So, a while ago I got super excited about this AU, then realized I didn't have an actual storyline and therefore couldn't make it an actual multi-chapter fic. So instead I wrote this...whatever this is. Um. Enjoy.

The title is from the song A Beautiful Mystery by Katie Costello. Also someone made a super cute karamel edit with it, which you can check out here:

P.S. This is the first time in my life I've written something that turned out LONGER than I thought. Take that as you will...

PROLOGUE

A thousand years.

She was sleeping in that place for a thousand years.

The couple sitting across from Kara is kind, warm. As they were when they found her. Lily and Rowan, she thinks they said their names are. Maybe. It's hard to pay attention to names when her world has just ended.

Now she sits in their kitchen, a blanket draped over her shoulders that she doesn't need, listening to their son gurgle in one of his mothers' arms. They tell her the year and she doesn't believe it. The 31st century. That means that...that...

That Kal is dead. Long dead. She failed. Tears start to stream down her face and one of the Star Havenites gets up to wrap her in a tight hug she barely feels.

"It's going to be okay, sweetheart," she tells her, her voice low and soothing. "We're going to take care of you."

Kara wants to shout that nothing is okay, that she's lost everything, but she doesn't. Instead, she buries her face in Lily's shoulder and cries for the future she never got to live.

Over time, she gets used to this planet. Somewhat. Her powers come from nowhere, but her adoptive moms are there. They stay up all night with her, as she practices blocking out the sounds of the world, shaking someone's hand without crushing it, and blowing out a candle without turning it into an icicle.

It's a small comfort, but one nonetheless, that she doesn't have to hide her powers, her species. The people of Earth understand. Most of them are immigrants themselves, or their children. There are even schools for people like her, from more advanced planets.

But as welcoming as Earth is, it is far from perfect. The government is corrupt, chaotic. There are no records, even, of the planet's past. Just a giant question mark hanging in the backs of the humans' minds.

So when Kara grows up, she decides to do something about it. She becomes Supergirl. Hero of Earth. A proud, strong title. But it's not enough. One person can't undo corruption, after all. But maybe together they can.

She recruits Imra Ardeen, a powerful Saturnian and Kara's long-time best friend. They build something for everyone like them—a way to fight back against the darkness. To preserve all that is good and to never harm the innocent.

The Legion of Superheroes.

Kara looks out the window of their headquarters and smiles. The world is far from perfect. But together, they can make it better.

Her

Of course something had to happen. They'd just defeated the "villain of the year" as Garth likes to call it, and they were celebrating with a party when a comet streaked across the sky. No, not a comet. A pod.

So here she is. Leaning against a counter in the med bay at 3 in the morning, narrowing her eyes at the unconscious person as if she can read their mind from sheer force of will.

Not that you heard it from Kara, but they're kind of attractive. Soft brown hair, well built...Kara clears her throat, blushing as if someone can hear her thoughts.

An allergy to lead (lucky the Legion has doses of the cure) reveals that they're a Daxamite, and Kara groaned when she heard the news. She knows she shouldn't be disappointed. That she should be glad they survived, whoever they are. Even if they're a Daxamite. But, and she really doesn't want to admit it, when she saw that Kryptonian pod a tiny part of her thought Kal. But her lost cousin is elsewhere...if anywhere at all.

The mystery visitor gasps as they lurch upright, looking around wildly as if searching for an assailant.

"It's okay!" Kara carefully walks forward, hands extended. "You're safe."

"Who are you? Where am I?" they demand.

"I'll answer your questions, I promise, but you're gonna need to calm down." Kara glances at the door, wishing Imra was here to rescue her. Her best friend always seems to be much better at calming people down. Must be a telepath thing.

The Daxamite nods, taking a deep breath. "Okay. First of all, where am I?"

Kara knows people can appear from just about anywhere, so she does her best to tell them everything.

"You're in National City, planet Earth. This is the headquarters of the Legion of Superheroes."

"National City? But..." They look even more confused than before, if that's possible. Then their mouth forms an o as it dawns on them.

"What year is it?" They ask in barely a whisper. Kara tells them, and they tilt their head back as if to contain tears.

