Adventure Time ran for 10 seasons and generated over 200 episodes of content for anyone to watch. The show, which is available on Hulu, follows Finn and Jake as they explore their magical world of Ooo, but often features episodes that focus on different characters in its universe and their stories as well.
All this being said, this show has tons of content, which can be intimidating for someone who wants to get into the show, now. So, in order to better understand the characters without watching every episode, it's important to watch the specific ones that focus most on the lore of the series.
"Enchiridion" (S1, E5)
This episode introduces the Enchiridion, a powerful book that Finn believes will make him a better hero. This book becomes extremely important later on as it powers a portal for the Lich to space where he can make a wish to destroy the world.
Even though this book is really relevant to the plot early on in Adventure Time, it becomes less so as the show moves on from the Lich. It is, however, one of the first episodes of the show that tries to create the multi-seasonal arcs of the series.
"I Remember You" (S4, E25)
This episode of Adventure Time follows Ice King, a perfect tragic villain, as he tries to get Marceline to help him write a song. When they interact, it's suddenly clear that they had a past together that only Marceline remembers.
This episode is extremely important for the main storyline of the show as it shows, for the first time, that Ice King wasn't always the clueless, yet powerful, wizard he currently is. Introducing audiences to this sad history makes for a deeply interesting relationship that opens up many more questions by the time it ends.
"Simon & Marcy" (S5, E14)
While the previous episode revealed their relationship, this episode about Ice King and Marceline further dives into their past together and shows the exact moment when Simon switched into his alternate persona.
This episode not only explains more of what the world was like when Marceline was a kid, but it also puts on display how much of a caring father figure Simon was to her as they found each other after the world seemingly ended.
"Be More" (S5, E28)
Beemo, one of the show's best characters, is Finn and Jake's friend who, in this episode, goes back to the factory where she was built in order to get a repair that is specific to her system and can't be fixed anywhere else.
The lore in this episode uncovers the history of Beemo's creator and shows just how special she really is. Overall, it creates an interesting mystery related to a character that audiences don't know much about and adds new layers to her importance in the main story.
"Blade Of Grass" (S5, E45)
"Blade Of Grass" follows best friends Finn and Jake as Finn finds a new grass sword that turns out to be cursed. When it originally aired, this episode wasn't integral to the plot, but because it later leads to Finn losing his arm and becomes Fern, it's an origin story that everyone that wants to watch Adventure Time should put on their list.
This episode sets up a lot of the later episodes of this show and also, in a way, tracks Finn's development and progression as he learns more about his past and faces the pain of the physical loss of his arm.
"Betty" (S5, E48)
Although it may be surprising to see so many Ice King plotlines on this list, his episodes tie into the past far more than any of the other characters because of his power. "Betty" is follows Simon who accidentally turns back into himself after a failed experiment.
Not only does this episode explain what happened to Betty, his lost love, but it also explores more of his capabilities as an academic with a vast knowledge of his crown and other magical objects he's used to studying.
"Evergreen" (S6, E24)
"Evergreen" is about the creator of Ice King's crown and is mostly told through the eyes of his side kick, Gunther. This episode is a necessary watch to fully understand why the crown exist and why it turns people into egotistical wizards with bright white hair, a long nose and beady eyes.
One of the coolest parts about this episode is that it doesn't follow any of the main character, yet it's one of the most important episodes to understand the show's lore. Not to mention, Gunther is adorable to watch as he follows Evergreen around filled with awe at his powers.
"Astral Plane" (S6, E25)
"Astral Plane" is a trippy time, as Finn goes out of his body and travels around space. What makes this fun episode important plot-wise is the fact that Finn visits Glob, a being that is almost god-like in his world, as the planet he is about to be destroyed by a comet just like Finn's planet was thousands of years ago.
In a way, the ending of this episode is world changing for Finn, as he realizes that he can't save everyone and that there are some matters that are out of his control even if he has all the powers in the universe. It's an essential watch to understand some of the more existential themes of the series.
"Islands, Part 6: Min & Marty" (S8, E25)
This episode is part of a multi-episode storyline when Finn goes with Susan to the human island to meet his mom. Finn's parents meet, bond, fall in love and have a baby in a montage that explains their past.
Finn is the main character of the show and his real parents are rarely ever mentioned, especially after his dad directly contributed to him losing his arm. It also shows a little of how the humans regrouped and protected themselves to survive after the apocalypse many years ago.
"Come Along With Me" (S10, E13)
This episode is one of the most important of the series, as it is the final one and wraps up the story in way that satisfies almost every major character story arc. It gives everyone the peaceful ending that they truly deserve after years of some pretty cool, yet intense experiences.
Without spoiling anything, this episode also dives into the importance of the treehouse, sets up the idea that Bubblegum and Marceline are perfect together, and resolves Betty and Ice King's relationship. All in all, this episode, in particular, shows that Finn and Jake leave a legacy behind that extends beyond their lives and their time defending Ooo from all the evil that tries to corrupt it.