In the previous episode, Princess Bubblegum was accidentally dropped by the Ice King into the Lich's Well of Power, which had melted her body. 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. They put two scoops of sugar on her just before she flat-lines, though the doctor and nurse manage to stabilize her. 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 face, 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 (His old one got ripped in "Mortal Folly."). 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 behavioral problems, including spasms, heavy breathing, convulsions, and crawling into the bathroom. 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, but she is unresponsive. Princess Bubblegum asks Finn in a whisper to bring her some things. When Jake and Peppermint Butler return with a cup of tea, Peppermint Butler freaks out, hisses like a cat, and flees the room.
While Finn is away, Jake tries to play some music for Princess Bubblegum, but Princess Bubblegum growls, spasms, and seems to kill all the flowers in the room. Jake begins a new song, but Princess Bubblegum spasms and summons magical fire, scaring him from the room. 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 opens the door and takes them from Finn and returns to her room.
Jake then tells Finn of the fire she summoned, which alarms Finn. He asks how, to which Jake replies, "With her brain, I think." 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 skin, pouring the chemicals into her bathtub, forming a pool like the Lich's Well of Power. 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 as tall as her tower.
She knocks the boys through the wall into the hallway, and as she begins to attack the Candy Kingdom, Ice King appears and asks, "Hey, Finn, are you dead or what?" He then explains that he saw the Lich possess Princess Bubblegum with his "Wizard Eyes", and explains that 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, as it was his fault for stalling them while chasing the Lich and for almost killing Princess Bubblegum in the first place.
After telling Ice King to "get stuffed," Finn is accused by EJ of being "a bastard who only fights just for princesses" and is ordered to let him work. He refuses, and 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. Dr. Ice Cream, Nurse Poundcake, and Doctor Princess attempt to reassemble her but they do not have enough pieces. Nurse Poundcake 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 years old instead of eighteen.
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, with eyes that glow like the Lich's, foreshadowing the Lich's return.