The episode opens with Princess Bubblegum and Marceline flying on The Morrow on the way to the Ice Kingdom in order to attend Ice King's dinner party. PB isn't very excited to go but Marceline ensures her that it will be fun and requests that PB address Ice King as Simon.
Upon arriving and sending The Morrow off, Ice King greets them both with a present as a bribe for coming. Marceline says that they don't need to be bribed into attending, and Ice King takes back the present. Gunther enters to offer red ice drinks. Marceline drinks the red from hers while PB's drink slides out of the glass and is wasted.
Ice King starts up a karaoke while Marceline gets a new drink for PB. As PB is about to start singing, Ice King wigs out while his crown sparks and circuitry lines pulsate red. PB yells for Marceline to come back and help Ice King but when she comes into the room Ice King is fine, making Marceline think they were playing a prank.
Gunther plays a triangle to signal dinner being ready. Ice King serves them "penguin food" which are just different shaped food items. PB insists that something is wrong with the crown as she keeps seeing sparks fly out of it while Marceline insists nothing is wrong, and is shocked when PB called Ice King "Simon" while inspecting his crown. Simon says to call him whatever as he enjoys this date being better than the last one. PB says that it is not a date, to which Simon replies "neither was the last one," it was a "robbery" with Betty stealing his heart and crown. This piques PB's interest, thinking the Crown has been tampered with.
Simon goes mad again, smashing plates with his head while Marceline watches in awe. PB uses a large icicle to knockout Simon as it would be easier to bring him back to the Candy Kingdom for study.
In PB's lab, an unconscious Simon is resting in a gurney while the crown is suspended in a tube full of wires and pink fluid. A reading comes out of the podium which PB reads but cannot understand. She heads over to a VR set which is connected to the crown, impressing Marcy who is curious as to why PB has never shown her the VR set before. PB says that she thought Marceline wouldn't like science, to which Marcy says she likes games.
They enter the crown, a large labyrinth with foliage everywhere and nothing around the outskirts of the maze. They enter and soon find Gunther, the dinosaur, playing bongos. He wonders two newbies could get into the crown at once, thinking they were skinny enough to fit in together at the same time. PB notices the walls are pulsating the same way as the crown was, changing the pattern of the labyrinth whenever it does. Gunther says it's been happening for a while. Gunther suggests they go to the Professor, the smartest person in the crown next to Evergreen.
They walk around the maze, passing by a young looking boy in a dark gray shirt speaking in some sort of Scandinavian (likely, Old Norse derived) language and an older looking man in a red suit listening. Gunther says that the boy is Svein, who only put the crown on once, and the other guy is some guy who calls himself the first "Santa" which excites Marceline, but PB has no idea who he is, nor cares.
They find The Professor, Simon, who is tinkering with the pulsating circuitry in the labyrinth walls. Marceline calls out to him and he runs to her, hugging her and asking if Betty found a way to fix the crown. Marcy says no, the reason they are there is because of PB. Simon assumes she's Betty's assistant, which angers PB. Marceline diffuses it and says they are in the crown because it's going haywire, affecting Ice King in real life. Simon says he's been following the path of destruction left by the malware.
Gunther suggests a quest, leading the group around the maze. Simon apologizes to Marceline for the previous time he was in Ooo, focusing too much on Betty and not on Marceline when Betty first came to Ooo to which Marceline responds with cheerful understanding. He also asks if Marceline has a boyfriend. She laughs it off, saying she did but she dumped him hard, and Simon wishes he punched him. He already has but didn't realize who Ash was at the time in Betty, while Marceline reassures him that she can punch her own jerks, lightly punching Simon in the shoulder. He laughs it off while whispering "ouch."
The group walks past a corner and find the source of the malicious coding: A disembodied head of Betty, referred to as Magic Man by PB, who corrects her as "Magic Woman" reveals herself cheerfully and continues to re-code the crown walls. Simon runs up to her, excited to see Betty while Gunther is underwhelmed. Simon asks what she's doing, and Betty says she's reprogramming the crown's wish magic in order to fix Simon. She exclaims "AHA!" as she enters a code which affects the Ice King in the gurney into putting his feet in his mouth, and rearranging the world in the crown.
