Sonic gets captured by Captain Gantu & Dark Oak, but escapes back to Earth and meets a young girl named Lilo and Experiment 626 aka Stitch (Sonic the Hedgehog belongs to SEGA, Lilo and Stitch belongs to Disney).
What was meant to be a simple rescue mission turns out to be a week-long trek across an island as Sonic and friends race to fix their powers. Knuckles and Rouge address the elephant in the room. Sonic may like one of his friends more than he thinks he does. Eggman's just generally annoyed.
Or,
I'm writing Sonic the Hedgehog fanfic now, I guess.
(Crossposted on ff.net and Ao3.)