They're born on a Rock, and grow up staring spaceward.
They swear to one another that one day, they will dance among the stars.
"I'll wear a tuxedo in all black," Merlin tells Gwen, squeezing at her hand as they gaze up. "And you'll wear a sparkling dress. And we'll dance and dance until the people watching from the Rocks below can't tell us from the stars and the sky."
"Merlin," Gwen laughs. She pulls him into a waltz.
He thinks of dancing when they lift off the ground for the first time, even though he's strapped to the engine room wall, praying that their sweet little bundle of metal and old-fashioned fusion core chips doesn't shake apart before they break the atmo barrier.
Abruptly the noise stops—the jangling of his body against the steel stops—and his limbs start lifting off any which way before he brings them in line again, a grin splitting his face.
"Gwen?" he buzzes up.
"We've done it!" she says, tinny and bright over the comm system. "Space!"
It's big enough that it gets boring after a while.
It's an embarrassing, cramped, ill-planned affair, despite having watched the old waltz discs so many times in their youth that they'd memorized them. They clutch each other's arms in the little space they can find between the nav deck and the bunks, the cargo hold being stuffed to the seams, and Merlin manages to step on Gwen's toes nearly every twirl with exceeding skill.
"It's just that, you know, there's not a lot to do out here," Gwen babbles. "Not that I'm only doing this because I don't have any other options—you're very handsome, and very sweet, it's just—"
"Right," Merlin says, nervously lowering the blanket he's been covering himself with. "It's just the one time."
The dancing hadn't gone very well, but it's not a half bad shag. So they keep it up.
"Oh god," Gwen moans. (Gwen isn't a talker during sex; Merlin would be proud of his apparent sexual accomplishment, but figures that two months of semi-constant shagging has guaranteed a measure of skill by this point.)
He presses her against the wall as he thrusts, tiny jerking motions of his hips, and a particularly firm one has her banging her head into the wall loudly enough that it echoes throughout the ship. She doesn't seem to notice.
She does notice when the proximity alert goes off.
Of course they rescue the survivors of the crash. It's a yachtship crash, so the survivors are probably rich arseholes, but space is immense, and also immensely boring.
"Honestly, I'm getting tired of having sex with you, Merlin," Gwen says, as they wait beside the airlock for the survivors to board. "No offense meant, of course."
"None taken," Merlin says. "It'll be nice to have new folks around."
They both pause, glancing at each other, then say at the same time, "I hope one of them is hot."
Then they both say, "Dibs!"
"Oh my god," Gwen breathes, when the two survivors exit the airlock. Merlin, stunned and mildly aroused, agrees wholeheartedly.
They're both hot, and they are apparently siblings. Adopted.
It is decided that there is no need for dibs.
It takes a while to make their intentions known to the pair, but space is big. There's time.
"Merlin!" Arthur shrieks, slapping his hands over his eyes as soon as he's walked in.
"What is it?" Gwen asks, peering over his shoulder. "Oh. Merlin, how many times do I have to tell you that the engine room is a clothes-on zone?"
"As many times as it takes for my annoyance at you reminding me to trump my need to keep my bits from melting off in here," Merlin says, from half underneath the power core dock. "We can't afford a coolant system, and I like my bits."
"They are very nice bits," Morgana chimes in, from behind Gwen. Arthur looks scandalized.
The only eventual problem is that there is really no suitable place on their tiny ship to consummate a foursome. They make do with pairs and threeways here and there—Arthur fucking Gwen in the back of the cargo bay, Morgana fucking Merlin over a half-wired console, Merlin taking turns eating the girls out as they sit in the two piloting chairs (Arthur lurking in the doorway)—but something is missing.
"Quarter-G sex," Morgana suggests. "Zero-G is unwieldy, but there's really nothing like fucking on the ceiling."
There is plenty of room on the ceiling. Morgana is a genius.
"One day we'll take you dancing on Camelot III," Arthur whispers in the aftermath, kissing first Merlin's hair, then Gwen's. "You'll love it. We'll dress you up nice—"
"I'll give Gwen my sparkliest dress," Morgana hums into Merlin's shoulder.
"We appreciate the sentiment," Merlin mumbles, petting her hair, "but you two're idiots. Camelot III is just another Rock."
"Here," Gwen says, before Arthur can get angry. She pulls him up and takes his arm, and they glide into a barefoot waltz down the space-lit corridor, clad in nothing but their shining, oil-stained skin.
Morgana breathes, "oh."
Merlin watches them with half-closed eyes, and thinks he can't tell them from the stars or the sky.
"Don't leave me out," Morgana says, so Merlin heaves up and takes her hand.
They dance for a long time, switching partners as easy as hats. Merlin always takes extra care to step on Arthur's toes.