While it wasn't surprising that the Supergirl part of the Heroes vs. Aliens event was mostly standalone, it was pretty disappointing that there was literally under 60 seconds of total crossover footage. This chapter serves to expand that footage a little bit, as well as tie into some of the details of the Supergirl episode so that the crossover at least has a ripple effect for Kara and company. It also explains why Kara seemingly forgot she had an alien-killing virus on her Earth the whole freaking time.

As always, characters belong to DC Comics and the CW, and this is written purely for fun. Thanks for reading. I appreciate it.

...

[Deleted scene, Kara's apartment, after Barry and Cisco make the jump to Earth-38 (Supergirl 2x08, "Medusa")]

"What're we up against?" Kara asked immediately.

"Wait," Barry blinked. He had expected at least a little more explanation would be needed before Kara would agree to help a whole other universe fight an alien war, but she seemed to be volunteering on the spot. "You don't even know what I'm going to ask," he said, nonplussed.

"Doesn't matter. You're my friend and you came all this way, so I'll help you."

Cisco let out a small, appreciative laugh. "I'm starting to see what's so 'super' about her."

Barry nodded in agreement as he took out and unfolded a glossy printed picture from his pocket, a screenshot taken from a grainy, black-and-white movie. The subject was a sideways shot of an emaciated monster. Bald head. Wrinkled, diseased-looking skin. A mouth full of wickedly sharp teeth. Evil, shining black eyes.

A Dominator.

"This," Barry said, as he handed her the photo. "This is what we're up against."

Kara's eyes widened as she took it from him, her hand trembling just the slightest bit. "Oh, that's not good," she muttered as she scanned the photo, apparently trying to convince herself her eyes were deceiving her.

"Do you recognize it?" Barry asked. "Our Earth doesn't have aliens, or at least, none that we know of. We're calling them the Dominators."

"Yeah," Kara replied worriedly. "Yeah, Barry. I recognize it. There's actually a Kryptonian name for them but," she paused, looking up and tilting her head from side to side, seeming to debate the translation in her head, "I'd say 'Dominators' is a reasonable English term."

"What do you know about them?" Cisco asked. "The Dominators apparently killed a bunch of people on our Earth sixty-plus years ago, and now they're back and we're fighting them blind."

"Not much, unfortunately," Kara replied. "Most of what I have are stories about them eating Kryptonian children at night and stuff like that. You know, the things parents tell their kids to make them behave. Half of Krypton didn't even believe they existed."

"But you do?"

"Yeah," Kara nodded. "My father didn't want my imagination running away from me, so he sat me down once and explained that the Dominators were real, but that Krypton…Krypton…"

Barry looked at Cisco, but his face was just as confused as his. Neither of them knew why Kara had just trailed off and gotten a faraway look in her eyes.

"Kara?" Barry asked, tentatively. "What about Krypton?"

"Huh?" Kara started, disoriented. "Oh! Sorry, uh, yeah. My dad told me that the Dominators were real, but that Krypton would always be kept safe. That," she paused again, and Barry could tell she was dealing with some emotions that still weren't clear to him, to the point that she momentarily turned her head away from him and Cisco. "That I would always be kept safe," she finished quietly.

Barry wasn't sure what Kara was currently going through, and his urgency in getting her help in protecting his Earth was warring with his concern for his friend. Luckily, Kara pulled herself together and put on a brave smile. "Anyway," she said, a little too brightly to cover her previous lapse, "my dad told me a little bit of what the Dominators are capable of, but they haven't been seen for generations, so your guess is about as good as mine."

He sighed. "I was afraid of that," Barry conceded. "Look Kara, I know it's a lot to ask, but we really have no one like you on our Earth. Is there any chance you'd—"

"Of course I'll come help!" Kara exclaimed. "I said I would, didn't I?"

"Yeah, Barry can be hard of hearing sometimes," Cisco deadpanned humorlessly.