The reaction reminds Kara painfully of her first day on Earth, and she gives her guest a sympathetic, if sad, smile.

"Hey. We're gonna help you. I promise." It's probably time to introduce herself. "Uh, my name is Kara Zor-El. I use she/her pronouns. And I'm a refugee from the planet Krypton." She holds out a hand for them to shake. They take it with a bemused smile.

"My name is Mon-El. Uh, he/him. And I'm the former Prince of Daxam."

Kara inhales sharply. She heard rumors on Krypton of a playboy, a Prince who couldn't care less about his people. Frat boy of the universe.

...but who's to say that they're true? She's met many, many people over the years from all sorts of planets with bad reputations. They were good people. Maybe he is, too.

"Nice to meet you, Mon-El."

As the months go by, Kara still doesn't really know what to make of him. First, he's a stranger. Then he's a cute puppy dog, confused by even the smallest things as Kara teaches him about life in the 31st century. She doesn't want to admit it, but she likes spending time with him. His smiles are infectious, his obliviousness adorable, and he is the only person Kara knows who can stay up until midnight with her talking about their fallen worlds.

In time, he opens up to her, too. As it turns out, he's from the past. 21st century, just like Kara. (She ignores the fluttering in her chest, the wistfulness of what could have been.) He says that he landed on Earth after escaping Daxam, that he has friends there. That's it's his home.

So they do their best to get him there. Preya and Brainy work tirelessly to find a way to open a breach, one that will take him back to the century in which he belongs.

But the thing is, Kara finds more and more that she doesn't want him to go. It's scary, the way he makes her feel. But good scary. Like free-falling through the night sky, only to shoot up at the last minute and soar alongside the stars.

She doesn't plan on telling him how she feels. It's too much to hope that he feels the same way.

But, well, fate has a funny sense of humor. A twisted one.

The Legion has been going up against this race of half-computer, half-organic life forms for the past year. The Coluans. They want to capture Earth for their own sick amusement, add it to a collection of conquered worlds that didn't stand a chance. Kara intends to make Earth their last stop. They have a kill code now. They can do this. She knows it.

That is, until they kidnap her and Mon-El.

The cell is cold and empty, Mon-El's silent presence in the cell next to hers the only comfort. Red sun lamps dampen their powers, and Kara swears she's never felt so helpless. The door clangs open, and those possessed by Brainiac enter Kara's tiny prison, their dull and blank eyes digging a hole in Kara's soul.

A Coluan strolls in behind them, hands behind her back, eyes full of cold pleasure in the terror before her. Kara knows this one. Indigo. She nods, and the guards haul Kara to her feet, too weak to struggle. She wouldn't get far anyway.

"What are you doing?" Mon-El's voice trembles with fear as he clambers to his feet.

The Coluan smiles the way a shark smiles, with too many teeth and a hunger that reaches far too deep.

"You think we don't know about your little kill code?" she coos as if the idea of defeating them is laughable. "Why don't you just tell us what it is, and we can get this over with?"

"That's never going to happen," Mon-El spits, and Kara can't help the warm feeling that spreads through her at the steel in his voice. She knows he can be a hero. Hopefully, he believes it too.

"We'll see," Indigo sings, tilting her head as if there's nothing more interesting than the Daxamite and Kryptonian before her. "I can't hurt you, anyway. Pesky lead cure. But your pretty friend..." she leers at Kara, who gives a weak attempt to escape the arms constricting her.

"She's oh-so powerful, but has an awfully big weakness..." Another guard walks in, carrying a syringe that glows green.

"No, no," Kara gasps, straining desperately to break free. Indigo nods, and Kara is forced to her knees. Green fire enters her veins.

What feels like a century (but is probably around ten minutes) and two syringes later, Mon-El tells them the code.

Kara doesn't know how to feel.

When Indigo gets what she wants, the wall between their cells is lifted. ("A token of appreciation," Indigo says with a smirk.)

Mon-El's panicked cries fill her ears as he rushes to her side, checking frantically for a pulse. Kara wishes she could tell him that it's okay, that she knew the risks, but her throat is dry and his face is blurry and everything just hurts.

