"Mortal Recoil" is the twenty-sixth episode in the second season of Adventure Time. It is the fifty-second episode overall.
Princess Bubblegum recovers from being dropped in the Well of Power by the Ice King; however, she is altered and her condition leads to an unlikely final showdown with one of the greatest evils in order to save all life in Ooo.
In the previous episode, Princess Bubblegum was accidentally dropped by the Ice King into the Lich's Well of Power. The episode begins with Dr. Ice Cream and Nurse Pound Cake, along with other candy hospital staff, rolling her into the ER trying to stabilize her. She is stabilized but does not seem to be acting normally, and is paler than her natural hue. Ice King tries to explain something strange he saw when Princess Bubblegum fell in the well, but Finn—still mad at the Ice King for almost killing her and for making insensitive comments during the surgery—interrupts and nearly kills him by throwing his Root Sword at him, telling him, "Just shut your mouth, old man!" The Ice King denies this appellation, but affirmation of the other princesses hurts him and he flies away. The other princesses give Finn a new backpack. Princess Bubblegum is still very ill and is confined to her room, with Finn and Jake acting as her caregivers. When she wakes up, Finn and Jake reveal they have picked every flower in the Candy Kingdom for her.
However, she exhibits strange behavior and is not acting like herself, such as trying to get cleaning supplies. Finn returns her to her bed and takes some time to confide his feelings to her, asking Jake to leave the room. Finn thanks her for the sweater she made him due to it have "the power of liking someone a lot" because she cared about him and tells her that they should discuss their feelings later, and she asks him for some "things." When Jake and Peppermint Butler return with a cup of tea, Peppermint Butler suddenly freaks out, as the princess is oddly whispering like the Lich. Peppermint Butler hisses and runs off, leaving Jake suspicious.
While Finn is away, Jake tries to play some music for Princess Bubblegum, but the princess just spasms and growls, even killing all the flowers collected in her room and sets it on fire, causing Jake to run out and wonder if the well did something to her brain. When Finn returns to the hall outside, he is holding an assortment of items, which he reveals are things like gasoline, bleach, ammonia, lighter fluid, plutonium and other hazardous chemicals. Jake urges Finn to not bring the chemicals to Princess Bubblegum, but she just calmly takes them.
Jake then tells Finn of the fire she summoned, which alarms Finn. The two enter the room to find it empty and dark, with the bathroom door closed. Finn believes Princess Bubblegum is just having private time in the bathroom, but a skeptical Jake looks through the keyhole. Finn protests that Jake is acting "pervy," but Jake makes him look in. They see Princess Bubblegum, now in a demonic form with disproportionate limbs and dark-purple-black skin, pouring the chemicals into her bathtub. When she starts drinking the liquid, Finn and Jake try to stop her, but she rips the tub off of the floor and drinks the entire concoction, growing into a monster as tall as her tower and powerful to the point where a gumball guardian must fight her.
She knocks the boys through the wall into the hallway, and as she begins to attack the Candy Kingdom, Ice King appears and explains that he saw the Lich possess Princess Bubblegum with his "Wizard Eyes," and but he had not spoken up earlier because "when you have stanky old wizard eyes, sometimes you see things that are real, and other times it's like crazy, crazy, crazy, in your face all the time." He wants to help Finn and Jake, but Finn angrily refuses his assistance.
Finn tries to defeat the possessed Princess Bubblegum with the sweater she gave him, but fails. As Princess Bubblegum attacks Jake, Finn finally accepts Ice King's help. As Finn distracts Princess Bubblegum, the Ice King freezes her from the feet up. The Candy Kingdom rejoices, but their jubilation turns to shock when the frozen Princess Bubblegum statue falls over and shatters into pieces. Dr. Ice Cream, Nurse Pound Cake, and Doctor Princess attempt to reassemble her but they do not have enough pieces of gum for her hair. Nurse Pound Cake reveals to the crowd waiting outside of the surgery room that Princess Bubblegum is alive, but, due to the lack of recovered gum, she is now thirteen.
Ice King immediately loses interest in her and leaves. Finn remarks that he is thirteen as well. Princess Bubblegum calls him a hero, and the two share an embrace. As everyone celebrates, the snail (who freed the Lich in "Mortal Folly") is seen waving menacingly in the background, sighing with eyes that glow like the Lich's.
