It is the final episode of Adventure Time. Which is four episodes time-wise.
In the distant future, many years after the outcome of the Gum War, two friends named Beth and Shermy recover a rusty old mechanical arm that once belonged to Finn after it is unearthed by a Prize Ball Guardian. In their home, formerly known as the residence of Marceline the Vampire Queen, the two decide to visit the "King of Ooo" in order to inquire about its origins. They visit the King in his hideout at the top of Mount Cragdor, and are greeted by BMO and their many artifacts and keepsakes of old. After Shermy and Beth accidentally break a few items, BMO asks them to leave. When they present BMO with Finn's lost mechanical arm, however, BMO agrees to tell the two the story of the Gum War.
In the past, according to BMO's retelling of the events, Finn and Jake are out on a reconnaissance mission for Princess Bubblegum when they discover Normal Man, Betty, and Maja hovering silently above Bonnie's army camp. Jake stretches the two of them to PB's command tent, where she is going over her battle plans with her allies, including Marceline, Flame Princess, Huntress Wizard, Lemongrab, Lumpy Space Princess, Duke of Nuts, Cinnamon Bun, Colonel Candy Corn, and Lady Rainicorn. Finn and Jake tell PB of Normal Man and Betty's sudden appearance, and they are interrupted by one of Gumbald's spies listening in to their plans. After Huntress Wizard incapacitates the Gumbaldian, PB decides that they must attack at once and prepares to sound the war horn. Before she does, Marceline asks her to reconsider, having suffered through the Mushroom War herself, but PB is resolute and sticks to the warpath. Feeling that PB is wrong in her intentions, Finn decides to hatch a plan in order to stop the conflict.
After PB sounds the war horn, Finn tries to convince her to give Gumbald one last chance to surrender before the fighting starts. PB agrees after being reminded of her old friend Shoko. As a last ditch attempt to stop the war, Jake unleashed the nightmare juice given to him and Finn by Nightmare Princess, forcing him, Finn, PB, Gumbald, and Fern into a hallucinatory slumber. Inside the dream realm, PB and Gumbald continue to be hostile towards each other. PB attempts to attack her uncle and Fern using an imaginary "love bazooka", but is interrupted by a singing Nightmare Princess, allowing Gumbald to escape. Jake pulls Jermaine into the dream with them, and despite Finn's protests, PB decides to pursue Gumbald. Fern transforms into a bird/pterodactyl-like and flies away, while Finn decides to follow him after turning into a butterfly.
Before Gumbald can wake himself up so he can gain the upper-hand on PB's forces, PB confronts him inside a nightmare version of one of the Gumball Guardians. The two begin to fight each other using giant toothbrushes, but are suddenly rendered immobile by some unseen force. Meanwhile, butterfly Finn tries to calm Fern down, and two plummet to the ground. Finn attempts to appeal to Fern by reminding him that they are "two sides of the same coin" and that they share the same torment. PB and Gumbald recreate a nightmare version of the moment that PB defeated Gumbald by dousing him with his own dum-dum juice, only this time with the roles reversed. PB is doused, and is transformed into a smaller, less intelligent version of herself.
Jake and Jermaine work together to find Finn's vault, where all of his repressed memories reside. They are able to find it after Jake hallucinates Lady Rainicorn and their children. PB and Gumbald are brought to tears after both witness the Candy Kingdom turn into a pile of goo. Finn and Fern continue fighting, but are able to find common ground after Jake brings them Finn's vault. Together, they dive in, and confront the Grass Demon. Fern kills it, ridding himself of its influence once and for all. However, with no Grass Demon to control him, Fern begins to deteriorate. After the four of them wake up from their dream, PB apologizes to Gumbald for not respecting his vision for the Candy Kingdom. Gumbald fake-accepts her apology and agrees to end the conflict, intending to betray her, but is tripped by Aunt Lolly and once again doused in his own dum-dum solution, turning back into Punch Bowl. Lolly accepts the peace agreement in his place, and PB proclaims that the two kingdoms shall live in harmony. However, they are all interrupted by the sudden arrival of Normal Man and GOLB.
