"Incendium" is the twenty-sixth episode in the third season of Adventure Time. It is the seventy-eighth episode overall.
The episode opens with Princess Bubblegum showing Finn and Jake her liquid pyrotechnics. After she draws Jake onto her lap, Finn makes an attempt to put his head on her lap as well, but she pushes him away, saying, "Come on, Finn, don't be weird."
Finn becomes heart-broken and, after Princess Bubblegum leaves and the resulting rainstorm comes and passes, he curls up in a corner with a piece of Princess Bubblegum's hair and two pictures of her. He then sings "All Gummed Up Inside" to express his misery and starts moaning in pain knowing that it is hopeless to win Princess Bubblegum's heart, leaving Jake and BMO feeling sorry for him.
In an attempt to cheer him up, Jake seeks a new love interest for Finn, leaving BMO to watch over him. While walking, Jake hears what sounds like a girl's scream and stumbles across Flambo, who is being attacked by Iron Owls. After saving him, Jake asks Flambo if he knows any princesses Finn's age. Flambo then tells him about a "hot" Princess in the Fire Kingdom. When they get there, Flambo casts a "Flame Shield" on Jake so it will be safe for him to go in. Jake then checks on Finn, who is desperately trying not to cry. Jake goes into the castle and introduces himself as the "Baron of the Grasslands" to the Flame King, and then attempts to win over the Flame Princess for "Prince" Finn. First, he offers a water bottle which he accidentally kills the Flame Jester with. Flame King states that he hated his jester. Jake's second attempt is to sing "All Warmed Up Inside" as a serenade. Flame King is not impressed with the song and demands that Jake leaves. Jake then checks on Finn with BMO's camera, as he sadly apologizes for failing to win the Flame King's favor. Finn is now bawling and tells BMO to go away. BMO's hands are shown in an arrow pointed toward Finn's open pants while BMO walks towards Finn, while repeatedly saying "Boop, Boop, Boop." The view switches to Jake looking weirded out and Finn is heard yelling in pain/surprise. The Flame King demands to know why Finn is not here, so Jake stretches his stomach to look like Finn, who greets the Flame King and offers him a koala bear (made with Jake's stretchy powers). Though koalas are the Flame King's favorite animal, he rejects the gift, saying he no longer likes koala bears. Fake Finn begins to cry and Jake tries to cheer him up, but he loses control of fake Finn who begins to attack him and (seemingly) kill him. This impresses the Flame King who states that a prince who is willing to kill his own buffoon is welcome to date his evil, destructive daughter, much to Jake's horror.
Flame Princess walks over to the Finn-shaped-Jake and embraces him. Some fire elementals and Jake remark that she actually seems normal and Flame Princess responds by shooting fire at them and turning them into flambits. Upon observing this, Jake concludes that she is an evil princess. Jake makes his Finn say that he does not like her anymore. Flame Princess is enraged, causing mass chaos with her destructive fire powers. Jake attempts to flee to the tree house, but she follows him in her flaming, monstrous form and sets the tree house aflame. Jake tries to warn Finn, but Finn does not notice or care because of his depression over Princess Bubblegum.
The fire soon spreads inside the tree fort, destroying Finn's pictures of Princess Bubblegum before his eyes. Finn becomes enraged by this, vowing to kill whoever started the destructive fire. He looks out the window to see Flame Princess's monster form scaling the tree fort. Flame Princess accidentally ignites Princess Bubblegum's remaining liquid fireworks. This makes Finn even more furious, and he shouts at her to not touch Princess Bubblegum's stuff.
The rain resulting from the liquid pyrotechnics hits Flame Princess, dousing out her flames. This reduces her to her normal girl form, which surprises Finn. She passes out and dies using Princess Bubblegum's lock of hair as a support, Jake suddenly falls and Finn catches him in his arms. Jake then asks Finn why he does not like him. Finn immediately replies that he does like him causing Jake to blush.
- Princess Bubblegum
- Fire Elementals
- Iron Owls
- Incendium means "the fire" or "burning" in Latin.
- Flame Princess' mouth hangs open when Finn says that he likes her, but on the Facebook photo version, her mouth is closed.
- The title card for "Incendium" shows Finn crying with pictures of Princess Bubblegum, but in the title card for "Burning Low," the scene is reversed with Princess Bubblegum crying with pictures of Finn.
- In the preview, Ricardio's voice (George Takei) seems to be narrating, which is most likely because the episode premieres the day before Valentine's Day.
- This is the first Valentine's Special of Adventure Time.
- Even though the episode airs at 7:30 PM Eastern/Pacific Time, the promo art says it airs at 8:00 PM, a mistake made because of a habit back when episodes aired at 8:00 PM.
