The showrunner for the Adventure Time spin-off, Fionna & Cake, explained the big twist in the fourth episode.
In Adventure Time, Fionna and Cake were the in-universe fanfiction of the Ice King, based on the heroes of Ooo, Finn & Jake. But Adventure Time and Fionna & Cake showrunner Adam Muto wanted to push Fionna and her feline friend to be more than a "girl Finn" and Jake.
To accomplish that, the two had to move beyond being the borderline creepy fanfiction of an old man, so the first thing that had to be changed was their origin in the new HBO Max spin-off.
Prismo Created Fionna and Cake
In an interview with Inverse, Adam Muto, showrunner of Adventure Time: Fionna & Cake, discussed the twist of Prismo the Wishmaster being the actual creator of the gender-swapped duo instead of the Ice King.
“The idea of them being fanfiction is from the very first episode,” showrunner Muto clarified, “but making it somebody else's fanfiction came a bit later.”
It was a twist over six years in the making, beginning in the ninth season episode of Adventure Time, "Fionna and Cake and Fionna," in which it was revealed that Ice King was being beamed "ideas" for Fionna & Cake in his sleep.
Fans debated for years what this revelation could mean, but the fourth episode of Fionna & Cake, "Prismo the Wishmaster," finally explained that the beam of light was Prismo downloading the universe of Fionna & Cake that he created into Ice King's mind.
Muto explained, "We didn't think we'd ever really pay it off within the run of [Adventure Time]" and how "we just didn't have the episode count to overexplain one element of what Fionna and Cake were:"
“It was something we had in mind, but we didn't think we'd ever really pay it off within the run of the show. It would've been cool, but we just didn't have the episode count to overexplain one element of what Fionna and Cake were.”
The decision to have Prismo be the creator of Fionna and Cake instead of Simon was made “after Prismo was introduced as a character" in the fifth season opener, "Finn the Human," and that "it just kind of shifted in that direction.”
Muto said that he had “resisted the idea of them being just fanfiction from Ice King because it would limit them forever:"
“I resisted the idea of them being just fanfiction from Ice King because it would limit them forever to just being his interpretation of Finn and Jake and him being obsessed with them. Which was funny as a joke but it felt weird after a while. It felt like we're kind of boxing them in.”
Another Unresolved Adventure Time Plot Point
Another unresolved plot point from Adventure Time that might finally be expanded upon in Fionna & Cake is the identity of Prismo's boss. The mysterious overseer was mentioned only once in the original show, in the seventh season episode, "Crossover," with great emphasis on the reveal that Prismo even had a boss.
Even Jake, who is usually disinterested in such lore, was curious about who could be the boss of someone as powerful as Prismo.
More than seven years after they were first mentioned, Prismo referred to this "boss" again in the fourth episode of Fionna & Cake and how they would "drop the hammer" on him for violating his job and creating his own reality.
Just like with the reveal of Fionna and Cake's true origins, longtime fans might finally find out the identity of Prismo's boss.
Adventure Time: Fionna & Cake streams on HBO Max with new episodes every Thursday.