>>150971336
All you really need to know is
>there's a multiverse involved
>like four or five new universes/timelines/what-have-you were created over the course of AT thanks to time travel/magic wishes/plot devices happening over the series
>Fiona/Cake's universe was one of these offshoot worlds (kinda sorta) that was hidden inside Ice King's mind as "fanfiction", but Ice King lost his magic crown and became Simon again at the end of AT so now Fiona's world is "our world" without any magic but still has human versions of all the characters
>crux of the show is Fiona rediscovering the true nature of her universe, shenanigans ensues
There's callbacks and such that you'd only really pick up on if you watched the original show, but as it's own series it stands up pretty well. I stopped watching AT about halfway thru and I was still able to follow along pretty well