Barry had to resist every urge in his body to wince as he saw Kara's eyes flick uncertainly between the two of them. Apparently Cisco's going to make sure everyone in two universes knows he's pissed with me. Not wanting to give Kara a chance to poke that particular bear, he gave her a relieved smile. "Thank you, Kara, really," he said, gratefully. "Okay, so Cisco can open a portal and—"

"Wait," Kara interrupted, holding up her hand. "We need to go to the DEO first."

"Why? We don't have a lot of time."

"I know," Kara assured him. "We'll be quick, but there might actually be something we can use to fight the Dominators. No promises, but it's worth a shot."

Despite his urgency, Barry figured he should defer to an alien's judgment in this case. "Okay, lead the way."

"Just let me get changed first."

Cisco turned and headed towards the door. "Sure, we'll go wait outside."

"Oh, no need." A whoosh of air passed by both Barry and Cisco, and suddenly Kara was standing in front of them, fists on her hips and "S" emblem emblazoned on her chest. Cisco whistled appreciatively as he checked out the details of her suit.

"My compliments to your tailor," he said, admiringly.

Kara looked at Cisco and gave a small, mischievous smile. "Actually, I think you're going to get along really well with him."

…

"Dude, sweet gloves!"

"Thanks. I see you're a man of good taste."

"What do they do, exactly?"

"Well, basically, they amplify my sensitivity to the molecular stability of the dimension I'm in, allowing me to more easily produce a breach to a different universe."

Winn stared at Cisco in silence for a few seconds before replying, in dead seriousness, "Okay. You need to explain to me how that works. Like, now," he finished, punctuating his excitement by pointing his finger to the ground.

Cisco grinned and put his left arm around Winn's shoulders while extending his right gauntlet forward for him to see. "It would be my pleasure." With that, the two tech nerds wandered away, chatting excitedly.

Barry shook his head in amusement. "You were right," he said to Kara.

Kara laughed. Cisco's excitement over her suit was all the evidence she needed to see that he and Winn were two peas in a pod. "I'm glad Winn has a kindred spirit. Most of his nerd rants are wasted on me."

Barry nodded slowly, clearly showing he knew what that was like. Kara couldn't help noticing there was almost a wistful look in his face as Cisco excitedly showed off every nook and cranny of his gloves to Winn. There had obviously been tension between the two of them back at her apartment, but given the circumstances, she wasn't sure now was the best time to ask.

He must have noticed her looking at him, because he quickly averted his eyes to scan rapidly around the DEO. They were standing at the foot of the stairs from the window in which she had flown them both in. (Kara figured Cadmus had eyes and ears everywhere, and she didn't know how they'd react to the Flash being back in National City, so she didn't let Barry run up the side of the building.) Taking in the floor-to-ceiling glass windows, polished surfaces, and bustling DEO agents, Barry remarked, "I have to say, your last DEO base was cool, but this one's even cooler."

"Thanks. Apparently, this has been here the whole time and nobody decided to tell me until a few months ago," Kara grumbled. "Did I ever tell you a bat bit me in the last one?"

Barry opened his mouth to respond, but Alex interrupted them as she called out from the other side of the room. "Supergirl!"

Kara and Barry turned around to see Alex and Eliza striding towards them, Eliza politely curious, Alex irritated. Before even reaching them, Alex spread her arms out in a "What the hell?" gesture.

"Seriously?!" Alex huffed as she stopped right in front of Kara. She gestured impatiently to Barry and Cisco with her hand. "There are procedures for bringing outsiders into the DEO, Supergirl!"

"Ugggghhhh," Kara groaned, playing the annoyed little sister role to perfection as she threw her head back petulantly. "Alex, it's fine! They won't even be here long, and by 'here,' I mean they literally will not be in this universe long enough to compromise us."

Alex stared. "What does that even mean?"

"Just trust me. They're friends of mine, they know my identity, and they need help."

Alex exhaled angrily through her nose, while behind her, Eliza put a hand to her mouth, amused to see her grown daughters still battling essentially the same arguments as when they were children. After closing her eyes and seemingly counting to ten, Alex relaxed. Slightly. "Fine. What can we do for them?"

"Well, that guy over there with Winn is Cisco Ramon, and this here," Kara said, gesturing to her side, "is Barry Allen."

