Warning: I do not own the Arrowverse or Smallville series. Fictional events that I make are mine.
Earth-1: Keystone City
"Okay, you guys, I'm here." Barry Allen's voice crackled through the comms as he announced his arrival. He stood in the middle of a vast, open grassy area, surrounded by nothing but the gentle rustling of the wind. Meanwhile, his team - Cisco Ramon, Caitlin Snow, and Iris West - were gathered in the cortex of S.T.A.R. Labs back in Central City, intently listening to his every word. "Do you have my location?" He asked, his voice laced with urgency. The tension in the air was palpable as they all awaited a response.
"You good? You don't need to, like, stretch or something?" Cisco asked in concern.
"No, no, no, I'm good, I'm good." Barry offered his friend words of comfort and reassurance, hoping to ease their worries and concerns.
"Let's take this Tachyon enhancer out for a test drive." Cisco said with an evil smile plastered on his face.
"All right." Flash said as his fingers trembled as he clutched the silver Tachyon device in his hands. He took a deep breath before attaching it to the white chest emblem on his Flash suit. As soon as he did, a sudden surge of energy flowed through his body, causing the device to glow bright blue. Barry couldn't help but let out a satisfied sigh as the electric sensation coursed through his veins. "Ooh, yeah, I feel that." he murmured, a grin spreading across his face.
"That is the Tachyon device powering up the Speed Force in your cells like a quick-charge battery." Caitlin explained. "How do you feel?" She asked.
"Different." Barry answered as he felt like he just got a shot of adrenaline while bouncing on his feet left and right.
"Let's see how long it takes for you to get back here." Cisco said as he put a timer on one of the monitors. "On my count. Three, two, one, go!"
In an instant, Flash vanished from sight, leaving behind a trail of yellow lightning that caused the 'Welcome to Keystone City' sign to topple over. With the Tachyon prototype in place, he was moving at an unprecedented speed, faster than ever before. This was the advantage he had been seeking to defeat Zoom, the nefarious speedster from Earth-2 who had been causing chaos in Barry's life for several months. Finally, with the Tachyon's power at his disposal, he was ready to take on his enemy and emerge victorious.
Back at STAR Labs, Flash's team were very impressed on how fast he was currently running. "Daaaang..." Cisco muttered as he looked at Flash running so fast throughout the city. "This thing got you cruising!"
"How fast does he normally go?" Iris asked as she was still new to all of this.
"Not this fast." Caitlin said, grinning.
"Not even close." Cisco added, before asking a question. "Are his vitals okay?"
Caitlin looked down at her computer screen. "Yeah, they're perfect."
"Cool." Iris commented. "It's like having a pit stop attaching to your chest."
"Guys!" Flash yelled in through the comm, all of them watching as he zig-zagged all over the city. "I can go faster!"
He sounded completely exhilarated, and it made everyone smile. Cisco looked like a kid on their birthday. "Do it." Cisco encourages his friend.
As Flash pushed himself to exceed his own limits, his heart pounding with adrenaline, a sudden and unexpected phenomenon occurred. A portal with a bluish-white hue appeared right before him. Without a moment's hesitation, Flash accelerated even more and darted straight through the portal, disappearing into the unknown beyond.
Earth-38: National City
Turns out, Flash ended up getting stranded on another Earth…with it's own hero such as…Supergirl.
So turns out Kara Danvers is rescued in mid-air by the Flash, who accidentally speeds her out of the city and is then confused when she turns into Supergirl and flies off. Both are confused that the other hasn't heard of them. The Flash unmasks and introduces himself as Barry Allen, saying he's on the wrong Earth.
Back in Kara's office, Barry quickly learns that there is no S.T.A.R. Labs on this Earth. Winn and James join them and explain Siobhan left soon after Kara. Barry explains about the multiverse, how different Earths occupy the same space at different frequencies and how he accidentally went fast enough to pass through the barrier. James is uncomfortable at Kara and Barry's sudden closeness, to Winn's amusement. They run into Cat, who has an image of the Flash rescuing Kara. Despite Barry, who the others claim is their cousin's, protests to the contrary, she wants to name this new hero "the Blur".
Siobhan meets her aunt Sinead, who runs a magic shop and quickly realizes what has happened to her. She explains that as a result of a curse by a banshee on one of their ancestors centuries ago, all the women in their family gain the power when someone wrongs them. If they don't kill the source of their anger, the power will grow until it destroys their souls. Siobhan knows Supergirl will protect Kara, unaware they are the same person, so decides to gain an ally. She uses her scream to break Livewire out of containment and Livewire easily overpowers the guards.
Lucy informs Kara of the escape and she tries to get Cat to go into hiding but Cat is confident Supergirl will defeat Livewire. Kara goes to Barry for help and she and Winn take him to the DEO where he offers his CSI skills. Lucy also notes James' jealousy and tells him not to stay away from Kara because of her. Livewire meets Siobhan at an abandoned warehouse where Siobhan suggests they team up to kill Cat, Supergirl and Kara. Livewire says Siobhan will need a makeover first.
Barry is able to use the atmospheric disturbance to track Livewire. He wants to come up with a plan first but Supergirl insists they need to contain Livewire quickly. She and the Flash head to the warehouse to find not only Livewire but Siobhan, now costumed and made up and calling herself Silver Banshee. The Flash's use of his powers merely charges Livewire up and causes her to knock him down, while Silver Banshee's scream leaves Supergirl's ears bleeding. Despite making a few hits of their own, the heroes have to retreat.
