Adventure Time follows two best friends: Finn (a 12-year old boy) and Jake (a wise 28-year-old dog with magical powers), and the surreal adventures undertaken by the duo as they traverse the mystical Land of Ooo. A world built for adventure, Ooo is filled to the brim with various landscapes for the two buddies to explore and bizarre characters to assist.
While returning home from their adventure in Islands, Finn, Jake and BMO take a nap after eating a lot of bananas. A mysterious black orb in the sky begins to subject the three to strange dreams that quickly become nightmarish. Their nightmares begin to intertwine as they realize that they are dreaming and come in contact with a being that Finn dubs Nightmare Princess. This entity simply wants their bananas, but BMO strikes a bargain so that they get something in return. The group wake up to the orb taking some of their bananas and leaves a pink vile of nightmare juice. BMO looks through the telescope to discover that Ooo has changed radically.
Finn, Jake and BMO arrive back in Ooo and discover that their home and the areas surrounding it have been entirely converted into candy. They also encounter Fern, N.E.P.T.R., Shelby, Lemongrab, Tree Trunks, Mr. Pig and Marceline who have been converted into happily brainwashed candy people. A frightened Sweet P is the only person who is still normal and directs the gang to a mysterious candy tower. The tower turns out to be Princess Bubblegum who has become a large candy being and attempts to convert the gang into candy people as well. While BMO is completely transformed, Finn and Jake are rescued by Ice King who takes them to the Sky Kingdom. As Finn and Jake look down, they discover that Ooo has been divided into four completely corrupted sections.
Ice King explains what happened to Ooo while Finn and Jake were away. After the group left on their adventure, Ice King was visited by Betty (voiced by Felicia Day) who, frustrated over Ice King not remembering her, asks him out on a date. Betty recreates a restaurant from Ice King's previous life as Simon. When Ice King still can't remember his old life Betty drops him off back home and leaves him. Tiny Manticore gives Betty some advice; rather than trying to adhere to Simon, she should just reach out to him as Ice King due to her having changed as well. Betty returns to Ice King and they start playing together. Suddenly, Patience St. Pim (voiced by Lauren Lapkus) kidnaps Betty and begins using her magic to empower mind controlled Princess Bubblegum, Flame Princess and Slime Princess while Ice King flees with Gunter.
Finn, Jake and Ice King travel to the Ice World and break through the large ice dome covering the center of the land. They encounter Carroll (voiced by Cameron Esposito), from "The Tower", who was converted to Ice and is genuinely pleased with her new appearance. The gang eventually encounter a powered up, and rather withheld, Patience who seems disappointed with the outcome of the new Ooo. While she succeeded in empowering the other elementals they have relegated themseleves to their sections of Ooo as Patience had forced them to power up rather than allow them to do so themseleves. The gang rescue Betty and leave while Patience bemoans the new world to an iced Choose Goose. Betty reveals that she can help them with the power of the Enchiridion which Finn has, causing Betty to ominously laugh.
In order to calm Finn down, Jake tucks him and himself into a cloud, but it floats away from Ice King and Betty. The two find themselves floating in the middle of endless sky while another cloud with a door flies past them. The two play pretend barber to pass the time and end up getting to the roots of their worries. The cloud flies past again and discover that it is actually a lard. They use the lard to fly back to Betty and Ice King where the former has discovered a way to return the land of Ooo to normal.
Finn and Jake arrive in the transformed Slime Kingdom to retrieve Slime Princess' crown for her jewel. They run into Lumpy Space Princess who is attending Slime Princess's roller skating party. The winners of the skating competition get absorbed into Slime Princess's whole being while the losers are caged up. Finn, Jake and Lumpy Space Princess enter, but lose, only to learn that by losing they still get absorbed. While Finn manages to grab the crown, Jake gets fully absorbed into Slime Princess forcing Lumpy Space Princess to save Finn, revealing in the process that 'her lumps' protect her from corruption. They are ejected from Slime Princess and forced to leave. Finn is saddened at the loss of Jake, but Lumpy Space Princess explains that Finn and Jake always win which inspires Finn to continue his mission.
Finn, Lumpy Space Princess, and Gunter arrive in a radically altered Fire Kingdom. The inhabitants, which include Wyatt (voiced by Andrew Daly) and Lady Rainicorn, attack the heroes. Finn and Gunter try desperately to restrain their violent tendencies as it would allow the kingdom to corrupt them. Eventually the group find Flame Princess who has transformed into a dragon. When she swallows the jewel necessary to save Ooo, Finn gives into his violence and begins attacking Flame Princess. Lumpy Space Princess angered by the violence yells at everyone to stop their fighting. The kingdom misinterprets her anger and decide to start a war with the Candy Kingdom. Cinnamon Bun, who was unaffected by the corruption, frets that this is the end of Ooo.