Alex tilted her head as she extended her hand to shake Barry's. "Barry Allen? Where have I heard that name before?"

"I visited your Earth a little while ago," Barry explained, shaking Alex's hand. "I'm the Flash."

Alex's expression softened. "Oh, right. You're the speedster who helped my sister fight Livewire and Silver Banshee. Apparently you're as fast as she is?"

"Maybe faster. Our last race was inconclusive," Kara interjected. "Barry, this is my sister Alex, and my foster mother, Eliza." Barry turned to smile and shake Eliza's hand. "We don't have much time, so let's go into Alex's lab and I'll try to explain as fast as I can."

…

"So the multiverse is real, Barry and Cisco have powers that enable them to cross over to our Earth, and an alien race called the Dominators is currently attacking them," Alex summed up.

"In a nutshell, yes," Barry confirmed.

"They came here to recruit an alien," Kara added, pointing to herself, "in order to fight aliens, but it occurred to me that maybe we already have the magic bullet for them. Could we reproduce the Medusa virus and use it on the Dominators? If ever there was a violent alien race to unleash it on, this would be it."

"Medusa virus?" Cisco asked.

"Originally, it was a virus designed on Krypton to kill all non-Kryptonians," Alex explained. "Unfortunately, a rogue group called Cadmus got ahold of it, and after bioengineering it to spare humans, they tried to kill all alien life here on Earth."

"Our Earth, anyway," Kara clarified.

"Luckily, their plan was foiled and the virus rendered inert. I'm not saying yet that we're just going to hand over something that dangerous, but in theory we could reproduce the Cadmus formula for you to use on the Dominators."

Barry and Cisco were silent, both thinking hard. "I guess it might work," Barry began. "Except…I'm just not sure. Stupid of me, we should have brought Caitlin too."

Cisco rolled his eyes and got to his feet. "Well, at least one of us thinks things through and plans ahead," he commented snidely. Kara glanced at Barry and shot him a questioning look. His silent reply was as plain as day. Don't ask.

Cisco fiddled with some controls on his right glove with his left hand, holding his right palm face-up. After a few seconds, a light went on in the middle of Cisco's palm and a holographic projection of a computer screen appeared a couple inches above his hand. In the screen, Kara could see a brunette woman staring out.

The holographic woman spoke. "Hey Cisco."

"Hey Caitlin."

Kara was shocked and impressed. "Is she on…?"

"Our Earth, yep," Cisco finished proudly. "I fitted the gloves to include some limited communication functions."

"That's so cool," Winn muttered from the corner of the lab, to no one in particular.

"Cisco, what's going on? Do you need something?"

"Yeah, just one sec, Caitlin. Stand by to receive a biochem data transfer. I'll explain in a minute," Cisco said to the floating head. Addressing Alex and Eliza, he continued, "Look, your virus sounds like it might work, but Caitlin's our bioengineering expert. Maybe if you put what you have on the Medusa virus onto an external drive or something, I can send the information to Caitlin, and she can figure out whether it's a good idea."

Alex, looking uncomfortable at the idea of giving information on such a deadly virus to people she didn't know, turned to Kara. "Tell me one more time, Kara. Do you trust them?"

She looked at Cisco, who was silently pleading with his eyes. She looked at Barry, and remembered how he ran right behind her to fight Livewire and Silver Banshee, not really motivated by trading his services for assistance getting back home, but because he simply wanted to help.

Kara nodded. "I do."

Alex sighed. "Fine. Cisco, Winn, get over here. Apparently we need to set up an interdimensional conference call."

…

In surprisingly little time, a workable setup had Winn transmitting data and figures while Eliza explained the nuances of the virus to Caitlin over the connection, which Cisco maintained. Kara stood off to the side, letting the experts hash it out, and soon fell deeply into thoughts that went uninterrupted until she realized Alex was at her side.

"Hey," Kara greeted, snapping out of her trance. "Giving the pros some space?"

"Yeah," Alex agreed. "Too many cooks in the kitchen. A kitchen that, apparently, spans multiple universes," she finished in a dumbfounded sigh.

"Hey, I told you about Barry before."