Kara apologises to Barry for going in without a plan, explaining she's trying hard to make up for recent mistakes. Barry says these things take time. Livewire and Silver Banshee turn up at CatCo, where Livewire stops James summoning Supergirl and Silver Banshee rejects Winn's attempts to reason with her. Back at the DEO, Barry has made some ear buds to block Silver Banshee's powers. Lucy tells him and Supergirl that Cat has been kidnapped.
Livewire and Silver Banshee have Cat chained up in the park where Cat begs Livewire not to kill her and leave her boys with nothing. Supergirl and the Flash arrive and the ear buds do their work, protecting them from Silver Banshee. Livewire and the Flash fight on a rooftop and Livewire triumphs, while Supergirl frees Cat and confronts Silver Banshee. Livewire tries to blast down a helicopter above them and Supergirl blocks the electricity with her body but is badly injured. Showing she has regained the city's trust, the bystanders move to shield Supergirl and a fire crew turn their hoses on Livewire, shorting out both her and Silver Banshee.
Kara tells James and Winn that Barry has given National City Police Department some advice on dealing with meta-humans, meaning Livewire and Silver Banshee are now in their hands, noting James once said her enemies deserved a fair trial. She asks Cat for permission to go and see off Barry; Cat has already guessed he is the Flash.
A little while later Supergirl and Flash were standing in the middle of a grassy landscape with mountains in the distance.
"You really think this is gonna work?" Supergirl questioned.
"Well, watching Livewire and Banshee work together reminded me of something that we tried on my Earth once. You and I join forces." Flash said causing Supergirl to look at him in confusion. "Literally." Flash answered as he walk forward. "We combine your speed with my speed. If you throw me forward at his fastest, then I might just break the dimensional barrier and get back home." He explains with a slight chuckle.
"What? What do you mean? Like a race?" Supergirl smiled, liking where this is going
"Yeah, I guess." Flash answered causing Supergirl to laugh. "Think you can keep up, Girl of Steel?" He ask with a sly smile.
"Just you watch, Scarlet Speedster." Supergirl chuckles as she walks up to him. "I'm gonna miss you, Barry Allen." Supergirl admits.
"I'm really gonna miss you too, Kara Danvers." Flash admits. "Or Kara Zor-El. Which is your alien name. Because you're an alien which I think is very cool." Flash said with excitement on learning she was an alien, causing Supergirl to laugh once more. "I think James Olsen thinks so, too." Flash admits.
"What makes you say that?" Supergirl asked nervously as she doesn't believe it.
"You remember before when I told you to take things slow? That's good advice for a superhero, lousy advice for two people who really like each other." Flash answered. "Maybe it's time to speed things up."
"Maybe." Supergirl smiles with a nod as she spreads her arms open. "Come here." She hugs Barry before the two release themselves
"All right. Let's do this." Flash said as he placed the device that he had on before back on his chest. The speedster then places his mask back on as the two heroes get in a running position.
"On your marks…" Flash started.
"Get set." Supergirl continued.
"Go!" Flash finishes.
Supergirl and Flash immediately shot forward along the road, the former running instead of flying. Rapidly increasing their speed as they ran and ran, yellow electricity surrounded the red blur of Barry, while Supergirl's cape fluttered wildly in the wind. The two smiled at each other as they both tried to outrace the other but is seemed to be at a stalemate.
That's when a portal unexpectedly opened up before they had anticipated. The two heroes couldn't stop in time as they ran toward the portal as it quickly closed on them, sending them to God knows where.
Earth-10: Metroplois
The blue portal right back up, but something was different, before Supergirl and Flash ran straight out and immediately stopped in their tracks.
"What the…?" Flash asked in confusion as he saw Supergirl next to him. "That was unexpected."
"Yeah, I think the portal opened up a bit too early." Supergirl guesses as Flash agrees.
That's when the heroes hears an unexpected voice. "Flash and Superwoman?" The world's finest suddenly become aware of a man standing nearby. He has a plump build, with a pair of bright blue eyes that sparkle in the light. His wavy, medium length brown hair frames his face, while a set of mutton chops adorns his cheeks. The most noticeable feature of his appearance, however, is the pair of round, purple glasses that rest on the bridge of his nose.
What truly shocks the heroes is that the man is wearing a gown, similar to those worn in surgeries. Even more concerning is the fact that he appears to be preparing to operate on a woman lying on a table nearby. The scene is eerie and unsettling, and the heroes can't help but feel a sense of apprehension wash over them.
"Who are you?" Flash asked first after a moment of silence.
The man they were facing had an unsettling demeanor. When Flash asked him a question, he seemed to completely ignore it. Instead, he spoke in a calm voice, almost as if he were enjoying the situation. "Have to say, Flash, digging the new suit and sudden resurrection." He said, almost tauntingly. It was as if he was trying to provoke a reaction. But then he pulled out a gun, pointing it directly at them. "You're too late." He continued. "Nothing will stop me from destroying my plans."
"Except for us." Supergirl told him with a glare.
As the man raised the gun ready to fire as he puts it's on the trigger, Supergirl appeared out of nowhere, moving faster than the eye could see. In less than a second, she had closed the distance between them, and with a swift and decisive move, she crushed the gun into pieces, rendering it useless. At the same time, she also crushed the man's hand, making sure he wouldn't be able to harm anyone with his weapon again.
The man let out a sharp cry of pain as he clutched his broken hand, looking up at the two heroes who had cornered him against the metal wall. He tried to speak, but before he could get a word out, the Flash appeared in a blur of motion and delivered a devastating punch to the man's face. The impact was so powerful that the man was knocked out cold, his body slumping against the wall with a sickening thud. As the heroes looked down at their fallen foe, they knew that justice had been served.