A mysterious knight crashes Finn's birthday bash and presents him with a dangerous gift.
Princess Bubblegum has the Candy Kingdom in lockdown until they find out what is happening with her Uncle Gumbald, so she arranges a small gathering mostly of banana guards and candy people, and a few of Finn's friends for his 17th birthday party, with carnival style games. A large green man enters and says that he has brought Finn an axe, but to win it, Finn must strike him.
Originally Finn declined, but then thought it may of been one of Jake's birthday pranks and chopped the man's head off, only realizing it wasn't a prank when Jake enters with a fake present. The man's head grows back, and he says it's now his turn to strike Finn. Finn suggests playing carnival games, and the person to get two out of three wins. If Finn won, he wouldn't get struck. The man put up a shield so it would be fair, and all of Finn's friends watched from afar. The first game was throwing coconuts at cans, which Finn scored four out of five, only missing the last shot due to the strange man's horse blowing air on the ball, causing it to go in a different direction. The man then knocked all five down on his turn, by cheating.
The next game was climbing a greasy pole, which Finn wins. The last game the man chose, which is arm wrestling, and Finn is certain he will win due to his new arm, however this is when it gets found out that the mysterious man is Fern, and he wins Finn by throwing him to the ground. Uncle Gumbald, cousin Chicle and Aunt Lolly then appear, as they were disguised as Fern's horse, as Bubble questions how they are back. Gumbald hints by saying "Oh my glob!" with Bubblegum realizing it is that they returned since Lumpy Space Princess reset the Land of Ooo in the Elements episodes, they were reset too.
Uncle Gumbald then goes onto rant that Bubblegum kept them imprisoned for centuries; she defends her choice by stating they were happy as candy people. Gumbald replies they thought they were but tells her not to feel guilty as he tries to change Princess Bubblegum into candy, as she has done to them many years ago. Before he can, Marceline shape shifts and guards PB and tells him to back off, in which he replies "the vampire girl is legitimately scary," though states as she has seen their superior technology and saw her champion fall, she'll realize she can't stop them. He ends by saying "We'll leave, for now" as they take off with Fern. Finn, disturbed by these events, is being consoled by Huntress Wizard as he says 'Worst B-Day Ever.' Right then, Jake and BMO trolley in his birthday cake which is a massive Finn Cake through the main entrance while singing happy birthday to Finn with the others joining in as well and circling around Finn who's sitting backed up against the wall and looking unhappy and Finn looks on, as the Ice King emerges from the cake dressed as Flame Princess, while the latter silently continues to look on in dismay, with weary eyes.
- Lumpy Space Princess
- Flame Princess
- Tree Trunks
- Mr. Pig
- Colonel Candy Corn
- Mrs. Cow
- Teenage Bear
- Sweet P
- Candy Cane Guy
- Mr. Cupcake
- The device Gumbald intends to candify his niece with is based on plasma globes.
- Chicle and Lolly strangely bear animosity to Bubblegum, seeing as how it was Gumbald's fault that they were transformed.
- Finn is called "Mr. Finn Mertens" by Ice King. This marks the first time another character has referred to Finn as such. Beforehand only Finn referred to himself as "Finn Mertens" while others called him "Finn the Human."
- Finn suspecting the thumping sound caused by the mechanical horse's hooves being produced by the elk's heart could be a nod to Edgar Allan Poe's short story "The Tell-Tale Heart."
- The beginning of this episode is a reference to the Arthurian story Sir Gawain and the Green Knight.
- The Green Knight's incessant cackling parodies the tendency of villains in many kids shows (and not only) to overindulge in what is colloquially called "evil laugh" or "maniacal laughter."
- Gumbald, Lolly and Chicle infiltrating Bubblegum's palace disguised as a horse is a reference to the famous scheme of the Trojan Horse.
- Ice King's line "Happy birthday, Mr. Finn Mertens" may be a spoof of Marilyn Monroe's "Happy Birthday, Mr. President."
- The Green Knight's armor resembles Alex Mercer's armor from the video game Prototype.
- This is the first episode since "Mystery Train" where Finn celebrates his birthday, the episodes take place four years apart.
- Upon hearing the sound of hooves approaching, Starchy claims it is produced by zombies, referencing the events from "Slumber Party Panic" and "From Bad to Worse."
- Finn's suspicion that the Green Knight is actually part of one of Jake's "classic" birthday pranks is a callback to "Mystery Train."
- Despite the events of the Elements miniseries, Bubblegum is still reticent to use the term "magic," as she called the invisible shield "advanced mystery tech."
- Gumbald found Fern's remains in "Three Buckets," and in "The Wild Hunt" a creature resembling him is seen in Gumbald's lab. This episode confirms that this is indeed Fern revealing that he has been revived or survived his supposed death.
- Fern's triumphant "Finally! I win!" references his frustration at perceiving himself as inferior to the "real" Finn, which emerges for example in "Whispers" and "Three Buckets," among other episodes.
- This episode reveals that Gumbald, Lolly, and Chicle were turned back to their true forms by LSP's anti-elemental effect at the end of "Skyhooks II," similar to Jake and Sweet P. In that episode, Punchy, Crunchy, and Manfried were seen standing together right as the anti-element wave restored Ooo, and none of them have been seen since then, as proven by the "Missing" poster showing Crunchy in "Always BMO Closing."
- Ice King bursts out of the cake dressed as Flame Princess, indicating he recalls them having been in a relationship but possibly forgetting that she and Finn broke up following the events of "Frost & Fire." However, it is also possible the entire ruse with Ice King inside the Finn cake is Jake's birthday prank.
- As of this episode, Finn turns 17 years old.
- This episode was released early on the Cartoon Network app on December 7, 2017.
- Right before Fern takes off the top half of the horse, the top of cousin Chicle's hair could already be seen.