It had taken two months of calculations, even with additional help from Twilight. It had taken weeks of negotiating and cajoling the princesses back in Equestria. It had taken dozens of tests to make sure the portal mappings were correct.

At last everything was ready.

And Sunset was terrified.

She sat looking at the great disc of pale blue haze as it descended into the water. A rhythmic patter of waves slapped the hull of her little boat. The soft breeze carried the smell of salt. The moon shone bright enough to cast shadows. A different moon. The original moon. Luna's moon, motion governed by magic and not mechanics.

Here were the rules: Take a creature from Equestria and send it through the mirror-portal to Earth. It will turn into a different creature – the same species as its counterpart in Earth. Take a creature from Earth and send it through the mirror-portal to Equestria – same thing.

But the sirens had not come through the mirror portal. Their forms, though human, had come from a combination of a spell from Starswirl, a spell from Sonata, and losing their gems twice. No mapping rules existed for that. Send them through the mirror-portal to Equestria and you wouldn't get a pony; you'd get a null result. In practice, that meant something Sunset didn't want to think about.

But that wasn't what scared her. She knew the changes she and Twilight had made would work.

The disc was now submerged. The water around it glowed like liquid dreams. The disc rippled. Sunset's heart pounded, but she didn't know whether it was fear or elation or both. She was on the precipice of the future, about to tumble.

The disc rippled again. And then, from the blue haze, a great golden shape emerged. Adagio slipped through the water with effortless grace. The moment she was clear of the portal she banked, swam in a great circle around the boat, then breached. She came down with a thunderous splash that soaked Sunset and sent the little boat dancing in the waves.

It took a moment for Sunset to realise she was shrieking and laughing. Water ran from her mane. She brushed it from her eyes. Beside her, Adagio's head – large as Sunset's whole body – emerged from the water. Moonlight glittered in her scales and fins, reflected in her eyes.

“Look what you've done!” cried Sunset. “I thought you were supposed to be graceful.”

Adagio grinned. Predator teeth, several inches long.

And on her neck, where her gem would have been, just an indentation. That was what the mapping rule gave her: The physical aspect of a siren, but bereft of aetherial presence. No magic. Just flesh.

“Oh, hush, little pony.” Recognisably Adagio's voice, and yet different. This was an overtone, dancing on a barely-audible contrabass rumble.

“How does it feel?” A fraction of her joy vanished, replaced by anxiety, as Sunset asked the question.

“Wonderful.” Adagio slid back into the water and dove.

Sunset peered over the side of the boat. Already Adagio was just a dark shape below, barely visible. “I'm glad,” she said at the water.

A moment later Adagio surfaced again on the other side of the boat. She moved her head over the side and nudged at Sunset. “Thank you, Sunset Shimmer,” she said. “I really do mean that.”

Sunset leant forward and put her head against the tip of Adagio's muzzle. Her scales were warm and soft. Her breath, soft and lulling.

Adagio reached out with a hoof and tried to stroke Sunset's mane, but she was too large, too ungainly to do it properly.

In time they broke away. Adagio frolicked again, diving, rolling, breaching. Sprays of water glittered in the moonlight like a million glowing tears. Adagio hooked the little boat with her tail and swam forward. They talked intermittently. All the while, Sunset bathed in Adagio's joy. This was her home, her true home, her true body, which she hadn't seen for centuries.

And she was beautiful. She was Adagio; of course she was beautiful. But she wasn't human.

Sunset asked Adagio to hold the boat steady. Then the slipped over the side, immersed herself in the ocean. The change in temperature made her shiver, but them she was there, swimming beside Adagio.

“May I?” she asked, reaching out towards Adagio's great, gauzy fins.

“Of course. Be gentle. They're sensitive.”

Sunset brushed her nose against Adagio's fin. In response, she heard something like a rumbling purr.

She swam for a while, but she couldn't match Adagio. She was barely more effective that when she'd been in the boat. And soon she returned to it.

Adagio travelled further and further afield. How much time passed? Sunset wasn't sure. Hours, at least; Overhead the moon swam through the sky.

Eventually Adagio came up and looked over the side of the boat. Water ran from her crest; reduced to rivulets, then drips, then nothing. “It's odd,” she said.

This is it, thought Sunset. You're home at last. Why return to Earth when you have all this?

“I can't put into words how much I've enjoyed this. I'm resplendent in a way humans couldn't even hope to grasp. Even without my gem. You've given me something wonderful Sunset Shimmer, but …”

“But?”

“Part of me wants to go home, so I can hold you properly.” Adagio looked into Sunset's eyes for a moment. Her frill twitched. Those immense carnivore teeth moved. “I don't know if I'll ever get used to feeling conflicted.”

Sunset was still struggling to find something to say. At last she jumped forward and gave Adagio the closest thing to a kiss she could. It didn't quite work, but that was okay. And it was all that needed to be said – Sunset knew enough of Adagio that she didn't need to patronise her to say it was okay to feel conflicted, or that she could come home any time.

So she just reached out and nuzzled Adagio's soft golden frills.

Author's Notes:

Aria and Sonata have also been through the portal – all this was arranged beforehand.

So … the end!

And what a journey, eh? So, some various thoughts:

I'd already been writing ponyfic for over a year when Rainbow Rocks came out. And, honestly, by that point the show had lost most of its glamour for me. I didn't really care about the long hiatus, and my ponyfic writings were more for the “fic” than the “pony.”

Coming away from Rainbow Rocks, I realised two things. First, that I really fancied Adagio. Second, that I'd felt more enthusiastic about it than I had about any FIM episode for a long while. At the same time, I was thinking about writing porn, mainly to prove to myself that I wasn't too prude. From there came this account, and with it, IWBYP.

… And then it kept going. Swept along by a much better reception than I expected on one hand, and on the other, the amazing chemistry these girls had.

And so here's the whole thing. Not without its cockups, I'm sure you'll agree. But on the whole, this turned out way better than expected. Like Adagio, I'm not great with the mushy stuff, so I'll just say: Thank you, everyone who read this far, for all your support. It wouldn't have happened without you.

Also – if you're interested in more stuff from me, you can have a look at my main account. I'll be around in a couple of days, to make edits and conversation and what have you.