Ultimately, it wasn't that much of a radical break from previous iterations, but it did allow the show's writers to take a shot at slightly more advanced storytelling while introducing more complicated visual concepts and fresh characters. In essence, "Adventure Time" got to showcase a fully realized adventure without compromising its goofier sensibilities. The contrast between the two main characters helped maintain that balance: Finn, stuck in a bleak alternative universe of his own making, endured a greater battle than he's ever faced before, while Jake made a new friend at the end of time and chilled out in a jacuzzi.
Picking up where the previous season left off, Finn and Jake chased the menacing undead villain known as the Lich into a time portal that provided access to the multiverse, a waystation between various worlds. In the middle of a floating nothingness, the Lich arrives at the bright yellow Time Room, where a lanky pink shadow figure known as Prismo (voiced, with an amusing amount of lackadaiscal charm, by the comedian Kumail Nanjiani) has the power to grant any visitor a single wish. Upon wishing the extinction of all life, the death-obsessed Lich promptly vanishes -- but Jake and Finn, protected by their presence in the Time Room, remain behind. Prismo, just doing his job, quickly bemoans the Lich's grotesque appearance: "Might be the nastiest thing I've ever seen," he whines. And just like that, another memorable "Adventure Time" character is born.
While a now-altered Finn wreaks havoc on his new world even as he tries to protect it, upping the ante for gloominess, "Adventure Time" sets the stage for a hilarious ongoing contrast: Back in the Time Room, Jake watches the action unfold with an unbothered expression. Instead of using his wish to save his pal, he asks Prismo for a sandwich. Fortunately, Prismo knows better, leading to one of the show's most memorable refrains: "Dude," he says, "I can just make you a sandwich." So he does, and Jake continues to pore over the possibilities of his wish while bonding with his newfound pal. Somehow they wind up in a hot tub. The Cosmic Owl, an enigmatic character who has surfaced in earlier episodes, drops by with board games. Eventually, Prismo just talks Jake through the wish he needs to make in order to restore things to normal.
Magically sent back to the Land of Ooo, Jake celebrates his achievement and plans to maintain his friendship with Prismo, while Finn barely questions any of the strange cross-dimensional events that just unfolded. But why should he? Jake's final exclamation, when Finn mutters "What just happened?," illustrates how "Adventure Time" manages to traverse an ever-expanding mythology while shrugging it off in favor of keeping the show's exuberant playtime in flux: "It already happened!" Jake gasps. "Nothing happened!" What he's really saying is that no matter what happens, the adventure continues.