Lumpy Space Princess chases Finn and attempts to turn him back to normal, but to no avail. Ice King and Betty try to intervene, but are swatted away by Flame Princess. The Fire Kingdom inhabitants 'fight' the Candy Kingdom inhabitants making Lumpy Space Princess upset. After eating some of Marceline, who is made of marshmallows, Lumpy Space Princess reaches out to Finn's hero heart by placing some of Princess Bubblegum onto him. Recalling happier times with PB, Finn returns to normal. PB then fires her candy powers into the air and converts everyone into obedient candy people, except for Finn who was protected by Lumpy Space Princess. Finn nabs the three jewels and gives them to Betty, but she leaves him behind. Finn finds himself surrounded by the Candy Kingdom.
Betty knocks out Ice King and reveals that she wants to use the jewels to go back in time and stop Simon from transforming into Ice King. Ice King, still oblivious to her plans accidentally ruins her spell causing her to get transported to Mars where she meets Normal Man. Lumpy Space Princess rescues Finn from the candy inhabitants as Princess Bubblegum enacts her plan to convert the rest of Ooo into candy while Patience freezes herself again. Finn realizes that LSP's immunity means that she is the anti-elemental. Ice King arrives with the gems and Lumpy Space Princess is able to transform all of Ooo back to normal. However, Jake comes back resembling his shapeshifter parent. Despite this, Finn is glad that he is reunited with his brother.
Finn, BMO and Lady Rainicorn are slightly disturbed by Jake's new alien appearance while Jake himself seems oblivious, or rather in denial about it. After having a bizarre dream about Jermaine (voiced by Tom Scharpling) painting abstract art, which according to Jake he hates, he decides to go look for him. Jake traverses the wasteland until he comes across Jermaine's studio. He tries to "rescue" Jermaine from changing, but Jermaine convinces Jake that change is a good thing as long as you remain the same on the inside. After examining and taking home one of Jermaine's paintings, Jake discovers that he has returned to normal and is greeted by a very pleased Finn.
Marceline is unexpectedly visited by BMO causing the former to ask BMO to download a USB drive. While they wait, BMO tells the story of when it, Finn and Jake traveled the seas. However, the story is mostly inaccurate and involves a cat named Ted and a blue jay that says "OOOOOOHH!" (voiced by J. G. Quintel). Marcy then tells the story of "Lollipop Girl" and "Rock Star Girl" and their encounter with a "Blue Tranch" that resulted in "Weekend Island" getting taken over by potatoes. When the USB drive finally downloads, it reveals pictures of young Marcy with her mother. She feigns knowing them and asks BMO to tell their story resulting in a tale about "the Child and the Moon Girl" which Marcy thanks BMO for.
At a Fionna and Cake book reading by Ice King, an old woman claiming to be Fionna (voiced by Charlotte Newhouse) arrives with a tape of an early Fionna and Cake adventure. Ice King is enamored by "Fionna" and invites her to stay with him. At night Ice King watches the remainder of the tape where Fionna and Cake find a mummy (voiced by Chelsea Peretti) only to learn that the mummy is really the Queen of Ooo. Ice King misinterprets the video and thinks that "Fionna" is a mummy. He confronts her and discovers that she was looking for more Fionna and Cake tapes only to find that Ice King doesn't have any. "Fionna" is really a bunny who picked up signals that played Fionna and Cake and wanted more. When she realizes what she has done she flees. Later that night, Fionna and Cake adventures are beamed into Ice King's head.
While fishing, Finn and Fern encounter Sweet P who was running away from home after having a nightmare. Sweet P reveals that he has been having visions of a "whisper monster" trying to tell him he is evil. Finn and Fern decide to camp with Sweet P when they encounter the hand of the Lich that was cut off in "Crossover". Finn and Fern fight the Lich's hand, but it escapes. Finn gives chase and follows it into an underground tunnel where he ends up getting rescued by Sweet P (he had knocked out Fern so he could escape) who slays the hand. Fern feeling useless for not being able to help, plots to become the true Finn.
Fern reveals his ability to change his appearance to look just like Finn. Impressed, Finn joins Fern to a dungeon hidden away in a mountain. While there, Fern traps Finn in one of the rooms and reveals his plan to replace him. Finn finds out that his robotic arm can do a variety of things and drills his way out of the dungeon. Finn and Fern fight out in the field where Finn attempts to reason with Fern, however his robot arm misinterprets the word "finality" with "fatality" and he ends up destroying Fern. Finn returns home traumatized with Jake and BMO comforting him. Meanwhile, a mysterious man (voiced by Fred Melamed) scoops up Fern's still living remains in a bucket.