"I know, but sometimes weirdness has to slap you in the face before it registers."

"Fair enough," Kara chuckled, before lapsing into silence.

"You okay?" Alex asked, nudging her sister with her elbow. "You look like you're in a universe all your own."

Kara shook her head, opened her mouth to explain, then simply sighed. Alex didn't press her and simply waited patiently. Finally, she said, "This whole thing with the Dominators just stirred up some memories." Kara paused to glance at Barry, who seemed lost in his own thoughts in another corner of the lab. "I didn't admit this to Cisco and Barry earlier," she continued, "but as a little girl I was actually really freaked out by the stories of the Dominators."

Alex nodded in understanding. "I don't blame you. That picture doesn't exactly scream, 'We come in peace.'"

"Yeah," Kara agreed. "I used to have nightmares about them. I didn't calm down until my dad sat with me, told me the truth about Dominators, and told me…told me that he would never let anything happen to me. That he would keep Krypton safe, and keep me safe, from threats like the Dominators."

Alex tilted her head back, finally seeming to get where Kara was going. "And you think he was referring to Medusa?"

Kara swallowed and nodded. "This whole time, I felt ashamed of my father for creating a virus that would target all other aliens. How could he make something so destructive?" She sighed again. "But Alex, these Dominators? They're bad aliens. If they had come to Krypton, in force?" she asked, shaking her head. "It's not like Kryptonians have superpowers under a red sun."

Putting an arm around Kara's shoulders, Alex hugged her close. "Well then, now you know. Your dad made that virus to keep his planet safe. To keep his daughter safe," she finished, squeezing her shoulders slightly for emphasis.

Yet another sigh. "I still don't know if that excuses him for making a bio-WMD."

"But what you do know," Alex countered, "is that he was motivated by his love for his family."

Kara turned her head and held Alex's gaze for a moment, then slid down a bit so she could rest her head on her big sister's shoulder, the Danvers sisters' equivalent of "thank you."

…

"Alright," Cisco sighed, shutting off the communications link with Caitlin. "I guess the virus is a no-go."

"Why? What's wrong?" Barry asked.

"As engineered by Cadmus, the virus is harmless to humans," Eliza began, "but according to Caitlin, metahuman physiology on your Earth is just different enough from normal people that the virus might recognize them as 'alien' and be just as deadly."

"We can't tweak the virus again to account for that?" Alex asked.

Cisco shook his head. "Unfortunately, there's still a lot we don't understand about the changes that occur when someone becomes a metahuman. There just isn't enough time."

Kara grimaced and looked apologetically at Barry. "I'm sorry, Barry. I thought this could work."

Barry shook his head. "Our Earth is facing its first-ever alien invasion. We gotta look at every option."

"So what now?" Winn asked.

"Now," Kara replied, "I go with Barry and Cisco back to their Earth, and we deal with the Dominators the old fashioned way." Turning to Alex, she asked, "You and J'onn good to watch over things while I'm gone?"

"Absolutely. Go do what you need to do," Alex assured her.

"I will," Kara said. "Don't be afraid to call Clark if you need anything. I still have so many embarrassing stories I'm holding over his head."

…

As Kara said her quick goodbyes, Barry and Cisco made their way to the outdoor platform onto which Kara had flown the two of them before entering the building. As Cisco got his shades and gloves ready, Kara caught up with them and turned to Barry. "So, what are we working with, exactly? Do you guys have, like, an army of metas?"

Barry tilted his head, considering how to explain. "Sort of. Metas. Vigilantes. A couple of criminals too, actually."

Kara did a double take. "Say what, now?"

"You'll see," Cisco assured her, his gear now in place. "Okay, next stop: Earth-1." With that, he raised his fist in front of him, and soon a swirling blue-and-white vortex of energy appeared in front of them. In a blink, Cisco had disappeared.

Just to be safe, as he was pretty sure she hadn't traveled via breach before, Barry reached over and took Kara's hand. "Might want to hang on," he said, in reply to her questioning look. "First time can be a little unpredictable." With that, he burst into superspeed, pulling Kara into the multiverse.