Who is Fern? Fionna and Cake resurrects Adventure Time's most tragic villain

The penultimate episode of Fionna and Cake season 2, “The Worm and His Orchard,” was full of reveals, including the answer to one burning question posed earlier in the season. However, the return of a certain character was perhaps the most exciting surprise for long-time Adventure Time fans. But for anyone who never finished the original series (to be fair, there are 283 episodes in total) or hasn’t rewatched it in a while, this unexpected cameo may be a bit confusing. Here’s a quick refresher course on what it all means and how Fionna and Cake picks up a loose thread from 2018.

[Ed. note: This article contains spoilers for Fionna and Cake season 2 episode 9.]

Fionna and Cake and Fern

Upon arriving in the Undergrowth, Fionna immediately becomes trapped in a venus flytrap-esque plant, only to be rescued by Finn’s long-lost grass counterpart, Fern. While there is a version of Fern in Fionna World, Fennel, this episode marks the first time viewers have seen the original Fern since the Adventure Time series finale, which aired in September 2018. When Fionna asks Fern if he’s Finn’s doppelganger, he replies, “Used to be, yeah. I remember sort of dying in Ooo, but my consciousness awoke here. I tried to be Finn. I wanted what he had, his things, his life, but I’m something else.”

That’s about all the context we get in this episode, but there’s way more history to Fern’s character. To understand the history of Fern, we have to revisit two of Finn’s swords: The Grass Sword and the Finn Sword. The Grass Sword came first, purchased from Grass Wizard by Finn in Adventure Time season 5 to replace his broken demon blood sword. The wizard had cursed the weapon to bond to its user forever, but Finn learns he’s able to fully control the sword once he accepts this fate.

The Grass Sword seems to have a mind of its own, however, and can be very reactionary to Finn’s emotions. This eventually culminates in Finn losing his arm in “Escape from the Citadel,” when it forms an aggressively strong vine in an attempt to stop Finn’s father from leaving him again, tearing his appendage off entirely and leaving a flower in its place. In the episode “Breezy,” the titular character — a bee — helps Finn’s arm regenerate with her singing, leaving him with a new version of the Grass Sword that manifests as a thorn in his palm.

The Finn Sword first appeared in the season 6 episode “Is that You?” because of a paradox. After Finn and Jake perform a memorial and dream ritual to honor the fallen Prismo, his pickle jar transports them into another dimension that Prismo reveals was his “plan B” if he ever died.

What occurs is a sort of time loop. Jake finds a bed within the dimension and falls asleep. He’s then awakened by Finn, and the pair hide as the same events repeat themselves. The next time, Prismo instructs Finn to stop the second version of himself from waking Jake. When he does so, the second Finn combusts, taking the form of a sword. (If this all sounds super confusing, well, it is. “Is that You?” is a very weird episode that works surprisingly well when watched, but can be difficult to explain.)

It’s later revealed that the Finn Sword has a form of Finn’s consciousness inside of it. In the season 8 episode “I Am a Sword,” the sword is stolen by Bandit Princess, and the Finn Sword asks her why she’s chosen to be a thief. Bandit Princess is eventually confronted by Finn, and in their fight the Grass Sword springs into action to protect him, breaking the crystal core of the Finn Sword. When Finn hangs the broken sword above his mantle, it glows green, implying a sort of merging between the two weapons.

In the season 8 episode “Reboot,” the two swords come into contact with each other again. This time the Grass Sword wraps completely around the Finn Sword, which both leaves Finn armless again and creates Fern. Though he initially appears as a simple grassy creature, during an argument with Finn in the next episode, “Two Swords,” he slowly morphs into looking exactly like Ooo’s hero and asserts that he is the real Finn Mertens. Because of how the Finn Sword came to be, Fern possesses all of Finn’s memories, genuinely believing he is him.

The rise and fall of Fern

While Fern is part Finn, he’s also part Grass Demon, an entity that got into the sword during the encounter with Bandit Princess and began influencing him inside the weapon. Though Finn and Fern initially are able to get along, the demon’s influence slowly takes over, leading Fern to be increasingly vindictive and jealous.

This culminates in the season 9 episode “Three Buckets,” wherein Fern traps Finn in a stone temple in an attempt to take over his identity. Finn escapes after discovering his new robotic arm (given to him by Princess Bubblegum after the sword-merging incident) is a multi-tool that includes rock smasher and weed whacker modes. When he confronts Fern, he tells him that the day “doesn’t have to have this kind of finality,” which the arm hears as “fatality,” activating the weed whacker function and shredding Fern to bits.

However, an unknown entity is shown sweeping up his remains and carrying them off in a bucket. In the season 10 episode “Seventeen,” this is revealed to be Princess Bubblegum's antagonistic Uncle Gumbald, who reforms Fern as the Green Knight to provoke the princess.

In the series finale, “Come Along With Me,” Finn uses nightmare juice from Nightmare Princess to bring himself, Jake, Fern, Princess Bubblegum, and Gumbald into a dream state to settle their differences and end the Gum War. While there, Finn and Fern go to Finn’s memory vault where they confront the Grass Demon, with Fern ultimately killing it. Fern is freed from the demon’s influence, but in turn loses his physical form. As he slowly disintegrates, he tells Finn to plant him at their tree fort, then turns into a seedling form of the Finn Sword.

Closure for an Adventure Time tragedy

That brings us to Fionna and Cake. In the show’s latest episode, we finally learn what happened to Fern after he left the material plane in Ooo: He was transported to the Undergrowth. The specifics of why this happened aren’t fully delved into, but it makes sense for multiple reasons. For one, in the most recent episode, Witch Wizard calls the Undergrowth the place with the “greenest magic,” meaning Fern likely has ties to it somehow as a magical grass entity.

The Undergrowth is essentially where everyone’s trauma goes to be processed and regrown as something new, overseen by the Karmic Worm (a new character who turns out to be the Cosmic Owl’s long-lost brother). During her time there, Fionna witnesses everything from Marshall Lee’s bad childhood memories to Finn witnessing Jake’s death (which was first established in the Distant Lands — Together Again special in 2021). Fern undoubtedly had a great amount of unprocessed trauma when he disintegrated, and it seems living in the Undergrowth has helped him come to terms with it.

Fern even tends to Finn’s own trauma tree, referring to the human hero as a “sad fruit medley” whose trauma balls won’t float up because he isn’t ready to deal with it yet. It’s unclear whether we’ll see Fern again, though it’s not out of the question, since Cake’s new bond with the Karmic Worm has left the door open for future visitation. But whether or not Fern ever returns to Fionna and Cake (or some other Adventure Time spinoff), learning about his fate after the events of the Gum War has brought some closure to a complicated character.

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Fionna and Cake is streaming on HBO Max.