of the League referred to as 'Earth-2'. "But if you have the same name as Bart and the same powers as well, then I guess that would make you his alternate universe counterpart?"

"Except Bart is from a thousand years in the future," Clark pointed out.

Barry turned to Bart. "You're from the future? I thought I heard you mention that earlier."

"It's a long story, amigo," Bart answered.

"So I guess you're not really my counterpart then, given that, unlike you, I'm from the present," Barry said. "At least you're not my counterpart directly. But this is the first universe where I encountered another version of someone I know from my Earth who is not a doppelganger." Turning to Oliver, he added, "Like you. You look very different from the Oliver Queen I know."

Oliver frowned. "Really? I look different on your Earth?"

"Like you wouldn't believe," Barry said, with a bit of a chuckle. "And your personalities are different too. I mean, compared to my Earth's Oliver, you seem more laid-back, outgoing, and not the type of guy who needs to be constantly told to lighten up."

"What are you saying? That, in your universe, I'm the all-serious and all-brooding Grumpy Pants type?" Ollie chuckled.

Barry also chuckled. "Honestly, that is how a lot of people would describe the other Oliver."

"Seriously? Because I was only exaggerating," Ollie said. "Well, if the Oliver Queen you know is anything like me, I highly doubt he's anywhere near as brooding and grumpy as Batman."

"Who you've been quite jealous of," Chloe teased.

Oliver rolled his eyes. "Quite jealous of? Okay, I admit, I was a bit envious of all the high-tech toys and gadgets Gotham's pointy-eared Caped Crusader possessed when we first met him and found out he wasn't just some myth, but I certainly don't feel that way now. If anything, all it did was convince me to step up my game as Green Arrow, so I guess I have him to thank in a way."

"How did you manage to travel to our universe in the first place?" Clark asked Barry, focusing on the matter at hand. "I assume it has something to do with that device on your chest?"

"Yeah, uh, I've been working on my speed so I could get faster to stop this evil speedster named Zoom my team and I have been dealing with, and apparently I'm faster than I thought," Barry explained.

"Let me guess, you ran so fast that you accidentally opened a breach between your universe and ours and that's how you ended up here," Chloe said.

"That's right," Barry nodded.

"I've done something like that before, but with time travel," Bart said. "But I never traveled to other dimensions before."

"Same here until today," Barry said. "Time travel is something I've done a few times already, but I never jumped parallel dimensions without meaning to. So, until I can figure this out, I'm... stuck here."

"Don't worry, Barry," Clark said, placing a hand on his shoulder. "We're gonna do whatever we can to help you get back to your Earth."

"Okay, thanks," Barry nodded, smiling optimistically. "Uh, first things first. Food. I have to consume about ten-thousand calories a day."

"Speaking of which, I'm feeling pretty famished too," Bart said. He turned to the other red-clad speedster. "What do you say, Barry? Big Belly Burger? Dude, if you don't have that joint in your universe, you're seriously missing out."

"Actually, I think it's safe to say that Big Belly Burger is the one thing that exists on every Earth," Barry said.

"I can already see the Flashes of two worlds getting along perfectly," Chloe chuckled.

"My thoughts exactly, sweetheart," Ollie added.

"What about you guys?" Bart said to Oliver and Chloe. "Anyone else down for a little Big Belly?"

"I'm good, Bart," Oliver said.

"Same here," Chloe added. "I already ate not too long ago, and the last thing I want is for a Big Belly Burger to give me a big belly."

"Well, I guess it's just gonna be us three amigos then," Bart said to Barry and Clark. "Last one to arrive is buying, by the way."

With that, Barry, Bart, and Clark sped out of Watchtower. Even if he was destined to be stranded on a different, yet very similar, Earth for a while, that didn't mean Barry Allen couldn't have a bit of fun in the meantime and enjoy the company of his new friends/fellow superheroes/potential allies, starting with chowing down on a tasty lunch from the Multiverse's favorite burger chain.

(Elsewhere in Metropolis)

At the site of the former LuthorCorp geothermal facility, several construction workers were on an afternoon shift. Suddenly, there was a loud rumbling noise accompanied by what felt like a tremor, causing the workers to react with alarm.

"What was that?" one of them asked, right before there was another tremor, this one more powerful than the first.

Out of nowhere, a large, hulking creature with dark red eyes, whose rock-hard, gray-skinned body was covered with many spiky white bones, burst out of the ground, sending chunks of debris flying in every direction and causing one of the bulldozers to get flipped over. Letting out a ferocious growl, the creature started grabbing the petrified workers and gruesomely tore them limb from limb like sheets of paper. After all the bloodied and dismembered workers were dead, the creature let out another growl. Now that he was free after being trapped miles beneath the Earth's surface for seven long years, it was time for the Kryptonian beast, known as Doomsday, to resume the mission he had started – destroy Kal-El, the last son of Krypton, then do the same to all the other puny life forms on the face of the Earth.