PB determines that Betty is causing irreparable damage and needs to be stopped. The groups tries and fails multiple times to capture Betty, who just keeps floating above and away to a different wall to keep on reprogramming. Simon suggests that the real Betty should be in there, which Gunther helps motivate him to do something. He approaches her, telling her that he's the reason she's doing this, to which she says "I'm busy." Simon tells her about the time they first met in a library and wanted to check out the same book. He let her have it, and she wrote a note to him. He remembers it fondly as a funny and witty note and beings to tear up. She continues to work and Simon cries that she can't help him like this. An angry Betty says she can and she will, addressing him as Simon. Simon, surprised by that she called him "Simon" tells her to follow that memory about him. She snaps to normal, and flies over to Simon to kiss him.
Marceline looks on as PB sees that the crown is stabilizing and reprogramming itself to push out foreign entities. Marceline asks if she can visit Simon, to which PB says no. Betty asks about herself, and PB says she'll be deleted. She apologizes to Simon for almost destroying his world, to which he says that she is his world. Gunther says goodbye and hugs Marceline. Marcy says goodbye as well and says she'll be nicer to the Penguins. Gunther is confused but still happy he got to meet new people.
As PB and Marceline are beginning to disappear back to the real world, Betty announces she has something to tell Simon and starts to glitch. Marceline and PB return before the audience hears what Betty has to say. PB checks the data printout, noticing remnants of Betty's code as a shadow print. Back in the crown labyrinth, Gunther is sitting by some bushes while Simon asks Betty's floating AI head what she wanted to tell him. She says it was just for dramatic effect if she was deleted so he could always think about her, but she remains, albeit with a glitched face. Simon doesn't mind and says he never could stop thinking about her as they kiss.
PB, Marceline, and Ice King are back at Ice King's palace with Gunter playing the on the karaoke. Simon doesn't recall anything and says it was the best dinner party he's ever hosted. PB agrees, with a surprised Marceline looking at her. PB asks her why she's looking at her like that, saying she does not like relaxing but likes science.
- This is the seventh episode (not counting the gender-swap episodes) in which both Finn and Jake neither appear nor are mentioned, the first being "Princess Day," the second being "Evergreen," the third being "Friends Forever," the fourth being "Be Sweet," the fifth being "Orgalorg," and the sixth being "Varmints."
- Princess Bubblegum calls Ice King "Simon" for the first time, as requested by Marceline.
- Marceline describes Princess Bubblegum as a "dynamo on the mic" suggesing that she is a great singer.
- The episode's title, aside from alluding to the Ice King's malfunctioning crown, may also be a reference to a line in the nursery rhyme "Jack and Jill" ("Jack fell down and broke his crown...").
- It appears the legend of Santa Claus did not pass beyond the Mushroom War, as Princess Bubblegum has no idea of who he is when Marceline becomes excited. Gunther's tone about him being the "first Santa" suggests that Santa Claus came into existence some time after the elementals encountered the comet in the Cretaceous period in "Evergreen."
- Several events that occurred in the episode "Betty" are common throughout the episode.
- Marceline mentions her breakup with Ash, first talked about in "Memory of a Memory."
- Simon tells Marceline that he wished he could have been there to punch Ash, which, unbeknownst to him, he already did in "Betty."
- Gunther makes his first appearance since his debut in "Evergreen."
- Ice King references his previous "date" as a robbery, the perpetrator (Betty with Magic Man's powers) stealing his heart and crown in "King's Ransom."
- This is the final episode in which Lena Dunham voices Betty before Felicia Day took over the role.
- It is revealed that others besides Gunther and Simon have worn the crown, and gone insane, including an adolescent named Svein (who only wore it once) and even a so-called "Santa."
- It is also revealed that the crown is actually a supercomputer, and Simon literally is trying to navigate a labyrinth in his mind as previously mentioned in "Holly Jolly Secrets Part II."
- Simon actually was there to punch Marceline's ex boyfriend Ash, in the episode "Betty."
- Princess Bubblegum and Marceline seem to have worked past their differences and are now closer, especially after the events of Stakes.
- Princess Bubblegum has never heard of Santa Claus.