- Candy People
- Doctor Princess
- Emerald Princess
- Wildberry Princess
- Lumpy Space Princess
- Raggedy Princess
- Slime Princess
- In the emergency room at the start of the episode, the heart monitor screen is displaying the first 15 digits of pi.
- All of the items Princess Bubblegum asks for (bleach, gasoline, lighter fluid, plutonium, and ammonia) are extremely sharp, flammable, explosive, poisonous, or radioactive. In fact, plutonium is so radioactive that it cannot be handled.
- Incidentally, Finn handles the plutonium he obtains in his bare hands. Plutonium emits alpha, beta, and gamma waves that are absorbed into bone marrow, and have a biological half-life of 200 years. The absorbed radiation stays in the bloodstream for decades, making the carrier much more susceptible to cancer. This level of unprotected exposure to the radioactivity that plutonium emits would most likely be enough to give Finn lethal radiation poisoning.
- However, given that Finn easily found the plutonium in the castle, it is likely that Bubblegum was making experiments with it and had previously contained its radiations in order to do so.
- When bleach and ammonia are mixed together, they form toxic and volatile products, such as hydrazine, chloramine and chlorine gas. 
- When the possessed Princess Bubblegum sets her room on fire, the flames in the background seem to have faces.
- The snail is visible in the title card.
- This episode marks the first time Adventure Time has had a season-ending cliffhanger, unresolved until Princess Bubblegum's next appearance six episodes later in season three's "Too Young". Since then, season-ending cliffhangers have been the norm: season three's "Incendium" was resolved in the season four opener "Hot to the Touch"; season four's "The Lich" was resolved in the season five two-part opener "Finn the Human" and "Jake the Dog", and season five's finale "Billy's Bucket List" continues into "Wake Up" and "Escape from the Citadel". Season six's two finales, "Hot Diggity Doom" and "The Comet" were not a cliffhanger to season 7's premiere, "Bonnie and Neddy". However season sevens two-part finale "Preboot" and "Reboot" was continued into season 8's "Two Swords" and "Do No Harm".
- When Princess Bubblegum is in the bathroom searching for the stuff she later asks Finn to bring her, the sound of glass breaking can be heard but when Finn enters the bathroom there is no broken glass around.
- Bottles and things she left on the floor are still there when Finn picks her up but in the next frame when he takes her back to bed those are gone.
- When possessed Princess Bubblegum hits Finn, his right shirt sleeve rips, but when he is flying with the Ice King and throughout the rest of the episode, the sleeve is back to normal.
- This episode was clearly inspired by The Exorcist, a 1973 horror film featuring a demonically-possessed little girl. Like the film, Bonnibel's bed shook violently, her voice became abnormally deep, her skin was pale, and her body suffered ungodly contortions.
- When Princess Bubblegum is eating from the giant vat of poisonous house cleaners, she resembles No Face from the Miyazaki movie Spirited Away.
- When Ice King ascends into the clouds, the clouds slam together like Monty Python sketches, especially that of God's ascent in Monty Python and the Holy Grail.
- When Jake opens the door to Princess Bubblegum's room, the door sounds like the door to the TARDIS from the British Sci-Fi show, Doctor Who.
- This episode and "Mortal Folly" would have been the final episodes of the season prior to the rescheduling of "Heat Signature". The two-part episode is still considered the season two finale by the show's crew and appears as such on the Complete Second Season DVD and Blu-Ray.
- This episode is a continuation of "Mortal Folly".
- In the scene showing Ice King's wizard vision, the spirits are the same as those in "Beyond this Earthly Realm".
- This episode's continuation is "Too Young".
- Finn's line "I'm a cat! I'm an agile cat!" is foreshadowing of the series finale episode "Come Along With Me", where Shermy, one of Finn's future lives, is a cat.
This episode was censored in some countries. See Censorship of Adventure Time for more information.
- Pen Ward refers to "Mortal Folly/Mortal Recoil" as season finale, even after the scheduling change. http://twitter.com/#!/buenothebear/status/65160425511202816
- Steve Wolfhard's Tumblr