OK.
Here is the short version of the Amphibia Sleeper au. I am not a writer and never will be, so this is just the barebones "and then this happens..." version.
It starts with the episode Olivia & Junnan, except when Marcy and her moms reach the staircase no Andrias shows up. So they escape the basement and sneak through the castle. Here the newts realize a "slight" flaw in their escape plan: the castle is still flying. The box might be an option, but it's in the throne room where the king most likely is and the patrols are more numerous in that direction While Olivia is busy convincing Yunnan that "No, your cool three point landing is not going to work from this height. No we are not going to test it!" Marcy grabs one of the frobots the general stealth killed on their way here. She gets up in the robots cables and hot wires it's jetpack. Shortly after all three of them rocket out of the castle window holding on for dear life onto a frobot wreck, while Marcy tries her best to steer the thing by crossing wires. the fligth isn't elegant and the landing hard but they are still over the shallow water and get away with no injuries. Yunnan manages to land kind of gracefully and catches Olivia bridal style, Marcy faceplants of course. They make their escape and here we get to see the destruction that has befallen newtopia and the surrounding area, burning towns and villages, frobots capturing amphibians for the mines etc. they make their way through the wilderness, Yunnan is in her element hunting, Olivia seriously misses her "juice" and Marcy begins to recover more and more (but very pointedly avoiding the whole "I got betrayed by my isekai father and killed" situation). As to where to go next, they agree that it would probably be best to find Sasha and the toads, even if Marcy isn't keen on facing Sasha (who she is convinced must hate her) but agrees that she needs to let her know she is alive and they need allies. They head to the toad territories in hopes of finding signs of remnants of the toad rebel army or Sasha (they don't expect Sasha and Grime to go to Wartwood since they think they would want to find more soldiers and the village most likely has a big target on its back since Andrias has shown himself to be petty and vindictive)
so when captain Grime gets the letter from his sister, it ends with "oh and Grimothy I found one of your hummus thingys. I am supposed to tell your daughter and I quote 'I lived bitch!'"
(would Mar Mar be able to resist a meme even in a serious situation, probably not) so Sasha and Marcy reunite at Wartwood. It's complicated. Marcy feels guilty and is afraid of Sasha wrath and rejection. Sasha is immensely reliefed that Marcy is alive but is also still angry, she tries not to show it because she wants to turn over a new leaf but it's hard. A lot of her anger with Marcy is projection, since she is mostly angry at herself and how she acted in the past. The following weeks are full of avoidance and miscommunication, while they try to get the resistance going. Thanks to the situation they are in and with some council from their amphibian parents they are forced to get their act together and talk things out. It takes some time and there is a lot of things the two resistance leaders can do to avoid facing the music but eventually Sasharcy happens. With Sashas leadership skills and cleverness and Marcys tactical skills, ranger training and insider knowledge (she wasn't able to see through Andrias lies, but she still knows him and kicked his ass at flip wart) the resistance is doing much better than in Canon. Marcy tinkers with some frobot remains for makeshift lasers and some good old hacking. Sasha and her spar occasionally, so Marcy also gets some more meele experience (short sword, dagger, speer?). There are some arguments when it comes to field missions since Sasha doesn't want to risk Marcys safety, but Marcy of course insists on helping and fixing what she sees as her responsibility (she is forced to wear a heavier armor to avoid other stabbing incidents). Marcy also has to endure Grimes "what are your intentions with my daughter" speech much to Sashas embarrassment. Sasharcy also bond over "we both messed up with Anne" there are probably some realisations in THAT regard too. All in all those two working together are a menace and Andrias is not having a good time.
But eventually things start to go south. Missions turn into ambushes, outposts vanish, patrols get attacked, supplies in the base get sabotaged. Andrias must have someone on the inside and from the damage it must be someone high up in the command chain. Sasha and Marcy lose a lot of sleep trying to find the traitor. It is Grime who gets behind the truth first, he investigates on his own to keep his daughter safe and nearly gets himself killed in the process. Only Sasha accidentally walking in at the right moment safes him. The captain lies injured and unconscious on the floor and the only other person in the room is Marcy...
Sasha is of course shocked and confused. Marcy only grins "well well well looks like the killerpilla is out of the bag" and attacks. Sasharcy had spared before, Marcy has gotten quite good but Sasha had the edge and physical strength , so she starts to push her friend back. But things start to change, Marcy seems to get faster and stronger more and more as the figth drags on until Sasha is hard pressed to defend herself. Sasha resorts to a risky feint that should have disarmed her opponent, but Marcy twists around in a way a human body shouldn't, parries and grins widely. Sasha starts to realize and takes a step back asking with trembling voice: "Marcy?"
Her adversary chuckles: "Marcy? what are you talking about Sashy?"
She turns her head on the side
"Marcy is dead. You KNOW that. You saw her die."
The grin gets even wider
"You LET her die...!"
Sasha feels tears in her eyes and anger rising:
"What are you?"
"You may call us... Darcy"
Half a dozen Orange eyes burning like fire open in the face before Sasha. The commander screams in rage and lunges.
Here the story ends. The figth ends inconclusive, maybe Waybrigth loses an eye as is the custom. The resistance has to evacuate from a frobot attack that starts as the same time as the duel and takes serious damage. Sasha escapes and has to face Darcy several times in the future. She doesn't know if Marcy is dead, possesed or if this Darcy thing is a shapeshifter. What she also doesn't know is that it really was Marcy up to the figth when the core activated, the sabotage was done subconscious, like sleepwalking. But Sasha cannot be sure how much, if any of their talks and affection have been real or a cruel and elaborate deception.