Ever so carefully, he helps her sit up against the wall of their cell, resting her head on his shoulder. Kara sighs and snuggles closer, ignoring the way her skin burns at any contact. It's worth it to soak up his warmth.

She's dying. She knows it. But his fingers are gentle as they stroke through her hair and this isn't such a bad way to go out, when you think about it.

It's funny, she thinks, how scared she was to tell him how she felt. Facing death certainly makes things seem a lot simpler. She chokes out a laugh at that.

"What?" Mon-El asks softly, obviously confused as to how she could possibly find humor now.

Kara tilts her head upwards so they're face-to-face. "Nothing," she whispers, her throat burning.

His stormy gray eyes are clouded with fear, eyebrows set in concern and it shouldn't be legal that he's so cute even now.

If she's going to die...

She may as well make it count.

With the last of her strength, Kara leans forward and presses her lips to his.

Him

Mon-El has always thought that the whole concept of love is rather stupid. They didn't have love on Daxam. You wanted somebody, you slept with them, you married someone else who helped your family's image. That's all there is to it.

But then he crashes onto Earth and sees Alex and Maggie, Winn and Lyra, J'onn and M'gann. The way they light up around each other, fight for each other, change each other. Okay, he allows, maybe love does exist. But definitely not for me.

And then the universe sends Mon-El to the 31st century. To Kara. And he finally gets what they mean.

It starts subtly at first. A wanting, no, a need, to spend as much time with her as possible, to make her proud. He'll do whatever it takes.

There are the little things too. The way her forehead crinkles when she lies. The smile that lights up every room like a supernova. The way she babbles when she's excited and then realizes it and blushes, embarrassed. Her brilliant, beautiful comet eyes.

It happens slowly, but all at once. He falls hard. He loves her.

Of course, things are never that simple. She would never see Mon-El that way, and there's no use trying. Hell, he's honored she even considers him her friend. He can be happy with that.

Maybe.

That is, until he and Kara are kidnapped. Her screams fill the room and gods help him, he doesn't know how he lasts this long but he does. They get to the third time and he just can't stand it. He yells out the code in a half-choked sob and Kara crumples to the ground.

Her head lies on his shoulder when he hears her laugh softly. What? She whispers her response and their eyes are locked and hers are so incredibly blue they shine like comets. He doesn't know if he can stop himself from kissing her.

Kara solves that problem for him.

The books he read in the 21st century always say that the first kiss is magical. That fireworks explode behind your eyelids, and everything suddenly makes sense.

Mon-El feels none of this.

Every thought (including the apparent secrets of the universe or whatever) is erased from his mind in shock as their lips connect. Her skin is sweaty and feverish, the poison that causes her veins to glow green painting patterns on the backs of his eyelids.

Then she's pulling away and collapsing unconscious in his lap and Mon-El doesn't know how to feel.

Except that he can't lose her.

He can't.

Exactly ten seconds later, Imra bursts through the door, a trail of unconscious bodies in her wake. Mon-El can't help but be secretly glad, even in this situation, that she didn't come a bit earlier.

When they arrive back at headquarters, it's a rush to get Kara into a healing tank to leach the kryptonite out of her system. He and Imra stand side by side, watching the bright green in her veins fade slowly away.

When the liquid in the tank drains, Kara is placed on a bed while they wait for her to wake up on her own.

Mon-El sits by her side, just watching her. He's too scared, too weak, to do something like kiss her forehead or take her hand but oh, he wants to.

The others go to fight Brainiac and he tries to help from the Legion ship, but they're losing. Badly. That is, until Kara appears in the doorway.

Her hair is still slightly damp, her skin a little pale, but her face is set in determination. "I've got this." She goes into battle and for the first time, Mon-El follows her.

When they get back, she asks to speak with him in private. It's important. They stand in the med bay, her twisting her hands anxiously and Mon-El has a pretty good idea of what she wants to talk about.

"So, we should probably talk about...what happened between us. While I was dying." Her voice is shaky, and she won't meet his eyes.

Of course she's nervous. She was feverish and delirious and did something dumb she didn't mean. How could he be so stupid as to think it meant something?

