A mysterious knight crashes Finn's birthday bash and presents him with a dangerous gift.
Tropes:
- A Birthday, Not a Break: Finn turns 17 and is almost killed by Green Knight Fern. Bubblegum discovers her artificial family is alive and plotting against her.
- Birthday Episode: As indicated by the title, Finn turns 17 in this episode.
- Bottle Episode: Apart from some games and Fern's new design, the entire episode is nested in a birthday party for Finn hosted in the main castle hall of the Candy Kingdom.
- Brought Down to Normal: The episode confirms that LSP's World-Healing Wave in Elements turned Gumbald, Lolly and Chicle back to normal. Lampshaded when he reveals this to Bubblegum.
- The Dragon: Fern is now acting as this to Gumbald.
- Jumping Out of a Cake: Ice King comes out of Finn's birthday cake dressed like Flame Princess.
- Misplaced Retribution: Even though Gumbald deliberately turned Lolly and Chicle into idiots, they rejoined him and entirely blame Princess Bubblegum for not turning them back.
- Mistaken for an Imposter: Finn at first thinks the Green Knight is Jake pulling one of his classic pranks, so he beheads him without a second thought. Then the real Jake walks in, and Finn is distraught that he beheaded someone for real.
- Ship Tease:
- After Gumbald threatens to turn Bubblegum into a dumb candy person, Marceline steps into the way and threatens to attack him if he does something to the princess. She also holds PB's hand when they sing happy birthday to Finn.
- Huntress Wizard gives Finn an elk's heart as an emotional and metaphorical way to show her affection towards him. She also stays at his side after Gumbald reveals himself, helping him recover after being defeated by Fern (and after forcing him to eat the elk's heart, causing him to vomit)
- Shout-Out: The Green Knight and the head-cutting-off game are taken wholesale from the Arthurian tale of Sir Gawain and the Green Knight.
- The way Fern smashes Finn through the table during their arm wrestling match is reminiscent of the {{Regular Show's episode "Playco Armboy," in which Skips does the same to Rigby, with even deadlier results.
- Stress Vomit: Finn is so overwhelmed by the events of the episode that he vomits in a bucket afterwards.
- Vomit Indiscretion Shot: Overlaps with Vomit Discretion Shot. After HW forces Finn to eat the elk's heart to help him recover, he pukes inside a bucket. While he puts his head inside the bucket to vomit, the liquid inside can be seen after Fern and Gumbald run away from the Candy Kingdom.
- Wham Episode: Uncle Gumbald's return has been revealed to the world, and Fern is now his dragon.
