The episode begins with Finn and Jake playing Sumo Scootz on BMO. Jake wins for the 79th time in a row. Finn is too bummed out about Flame Princess to care, so Jake says he needs a "girl's night out, boy style!" As Jake accidentally begins to tell Finn about going shopping and getting their hair and nails done, they hear the toilet flush. They find Ice King in their bathroom, who says he knew they would not mind him staying there since it is Finn's fault his kingdom is ruined. Ice King informs them that he has given Gunter temporary reign while he is away (a "meanwhile" shot shows Gunter messily making ice towers). Finn leaves the room, and Jake says "breakups are tough!" to which Ice King replies "yeah, like, remember how you and I were married?"
The scene switches over to a forest, where smoke is rising from Flame Princess. She pulls up blades of grass and burns them, clearly frustrated over her breakup with Finn and comments about how people in the Fire Kingdom are "all deceitful and Shakespeare." A spider crawls on Princess Bubblegum who is invisible with the aid of her gadgets, and she knocks it away. After Princess Bubblegum turns off her invisibility, Flame Princess accuses Princess Bubblegum of spying on her, which Bubblegum denies, claiming she was "observing" her. At that point, her invisible cameras suddenly become visible. Princess Bubblegum explains to Flame Princess that she is a threat, and that if she (Princess Bubblegum) could create a scientific model that described her complete elemental matrix, she could isolate and suppress Flame Princess's violent feelings. Flame Princess agrees.
In the next scene, Flame Princess is strapped down to a machine that Princess Bubblegum says will help "understand [her] in a very scientific way." Princess Bubblegum is about to initiate the test when Jake calls her, telling her that Gunter has created a "race of strangling snow snakes." The phone then disconnects and Princess Bubblegum lets it hang before pressing a button on her grid to initiate the test. She then instructs Cinnamon Bun to write the numbers that he sees on the monitor. She then looks at the door to the testing chamber, and tells him not to open the door, which he promptly opens the minute she is gone. He then proceeds to make conversation with Flame Princess, who tells him that she is sad but mostly confused. Cinnamon Bun invites Flame Princess to his house to pet his "dog" (a lump with a mop on its head), and they go to his house.
At Cinnamon Bun's house, Cinnamon Bun dances and sings a song but then walks into a table. Flame Princess heals his injury by pressing her hand onto his leg, and Cinnamon Bun calls her one of his friends. Flame Princess agrees, saying that she "really gets" him, but then goes into a speech about how friends are "hard to understand," and that they "play mind games." Cinnamon Bun asks if Princess Bubblegum is her friend, and she responds that she is not sure. Cinnamon Bun then goes on to ask about the princess's intentions when she put Flame Princess into "baby jail," noting that it was a "weird friend thing."
Flame Princess gasps, and the scene cuts to a flashback fifteen years in the past, when Flame Princess was a baby. A messenger of the Fire Kingdom gives Flame King word that his offspring will be more powerful than he could ever dream to be. The Flame King tells the messenger to leave the baby in the woods to perish. The messenger could not bear to let the baby perish, so he left her with a Rock Woodsman, whose house Flame Princess promptly burns. The lumberjack screams in shock, and Flame Princess runs into the forest, burning trees. Princess Bubblegum sees this, and, wearing a protective suit, returns Flame Princess to the Fire Kingdom. She tells Flame King that he should not let her wander in the woods because she is too dangerous. Princess Bubblegum tells Flame King that if he does not do something about Flame Princess, she will. The scene changes to Flame Princess in her lantern, and the flashback ends.
Flame Princess gasps, stands up, and angrily wonders if everyone is "hiding something." Cinnamon Bun then tells her that it is fun to hide. Flame Princess wonders why everyone cannot be honest like Cinnamon Bun. She then says that she should go back to where she belongs.
Back at the lab, Finn and Princess Bubblegum enter the testing chamber, so that Princess Bubblegum can observe Flame Princess's reaction "when confronted by the guy who broke her heart." Finn looks down, ashamed. Princess Bubblegum then opens the door, and, seeing that the chamber is empty, screams "CINNAMON BUN!" They go to Cinnamon Bun's house and read a note saying that he went to the Fire Kingdom.
At the Fire Kingdom, Finn and Princess Bubblegum try to enter, but the guards stop them. Another Fire Kingdom citizen successfully enters the Fire Kingdom, saying that he "needs to file [his] taxes," also saying he likes to use the bathroom there. Princess Bubblegum and Finn then honestly tell their purpose, and enter the kingdom, seeing Flame Princess is the new ruler.