Upon seeing the malevolent entity, Ice King has a flashback from before the Mushroom War, Simon and Betty discuss GOLB as described in one of Simon's many books while making ice cream sundaes. Simon states that GOLB's presence is felt in "every crevasse where chaos lurks". In the present, GOLB unleashed from his maw a dark green energy, which transforms a large portion of Gumbald's army into a hideous four-legged monster. Marceline tells PB that the beast must be stopped, or it could mean Ooo's destruction. PB orders her Banana Guards to run away, but Finn, Jake, Marcy, Flame Princess, Huntress Wizard, LSP, Lemongrab, Lady Rainicorn, and Fern stay behind to fight GOLB and defend Ooo. The Gumball Guardians attempt to stop the monster, but one of them is destroyed. Lolly tries to convince Gumbald's army to stay and fight, but they run away instead. Finn, Jake, Normal Man, and Ice King attempt to snap Betty out of her trance to she can help them banish GOLB. The remaining Gumball Guardian attempts to restrain the monster, but is mutated into another monster instead. PB uses her elemental magic on the beast, releasing a hoard of evil bird-like creatures from the turned Guardian's mouth. Amidst all the chaos, LSP gets romantic with Lemongrab and the two kiss.
Ice King is able to reach through Betty's trance and awaken her by singing "Oh, Fionna" from his fanfictions. Realizing that he is trying to stop her, Betty becomes angry and releases an emotional supercharge that destroys Maja and hurls her and Ice King into GOLB's mouth. Finn follows them, but GOLB's jaw closes on Finn's mechanical arm, which falls to the ground below. Flame Princess is incapacitated by the mutated Gumball Guardian, and Marceline attacks it. The other mutated monster attacks PB and Fern, and although Fern tries to protect her, he is knocked aside leaving PB vulnerable. The creature stomps PB, and in retaliation, Marceline temporarily releases her vampire essence, transforming into a more monstrous form, and is able to destroy the mutated creature before it can cause further harm. PB, protected by her armor, is embraced by Marcy. Marcy confesses that she never wants to lose PB again, and the two kiss each other lovingly.
Inside GOLB, Ice King and Betty are slowly digested and broken apart into their "essential forms". Ice King reverts back into Simon, while Betty reverts back into a human. Reunited once again, Simon and Betty embrace each other as the space they are trapped in begins to shrink. With the bulk of PB's forces defeated, the mutated Gumball Guardian and revived mutated Gumbaldian army begin to wreak havoc on the rest of Ooo. Jake, transforming into his blue alien form, tries to stop them, but the treehouse is destroyed with BMO inside. Jake is devastated after seeing the wreckage, but is comforted by BMO, who has suffered damages to his face screen. BMO begins singing "Time Adventure ", which creates an invisible force field that prevents GOLB's mutated creatures from touching them. Eventually, realizing that GOLB's essence is vulnerable to music, Finn and Jake's friends and allies join in on the song. Jake is able to amplify the sound of their voices and help Finn and Ice King escape from inside GOLB, although Betty decides to stay behind as a sacrifice and use the Ice crown (which was restored to its essential form) to wish GOLB out of existence. When this doesn't work, she wishes instead that Simon be protected at all costs. As a result, Betty is fused with GOLB and the two become one singular entity. Devastated at Betty's decision, Simon cries out for her as the new GOLB leaves the planet through another portal, and is comforted by Marceline. Before the portal closes, the Ice Crown falls to the ground. Gunther decides to wear it, and is transformed into Ice Thing after wishing to be more like Ice King.
Fern begins to disintegrate, and Finn promises to plant him in the remains of the Tree fort before he dies, leaving Finn a small seedling in the shape of the Finn sword. That night, Finn and Jake plant the seedling together, which causes a massive new tree to sprout with a remade Finn Sword loged in the top. They are then visited by PB, who thanks Finn for convincing her to stop the war and disobeying her orders. PB comments that Finn is getting taller, before bidding them a good night. In the future, BMO concludes his story. Beth and Shermy ask what happened to Finn and Jake afterwards, to which BMO replies that they went on and "lived their lives". After departing Mount Cragdor, Beth and Shermy go looking for Finn and Jake's tree fort, or what remains of it.
