"Belly of the Beast" is the twenty-first episode in the second season of Adventure Time. It is the forty-seventh episode overall.
Finn and Jake go inside the belly of a monster to rescue a bunch of bears who have made its stomach their new party cave.
Finn and Jake are awoken early one morning by a disgruntled, firework-spewing Monster whose rampage is shaking their Tree Fort. After discussing the possibility that they are both still asleep, they set off after the monster, determined to fully enjoy the dream they believe they are sharing before they wake up. Only after the monster kicks Jake far into the distance do the two come to the realization that this is not a dream and begin attacking him. Hearing cries for help coming from the monster's stomach, and resolving that they must get inside to help whoever they're coming from, Finn and Jake grab onto a tree which is then eaten by the monster. Once inside, the two come upon the source of the cries: a bear is asking for help not to escape, but to hang streamers for an impending party.
The culprit leads the two friends to a massive rave with flashing lights, fireworks, techno, and bears en masse. The bears have a striking resemblance to Care Bears, with pictures of different objects on their chests.Fearing that the bears will end up getting digested and turned into "the stuff," Finn and Jake decide they must get the bears to leave. After unsuccessfully trying to inform a few party goers that they are dancing inside the stomach of a monster, Finn and Jake are approached by a young bear named Cubby. Cubby has likewise been trying to make the other bears realize the gravity of their situation, but has been ignored because he is a child. He tells Finn and Jake to talk to Party Pat, the chief of the bears, and that if they can convince him to leave the others will follow.
After meeting Pat's requirement of partying with him for a few hours, Finn breaks the news to him: if he and his colony do not leave, they will die. An unconcerned Pat informs the other bears of the news that they are in fact in a monster's stomach, and is met with great applause; the bears love it.
While enjoying the view from the monster's open mouth, Finn unwittingly makes a comment about how the only way the monster could kill the bears for sure would be for him to swallow "some deadly volcano lava." Overhearing this, the monster makes way for the nearest volcano, while Finn and Jake race to get the bears out as fast as possible. After killing the bears' collective party drive with a somber ballad about "stuff," the two manage to rally the now attentive beasts just as the lava arrives and make their hasty escape the only way they can: through the monster's bum.
The scene skips to Finn treating the bears' burn wounds and informing Party Pat that his clan is now free to party outside. This latter offer is met with discontent from the bears, and Pat responds that, in order to survive, his people need a "phat party club to grind in," and that "that monster's gut was totally excellent." Setting a deal with the monster that they will replace their fireworks with "gentle laser pointers," Party Pat and the bears are let back into the now content monster's stomach to party once more.
- Cubby's father
- "Party With The Chief" (instrumental track)
- "The Stuff Song"
- "Bear Rave" (insturmental track)
- "Werk Dat Belly" (instrumental track)
- "Hardcore Will Never Digest (Original Teddybear Mix)" (instrumental track)
- Party Pat was based on Pat McHale, previous creative director for Adventure Time and "Pen's right-hand man."
- When Finn whispers his plan to Party Pat at the end of the episode, he can be heard saying "party" and "pancakes."
- The activities Party Pat require Finn and Jake to do include drinking honey energy drinks, break dancing on a mat on the floor, wrestling while being lifted and supported by another person, eating pancakes, and singing with a karaoke machine.
- The bears resemble Care Bears as they both come in different colors and have pictures on their stomachs.
- This episode references lucid dreaming, which is the concept of being aware that you are dreaming and thus being able to influence the dream's outcome.
- Party Pat tells Finn to open his mind eye, a spiritual concept that provides perception beyond ordinary sight.
- When the monster roars before punting Jake over the horizon near the beginning of the episode, the sound it makes is a heavily altered elephant bellow. The same sound clip was combined with the sound of a car driving by on a wet road to make the flyby sounds for the TIE fighters in the first Star Wars film.
- When Party Pat talks to the monster at the end of the episode, the music that plays is the same music from the title card of "Power Animal".
- Finn uses the root sword in this episode, because his previous sword was destroyed in "The Real You".
- The monster, along with the Party Bears, appear in the Season 5 episode "Billy's Bucket List", attending Rap Bear and Finn's Rap Battle.