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When Megatron wakes up fifty years after crash landing on earth, he finds that he's in some kind of lab owned by Isaac Sumdac and doesn't have a body. He calls a protoform to him so he can get a new body, but Isaac Sumdac finds the protoform first and now the two of them have a baby on their hands that they awkwardly raise together. Platonic co-parenting, please, I headcanon them both to be aromantic.
Sari is a techno organic and she's perfectly fine with having a robot dad with human DNA from her human dad. She's aware that it makes her weird compared to normal people but one of her dads is a giant alien robot. That's just so much more awesome than other people.
Sari grew up hearing stories about the Cybertronian war from Megatron and how it started. How Cybertron had a caste system and the lowest of the low were slaves on which the rest of society was built upon. Megatron being one of them who fought in the fighting pits and started making rallying speeches to the other slaves, saying that if the other Cybertronians get to live easy peaceful lives then so should they.
But talks and rallies never changed anything. So in the end Megatron and his band of Decepticons turned to war to fight for their freedom.
Sari personally thought they went too far but she didn't know how it felt to live back then so she gave her robot dad the benefit of the doubt.
And then the Autobots woke up and were on earth too and Megatron has Sari and Isaac join them to be spies to help the Decepticons take them down.
When they're joined by the other Decepticons, Sari takes a great delight in bothering Starscream all the time because Starscream hates the organics and keeps jumping away whenever Sari sneaks up on him and taps his leg or something. Anything for petty revenge for blowing up her robot dad, but then again if Megatron never came to earth then Sari wouldn't exist. Starscream really just created his own problems.
Meanwhile, Lugnut treats Sari like a princess because she's Megatron's daughter and Sari exploits it as much as she can, much to the dismay of her dads.
But slowly Sari learns the Autobot side of the war. She notices Megatron going too far when she thinks it's unnecessary. She finds out about the horrible things Megatron has done from destroying protoforms to experimenting on Blitzwing and that's why he's a triple changer with multiple personalities who works for Megatron.
When Sari tells Lugnut about it, he's distraught and in denial, but he's starting to see what the Deception princess means. Megatron is pushing his troops too much and Lugnut starts to wonder why the Decepticons still work for Megatron. His goals changed from getting freedom to ruling over worlds and they're starting to disagree.
Sari eventually betrays Megatron, tearfully saying that he's gone too far and that she can't let him do this anymore. Megatron, upset, tries to win her back but Sari repeated all the bad things she knew he did and then all of the current actions that he's doing like trying to take over earth and hurt people. This is too much. He's got his freedom hasn't he? Why does he have to become the corrupt ruler that he'd been trying for so long to overthrow?
Megatron quietly sees her point and mused that she's got his way with words. He agrees on a truce with Optimus Prime. The Decepticons stay on an island, which is dinobot island but is uninhabited because the AllSpark doesn't give Sari her key so the dinobots don't exist, and the Decepticons with the help of Isaac Sumdac can build their own place to live. The Autobots don't tell anyone about this and no one gets hurt.
The truce is set and Sari constantly visits the Decepticons. She still bothers Starscream. She still exploits Lugnut treating her like a princess. And she's helping her dad try and fix Blitzwing from being a shattered mess and maybe regain his original personality while keeping the other three.
And that's when she somehow meets the AllSpark and gets her key just because chaos is fun and now the other characters who were only born because of the key can exist.