Neither know how it happened. Prowl snuck in to see Knockout, and instead he bumped in Soundwave. Sarcastic bickering and not-so-unfriendly banter followed them down a hallway, and the bickering slowly gave way to something else. Something a bit more...personal.
All Prowl knows is that he never felt so safe, waking up in the arms of a sworn enemy. He sometimes regrets running before Soundwave ever woke up...
In a weeks time, he knows he's carrying. Can feel a bright, strong but small newspark beating against his own, as vibrant and bold as her sire. It wrecks his nerves, knowing that one night of reckless want and giving into an offer of affection may very well ruin what little he has to cling to. The Autobots would never let the bitlet of Soundwave live among them. She'd be hated. But he keeps her. Because of course he does. He loved her the moment he first felt that blip in his spark chamber.
But he doesn't tell Soundwave. No, he hides the bitlet, at least while he carries her, while he tries to think up some way to keep her safe from Prime and the other Autobots.
Eventually he ends up back where it began; standing in front of Soundwave, baring what little of him he's kept private. He can't live among the Autobots, not with Moonsong at his hip; she's the spitting image of Soundwave, all pale, iridescent plating and long limbs. Her slender hands end in sharp claws, though they'll only become sharper and more wicked as she grows.
The bitlet has Prowl's optics, narrow slits of an accusing red, though her face is nearly identical to her sire's, and her helm finials and fins are from him as well. She can bare sharp, wicked fangs as well, passed down from her carrier, and although she emerged utterly tiny and weak, barely able to feed from her carrier's fuel lines, a few days is all she needs before she finds her pedes beneath her.
Moonsong is wild and full of energy, bouncing off the walls and running around under her parent's pedes. She has yet to speak, communicating solely in beeps, clicking, and high pitched honks like one of her older brothers. But her parents don't mind.
Prowl teaches her how to talk through wingspeak, showing her all the right ways to move her tiny nubby doorwings to tell others what she wants. Soundwave teaches her how to speak through an EM field, how to manipulate it to show messages without the need for words. To speak through emotion where a voice would fail.
She grows up to continue being the spitting image of Soundwave, though she is just shy of his shoulders and her doorwings grow in to be long, broad and her claws are wickedly sharp and dangerous, like Prowl's. She still doesn't speak, letting an explosive, expressive EMF do that job for her, and she's wears a visor over her optics to hide the red Autobot optics of her Carrier.
She's close to both of her older brothers, and even as an adult insists on swinging from Springer's arms and perching on his shoulders like a horribly flexible, bright white bird. She and Barricade hang out often, using their EMFs to chatter together while their carrier and...her sire talk.
The relationship between Prowl and Soundwave remains strained, at least to outsiders, and Prowl still acts the same and speaks to his co-parent the same...but he's far more genuine, and is open and caring when its just them, or when Moonsong is young and asleep on top of them.
Prowl doesn't know what Soundwave wants of him, but.....he doesn't sleep alone, anymore, and for that he finds himself thankful for...