Finn and Jake discover a train where every car is a dungeon filled with a new foe for Finn to fight. Jake quickly gets bored and wants to leave, but Finn is having the best time and doesn't want to leave... ever!
Tropes
- Affectionate Parody: The Dungeon Train is based on dungeon-crawling video games, complete with respawnable enemies who drop loot and equipment when the player wins.
- Analogy Backfire: While thinking on his break-up with Flame Princess, Finn remarks "Dating girls is like riding a bicycle. If you mess up, you could really get hurt forever, or hurt someone you care about."
- Bittersweet Ending: Even though he decides to stay in the titular train for at least two weeks, Finn lets Jake go, promising he’ll be back once he sorted his issues out.
- Blood Knight: Finn doesn't want to leave the Dungeon Train because he gets to fight an endless number of enemies. It's ultimately deconstructed; Finn states that "things make sense there", meaning that fighting forever is easier that dealing with his depression over his breakup with FP.
- Continuity Nod: Finn is still unaware of the meaning of "spooning" from the "Love Games" with Slime Princess.
- Cool Crown: Finn gets a flame-spewing crown after defeating the first "boss".
- Cross-Referenced Titles: The title is a combination of Mystery Train and Mystery Dungeon.
- Distinction Without a Difference: Upon finding another train car filled with monstrous crystal ants, Jake bemoans the fact that they had already encountered an ant car, and so this is a sign that they have seen all there is to see on the train. Finn rebuts that these are red ants, and the previous ones were blue.
- Eldritch Location: The Dungeon Train, scaly with infinity symbols, it contains an infinite amount of dungeon dimensions filled with opponents to fight. Some of the monsters even drop food and drink, and monsters apparently don't attack the tired or sleeping, to give one even fewer reasons for leaving.
- Fate Worse than Death: The train is this for the Greedy and the violent, but only from an outsider's point of view. Those trapped on the train are doomed to fight opponents forever. In a twist, it's not the Train that traps them, but their own inability to leave.
- Hurricane of Puns: Finn fires off a series of ant puns the second time he fights a set of crystal ants.
- Jerkass Realization: After taking Jake's "treasure", Finn sees a vision from the future crystal where Jake is still by his side, despite growing old and decrepit. This reminder of his friend's Undying Loyalty is what makes Finn realize he's been neglecting Jake, and convinces him to, whilst not depart from the train just yet, moderate his time and pledge to return to his usual life. The future-seeing orb confirms the lesson has stuck with him.
- Karmic Transformation: Those fighting in the Dungeon Train eventually become monstrous-looking; others will recognize them as boss monsters and slay them for their loot.
- Ouroboros: The train is a rather obvious representation of it, in one of its less explored meanings: a symbol of endless/futile effort.
- Playing with Fire: The first "boss monster" can use fires from his crown to attack Finn and Jake.
- Pleasure Island: The Dungeon Train lures warriors into wasting their lives in eternal meaningless combats.
- Power-Upgrading Deformation: The gauntlet that Jake finds is what made the first "boss" so ugly in the first place.
- Shock and Awe: The bolt shaped sword that Finn finds after defeating his opponent has the power to shoot electricity.
- Shout-Out: The train is reminiscent of the Battle Subway from Pokémon Black and White.
