The gang (comprised of Finn, Jake, PB, Lumpy Space Princess (LSP) and Hot Dog Princess (quite literally a hot dog, princess) are all partaking in a special tea party called a Mallow Tea Ceremony. Finn finds it quite difficult to bounce on marshmallows whilst drinking tea and calls out LSP because she can float, and it makes it easier for her. For an instant she turns off her floating powers and accidentally bites Jake during her fall. The bite turns into a werewolf-like infection called "The Lumps". LSP informs Jake that he only has until sunset to reverse the effects, or he will remain "Lumpy" forever. LSP then agrees to take both Finn and Jake to Lumpy Space in search of the antidote.
The Portal to Lumpy Space is located in the Cotton Candy Forest where a frog guards the mushroom passageway. LSP knows the password and the three are transported into Lumpy Space. Because Lumpy Space is comprised of a series of widely spaced clouds, the group requires a car to safely make their way to Makeout Point, where the antidote awaits.
LSP constantly gets distracted with mundane, day-to-day gossip in Lumpy Space, so Finn takes her phone and impersonates her so that her friend Melissa will take them both to Makeout Point.
Jake's Lumps worsen and he begins to acquire traits of Lumpy Space people: he acts selfish and gets caught up in everyone's drama.
They make it to Makeout Point where they meet three Lumpy Space people who hoard the antidote. They are willing to share it, until LSP insults them, calling them "posers". At this point, Jake has become completely Lumpy and he leaves with LSP to attend promcoming. Finn needs to get to promcoming, but the only way to accomplish this without falling into the Lumpy Abyss is to be bitten by one of the "posers" and try to float to the distant cloud.
The posers respect Finn's passion and bite him multiple times to accelerate the Lumpification process. With the antidote in hand, Finn floats over the clouds to promcoming and rushes in to find Jake and LSP dancing. Lumpy Jake refuses the antidote and a fight ensues between Lumpy Finn and Jake.