- "All Warmed Up Inside" originally came from "All Gummed Up Inside," although it is changed talking about Flame Princess rather than Princess Bubblegum, and it is changed with specific words that have the same singing tune as "All Gummed Up Inside." The first four measures of each song sound different melodically.
- When Jake threw things out of his backpack, a magazine with a burger on it (Ble), an orange peanut, and a bear doll can be seen.
- According to Comic-Con 2013 interview with Jeremy Shada(Finn) and Tom Kenny(Ice King & Flambo), it is Jeremy Shada's personal favorite episode. 
- When Jake is fighting the second Iron Owl, his attack resembles one of Monkey D. Luffy's attack Gomu Gomu no Ono (Gum Gum Battle Axe in English) in the popular anime One Piece.
- BMO's game was a reference to Nintendo's Excitebike.
- When Finn tells BMO to go away, BMO performs a Kancho prank on Finn.
- When Jake sings he sings one line in a voice similar to Barry White.
- The Iron Owls somewhat resemble Bubo, the metallic owl from the 1981 film Clash of the Titans.
- Finn still keeps the lock of hair from Princess Bubblegum that he got in "To Cut a Woman's Hair", and was seen again in "What Was Missing".
- The plot of this episode is continued in "Hot to the Touch."
- Flame Princess is kept in a giant lantern because Princess Bubblegum asked her father to put her away so she would not experience extreme romance and destroy the world. This was not to be explained until "Burning Low."
- It was later revealed in "Earth & Water" that the true reason why she was locked up in the lantern was that when she was a baby, Flame King denies her as his daughter and sent her into the woods to perish. However, the messenger took pity on her and left her with a Rock Woodsman, whose house was burnt by Flame Princess. She runs into a nearby forest and burnt down trees while running around like a fireball. Princess Bubblegum sees this and returns her to the Fire Kingdom. She tells the Flame King that he should not let her wander in the woods as she was too dangerous and advises him to lock her up into a lantern.
- Rebecca Sugar said this was the episode when she could finally make Finn cry, although he cried briefly before in "Jake vs. Me-Mow."  He also seems to cry in "Dream of Love" and "The New Frontier." Even though this was not seen on the show, Finn says he spent thirty minutes crying in his bed after Jake almost suffocated him in "Crystals Have Power." This is the second episode where Finn is seen crying (not including him crying as a baby).
- In the first scene, on Jake's right side, there is a radio (Labelled "ROCK"). Later when it starts to rain, and Finn and Jake run indoors, the radio disappears.
- In the scene right before Finn sees Flame Princess on the roof, the thumb on his left hand is misplaced.
- When Finn is depressed he holds his wad of Princess Bubblegum's hair. In the scene when Jake walks away talking about Finn's love-life (right before Jake puts his boots on), the wad of hair is gone but Finn's hands are still in their previous position.
- When Finn leans over to lie down on Princess Bubblegum's lap his left shirt sleeve is the same color as his skin for a few frames.
- When the Tree Fort begins to shake, the arm of another BMO can be seen next to the original BMO, as if BMO is standing in front of a mirror.
- Jake sings a song with what looks to be an acoustic guitar (on fire), though it sounds like a ukelele.
- There are various continuity errors regarding the rockets and the matches that were on the roof in the first scenes with Princess Bubblegum as well as when Flame Princess is on the roof.
- When Jake tells Finn the rain is letting up, there is no red plant on the window; however, one appears when Finn start to sing.
- Even though the Tree Fort was on fire nothing seems to have been burnt, except for the picture of Princess Bubblegum.
- At the beginning of the episode, Finn is looking at a picture of Princess Bubblegum without a flower, but later on, he is still in the same position where he was looking at the last picture, but this time, the picture is Princess Bubblegum with a flower.
- When Finn breaks the glass, he grabs the window frame with his right hand, holding Princess Bubblegum's hair in his left. After seeing Flame Princess, his hands switch, holding on to the treehouse with his left arm, and holding Princess Bubblegum's hair in his right. After Flame Princess falls, his left hand is holding the window frame, and his right hand is holding Princess Bubblegum's hair, which he sticks onto the treehouse.
- This episode was previously titled "Heart of the Fire Kingdom."
- This is Adam Muto's and Rebecca Sugar's last episode as a team (see promo art). As from this episode, Adam Muto was promoted to creative director and Rebecca Sugar will work together with Cole Sanchez.
- The original storyboard shows Flame Princess shooting more Fire People.
- In the original storyboard, Flame Princess calls Finn freaky, after advising him to never mess with her again.
- On iTunes, this episode, along with the episodes "Another Way," "Ghost Princess," and "Dad's Dungeon," is incorrectly labeled as part of season four.
- The noises coming from from the instrument Jake is singing the serenade with is a guitar(six strings)but the sounds coming from it are a ukulele
This episode was censored in some countries. See Censorship of Adventure Time for more information.
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