In the past, Finn and Jake are relaxing with the music hole, reflecting on the outcome of the war. Jake comments that music is indeed a powerful tool, to which the music hole agrees. She then sings them a new song that she had been working on, about a really specific feeling that is hard to describe. As the music hole sings, a montage plays showing what the cast gets up to.
Back in the future Beth and Shermy can be seen exploring the remnants of Finn and Jake's old tree fort. They climb the massive tree that has sprouted from Fern's seedling, and after pulling the remade Finn Sword, strike a very familiar pose similar to Finn and Jake's at the end of the show's opening sequence.
- Princess Bubblegum
- Ice King/Simon Petrikov
- Betty Grof
- King Man
- Fern/Green Knight
- Gumbald/Punch Bowl
- Banana Guards
- Flame Princess
- Huntress Wizard
- Lumpy Space Princess
- Lady Rainicorn
- Slime Princess
- Duke of Nuts
- Cinnamon Bun
- Colonel Candy Corn
- Gumball Guardian
- Prize Ball Guardian
- Peppermint Butler
- Candy People
- Music Hole
- Minerva Campbell
- Sweet P
- Turtle Princess
- Susan Strong
- Bandit Princess
- Frozen Yogurt Princess
- Crabbit Familiar
- Jake Jr.
- Kim Kil Whan
- Pete Sassafras
- Peace Master
- Sir Slicer
- Tiffany Oiler
- Cinnamon Bun
- Bun Bun
- Space Wedding People
- Grass Demon (Nightmare Juice cameo)
- Martin Mertens (Nightmare Juice cameo)
- Shoko (Nightmare Juice and Princess Bubblegum's flashback)
- Mr. Fox
- Lumpy Space King
- Lumpy Space Queen
- Engagement Ring Princess
- Hot Dog Princess
- Breakfast Princess
- Raggedy Princess
- Water Nymphs
- Tree Trunks
- Mr. Pig
- Cosmic Owl
- Root Beer Guy
- Banana Man
- Mr. Cupcake
- Ancient Psychic Tandem War Elephant
- Ricardio the Heart Guy
- Moe (hologram cameo)
- Margles (picture cameo)
- Hunson Abadeer (picture cameo)
- Cheryl (pictured)
- Billy (carcass, nightmare juice cameo)
- Fionna (mentioned)
- The structure of the episode involving the final continuity story being told long after it is over resembles two other notable works of fiction:
- The 1939 MGM cartoon Peace on Earth, which tells of humanity's extinction by way of a massive war, similar to the Gum War and the GOLB incident bringing an end to the final status quo of the series.
- The 1971 Dr. Seuss book The Lorax, about a massive ecological disaster associated with the destruction of trees and the hope of replanting them to restore balance; similar to Fern's final fate and the destruction of the Treehouse Fort and Fern "returning" as the massive willow tree in the final scene.
- The title sequence for this episode features three-dimensional elements, which was previously featured in the title sequence for Adventure Time Mini-Series: "Elements".
- Beth and Shermy live in the future of "Adventure Time", in a duller Land of Ooo, this future had been seen in "Lemonhope Part 2" and "Graybles 1000+."
- Beth and Shermy encounter a Prize Ball Guardian, which made their debut in "Graybles 1000+".
- When they arrive at their house after finding Finn's robotic arm, Shermy is heard singing a part of "On a Tropical Island" which is first sung by Jake from "Morituri Te Salutamus".
- Shermy and Beth living in Marceline's old house is similar to Finn and Jake living in a tree house also previously resided by the vampire queen as revealed in "Evicted!".
- The ice-sphere Patience St. Pim froze herself into can be seen in the Ice Thingdom at the very beginning of the intro.
- Slime Princess's helmet heavily resembles the one worn by Slimy D from "Evergreen."
- This is the first appearance of the Duke of Nuts since season 1. Despite Princess Bubblegum's hatred for him, he appears to be fighting on her side of the war.
- When Jake stretches in order to transport Finn to Bonnie's tent, Jake stretches in cursive, similar to the season two episode "The Eyes".
- Finn uses the concoction he received from Nightmare Princess in "Orb" to de-escalate the fight.
- The dream they were transported to is similar in nature to the dream Finn, Jake, and BMO had in the aforementioned episode.