He sighs inwardly and decides not to make things harder for her. He plays dumb. "What happened? What did I do?"

Her mouth opens and closes soundlessly, like she can't tell if he's lying or not.

"You saved me," she says finally. "Thank you."

"Of course," Mon-El mumbles. He hesitantly meets her eyes. "You're not mad at me for giving them the code?"

She shakes her head. "Of course not. I would've done the exact same thing." Of course she would. She's Kara.

He nods, but the question remains. "What do we do now?"

She smiles. "We kick their asses. Together."

Throughout the weeks that follow, Mon-El tries his best to forget about the kiss as they fight the Coluans again and again, but he can't. And now there's this...tension between them. They're still good friends, but a small part of Mon-El's mind can't help overanalyzing everything she does, looking for clues that his feelings are returned.

Nearly a month later, the Coluans are finally defeated. The other Legionnaires are jovial, congratulating one another as they down their drinks. Though she smiles as brightly as always, something seems off with Kara, and his suspicions are confirmed when she leaves the party early with polite excuses and a side glance at Mon-El.

He taps the little screen next to her apartment door, and it beeps and slides open, revealing Kara standing within.

Her hair is tied back in a ponytail, she's wearing a soft loose shirt and leggings, and looks so gently beautiful he can't help staring as he steps inside.

"Are you okay?" Kara fiddles with her sleeves, the lightest of blushes adorning her cheeks. A faint yeah escapes Mon-El's lips, his throat dry.

It's time to come clean. She may not feel the same way, but lies are no way to keep a friendship. "I just...need to tell you something."

"Of course." She looks at him with concern. "What is it?"

"I lied. Before." His gaze is firmly glued to the floor. He can't bring himself to meet her eyes. "I remember the kiss."

Kara exhales shakily and Mon-El hesitantly meets her gaze. "I shouldn't have lied about it. I'm sorry. But I know that you were sick and—and feverish and you didn't mean it, so I didn't want to make things harder for you." He clears his throat. "I told them the code because I can't stand seeing you in pain. Because I care about you. A lot."

Kara's mouth opens and closes soundlessly, clearly at a loss for words. Mon-El swallows hard. "I'm not here to change your mind. I just, uh, thought you should know that."

She still says nothing and Mon-El suddenly feels an urge to escape. He starts to walk around her. "I'll just—"

Kara steps in front of him, blocking his path. "Wait, uh, don't go yet." The next word comes out as barely a whisper. "Please."

When did they get so close? Her gaze is piercing, her scent intoxicating, and Mon-El feels rooted to the spot, deeply confused as to what is happening.

"I get that you didn't mean to kiss me and—"

"No," Kara interrupts, an amused but determined look on her face. Mon-El blinks. What?

"I didn't kiss you because I was delirious," she continues, walking behind him. He spins around to face her, frowning in confusion, but he lets her speak.

"I kissed you because..." a smile tugs at the corner of her mouth. "Because I wanted to," she concludes simply. "I like you, Mon-El. A lot. And it was, uh..." Her smile turns shy. "It was okay that I was gonna die because I'd gotten to kiss you."

Mon-El's brain is running at light speed. This isn't real, right? He's going to wake up and be stuck in the Friend Zone as usual, trying to ignore his unrequited feelings...right?

His face breaks into a smile as he finally lets the words sink in. She likes him. She likes him.

"Well, you..." he starts, taking the smallest of steps closer, "Defeated the Coluans..."

She smoothly follows his lead, a smirk dancing across her face. "We did."

"That means nothing is..." he gulps. "Nothing's stopping us."

"Nope." Kara's face is dead serious now. "Nothing's stopping us."

When Mon-El wakes up the next morning in her bed and accepts flowers with a smile and a kiss, the only thought in his mind is that if this is a dream, he never wants to wake up.

And he doesn't. There are some struggles, of course, as they try and figure out how to navigate this radical change in their relationship—as it turns out, loving her was the easy part. But to wake up everyday to her smile, to watch the way her brow furrows when she's focused on one of her many projects in the science lab, to be the one she turns to—it's worth it.