Flame Princess tells the two that she overthrew her father, and that honesty is the new law in the Fire Kingdom. Flame King then threatens Flame Princess with grounding with no snacks, friends or pop music, and in reply, Flame Princess says that he never gave her any of those things and that he was a selfish king and a bad parent. He then tells her that he did it so that she would not overthrow him. She tells him that he belongs in the lamp, and that, if he is not quiet, he will have to answer to Cinnamon Bun. Princess Bubblegum tells him to go home, but Cinnamon Bun refuses, telling her that he finally has a real dog.
Finn apologizes for his actions before, and Flame Princess forgives him. He asks whether this means their relationship is still intact. Much to his dismay, she says it is hereby dissolved but that they can still be friends as long as he is honest with her, to which Finn merely says "Hmm," looking discouraged.
- Ice King
- Flame People
- Peppermint Butler
- Jake 2 (name not revealed until "The Red Throne")
- Rock Woodsman
- This is the first episode in which Flame Princess and Princess Bubblegum are seen directly interacting with each other. Flame Princess (before she has her flashback) states that she does not know whether Princess Bubblegum is her "friend."
- It is revealed that Flame Princess is 15 years old rather than 14.
- Flame Princess becomes the new ruler of the Fire Kingdom.
- Princess Bubblegum remains the same, if not a similar age in the flashback 15 years ago, which suggests she has a different aging pattern to Finn and Flame Princess. This is demonstrated in "The Vault" as she is shown to be hundreds of years old.
- Cinnamon Bun states that he thinks Princess Bubblegum is "bad" but he is not sure.
- Cinnamon Bun leaves the Candy Kingdom and now lives in the Fire Kingdom as Flame Princess' right hand man.
- The actions of Flame King, and the resulting early life of Flame Princess, mirror the beginning of the Ancient Greek play Oedipus Rex. In this play, a prophecy is delivered to the rulers of Thebes, telling that their son Oedipus will act against them. They send the child out into the wilderness with a servant, instructed to restrain the child and leave him there to die. The effort would have succeeded, had not a wandering shepherd found the young child. Oedipus, unknowingly and reluctantly led by destiny (perhaps not too different from 'purpose'), fulfills the prophecy.
- While talking about the Fire Kingdom's citizens' deceitful ways, Flame Princess makes a reference to the famous English poet and playwright William Shakespeare. This matches up with the last time Finn and Jake entered the kingdom in "Ignition Point."
- When Princess Bubblegum is reading Cinnamon Bun's note, there is a magnet that bears a resemblance to Totoro from My Neighbor Totoro.
- Flame Princess's flashback of a servant taking Flame Princess into the woods to make sure she dies but lets her go instead is similar to the fairytale Snow White.
- Flame Princess's ruler outfit bears some resemblance to the one worn by Princess Azula of the Fire Nation in Avatar: The Last Airbender.
- This episode is a direct continuation of "Frost & Fire" and "Too Old." Finn's break-up with Flame Princess is discussed, and Ice King reminds Finn about his deceit.
- Ice King has temporarily moved in with Finn and Jake after the events of "Frost & Fire," in which his house was melted by Flame Princess.
- The pond that Flame Princess is sitting next to is the same pond that was in "Hot to the Touch."
- Ice King's Banana Phone from "Loyalty to the King" returns. However, it seems to be a lighter shade of yellow, and now has a penguin keychain on it.
- Ice King reminisces with Jake about the time they were married, which was in the episode "When Wedding Bells Thaw."
- Jake mentions Princess Bubblegum's Ball Blam Burglerber, which she used in "Lady & Peebles."
- The title card for "Earth & Water" is similar to "Frost & Fire's. They both have a major character in the center and is split into two parts: a warm themed side and a cool themed side.
- Cinnamon Bun's minor hatred of Princess Bubblegum is shown again, from "Another Five More Short Graybles."
- Flame Princess once stated that friends are "hard to understand" and that they are just "playing mind games", may be due to Finn keeping all the ladies in confusion so that he has options in case Flame Princess doesn't work out in "Reign of Gunters."
- During Flame Princess' flashback when she was a baby, Princess Bubblegum was shown to have the haircut in "Burning Low."
- The burned area near the pond where Flame Princess is sitting, is gone when she was talking to Princess Bubblegum.
- When Princess Bubblegum and Finn arrive at Cinnamon Bun's house, there is clearly nothing at the door. However, when they walk up to the door, Cinnamon Bun's notes and 'dog' with dog food are there.
- Cinnamon Bun's house looks different than it did in "Another Five More Short Graybles."
- When Finn says "anything," the pattern on Flame Princess's ruler outfit disappears.
- When it first shows, the Flame King in the lantern his crown is on his head when it should be Flame Princess.
- the Ice Kingdom appears in the background when the episode starts desperate it melted, however it could be that Gunther created some of the ice towers already.
- "Earth & Water" was the first episode to air in the new Adventure Time timeslot at 7:00 PM EST.