- Shermy can be seen wearing Marceline's anti-smoking t-shirt from "I Remember You".
- BMO's cave is situated on top of Mount Cragdor, first introduced in "The Enchiridion!".
- Princess Bubblegum recognizes the robotic arm she made for Shoko in "The Vault" when Finn pleads to her to negotiate.
- Many items and artifacts from throughout the series make an appearance in BMO's cave, including:
- Finn and Jake's treehouse clock
- Jake's golden tooth from "Gut Grinder"
- Lumpy Space Princess's star "jewel"
- Birthday banners that say "Happy Birthday" and "Kim Kil Whan" from "Ocarina"
- Princess Bubblegum's war horn
- A box of Super Porp
- Princess Bubblegum's crown
- Engagement Ring Princess's ring
- A King of Ooo statue
- Marceline's t-shirt from "I Remember You"
- Lemongrab's sword
- One of the speech cards from "Shh!"
- Margaret's music box
- Grob Gob Glob Grod's helmet
- The Fionna and Cake Omnibus
- The Rainicornicopia
- One of the wood cutouts from "Orgalorg"
- A remnant of Finn's grass arm
- Princess Bubblegum's Bod-Rod from "Bonnie and Neddy"
- A Ble magazine
- Jake's vuvuzela
- The beach balls Jake and BMO collected from "Horse and Ball"
- The breadstick wand
- Ice King's drumset
- One of Ice King's tapes.
- A universal translator (presumably from "My Two Favorite People ")
- The licence plate from Melissa's car
- AMO's destroyed body
- A copy of Heat Signature
- Simon Petrikov's glasses
- A prism-gram
- Viola's necklace
- A copy of Jay T. Doggzone 's "Mind Games"
- King Man explains that Betty linked herself to Maja and was attempting to harness the power of GOLB. This was likely part of the plan to bring Simon and Margles back that she mentioned to him at the end of "Temple of Mars".
- Marceline utilizes the essence of the Vampire King she obtained in "The Dark Cloud" to save Princess Bubblegum.
- The windmill of the village seen in "The Monster" and "Marceline The Vampire Queen" was destroyed.
- When Jake tries to stop Gunther from stealing the Ice Crown, he reminds everyone that Gunther is the space deity Orgalorg, which was revealed in “Orgalorg”.
- It is revealed that the crack in Simon's glasses wasn't a consequence of the Mushroom War, but simply of Betty throwing a jar of cherries.
- In the final montage, T.V. has become a detective. He first displayed detective skills in "The Diary", where he was able to deduct the author of the titular book.
- The nightmare sequence explored the interpersonal conflicts between Finn and Fern and Princess Bubblegum and Uncle Gumbald.
- The sequence saw the roles of Princess Bubblegum and Uncle Gumbald switched. The first part was similar to the events of "Bonnibel Bubblegum" but it was Bubblegum threatening Gumbald with the dum dum juice as well as the one succumbing to its effects when Gumbald destroys its flask. It is also here where Gumbald assumes the role of princess and builds up the Candy Kingdom, eventually becoming paranoid of its destruction and failure.
- Fern continues to insist his tormented situation in contrast to Finn but is offset when Jake unleashes their repressed fears. Fern reaches an understanding with Finn as he realizes that they have the same fears. With their newly fixed bond, they set out to free Fern from the Grass Sword Demon which he kills. Fern (Finn Sword Finn) is freed from the curse but without the Grass Demon, he no longer has a body and starts to wilt in the real world.
- When Finn and Fern's vault opens, evil, grass demon versions of Princess Bubblegum, Martin Mertens, Susan Strong, and the Lich-possessed Billy emerge. It is not clear what they each represent.
- Martin likely represents Finn and Fern's fear of becoming him (via "Don't Look").
- Princess Bubblegum could represent their embarrassment of their former romantic feelings for her.
- The Lich-possessed Billy likely represents general fear of the Lich along with the sadness of losing Billy.
- Susan might represent the "soul-searchiness" and "weirdness" of supposedly being the only human in the world.
- Inside GOLB's stomach, due to digestion, Ice King was reverted back to Simon, Betty was reverted back to her sane self before the events of "You Forgot Your Floaties", and the Ice Crown was reverted back to its pre-wish state. Finn, however, did not grow back his arm.
- Since the Ice Crown was reverted to its pre-wish state, it was able to grant its first wearer its truest deepest, deepest wish. Betty wore, intending to wish GOLB away, but her truest wish was to protect Simon. GOLB and Betty were fused into one being and left Ooo through a portal.
- The Ice Crown was quickly reverted back to its pre-wish state again after the new Betty infused GOLB left. It landed near Gunter, who was able to grab and wear it despite Jake's alarm. Gunter was then transformed into the Ice Thing, as his truest wish was to be like Ice King, similar to Gunther the dinosaur who wished to be like Urgence Evergreen.
- The tall willow tree seen in "Graybles 1000+" and "Lemonhope Part 2" is actually from a seed from Fern's remains, grown in place of the destroyed Tree Fort.
- In the ending sequence, there are a montage of events that follow:
- The willow tree from Fern's remains grows from the remnants of the Tree Fort.
- Lady Rainicorn and Jake (in his shapeshifter form) explore a dimension.
- Lumpy Space Princess is crowned by Lumpy Space Dad and Lumpy Space Mom.
- Gunter (as the Ice Thing) and Turtle Princess get married.
- Some Banana Guards, Colonel Candy Corn, Rattleballs, and some Candy People from Gumbaldia watching in a living room.
- Some Water Nymphs along with Carroll, now in her water form, and a Sea Lard from the alternate dimension from "Beyond the Grotto".
- T.V. moving into Joshua and Margaret's old office where he opens his own investigation service, "Detective T.V. Investigations".
- Sweet P graduating.
- Princess Bubblegum and Aunt Lolly singing a lullaby to Neddy within the Candy Castle.
- Simon visiting Prismo in the Time Room where he presumably wishes to bring Betty back, only to have a similar result to when King Man wished to bring Margles back.
- Jermaine painting a new mural in Lemongrab's quarters.
- BMO launching Moe's memory drive from a rocket with help from Banana Man and ALLMO.
- Shermy and Beth arriving at the tall willow tree in the future.
- Neptr and Flame Princess rapping.
- A modded Tiffany in presumably the Badlands.
- The Jiggler from "The Jiggler" grown up with a suit and suitcase.
- Crabbit Familiar performing in a theater.
- Susan and Frieda exploring a town in a boat and encountering Lemonhope.
- Starchy, Crunchy, Punchy, and Dirt Beer Guy sharing toasts.
- Tree Trunks offering an apple pie in a crop circle, presumably to her alien family from "High Strangeness".
- King Man looking at a picture of him and Margles in his office at Mars.
- Princess Bubblegum, Lumpy Space Princess, Marceline, Slime Princess, and Flame Princess hanging out.
- Huntress Wizard meditating.
- Simon making popcorn with Turtle Princess while Gunter (as the Ice Thing) tells a story to Princess Bubblegum, Marceline, and Peppermint Butler who is reading about dark magic.
- Finn and Jake witnessing the arrival of humans (including Finn's Mother) in Ooo from a beach.
- The final scenes of the series parallel back to the original animated short.
- The original short begins with Jake teaching Finn a new song he learned from the internet; and the final scene with Finn is him learning the ending theme from the Music Hole.
- The final shot of the entire series is Shermy and Beth enacting the title card of the entire series, which was also the very first shot of the entire franchise with the title card of the short.
- This episode is the series finale of Adventure Time.
- The episode features an alternate version of the intro sequence sung by Willow Smith, who also plays Beth in the episode.
- This episode is the longest episode of the show with a length of 44 minutes.
- The episode was promoted with the title "The Ultimate Adventure", instead of using the episode's official name.
- According to Adam Muto, Princess Bubblegum and Marceline's kiss wasn't in the original story outline. The scene's direction was left to Hanna K. Nyström's discretion, as the outline simply said they "have a moment". When the scene was storyboarded, there was a note in the margin simply saying "Come on!" with a large exclamation point.
- The title of this episode is derived from the Adventure Time ending theme:
| Come along with me|
And the butterflies and bees
We can wander through the forest
And do so as we please.
Ashly Burch's name is misspelled as "Ashley Burch" in the opening credits.