City of Thieves aired on May 24, 2010.  It was boarded by Sean Jimenez and Bert Youn, who as a team had boarded Tree Trunks, Ricardio the Heart Guy, Memories of Boom Boom Mountain, and Evicted.  Bert Youn also co-boarded Wizard. What's interesting about City of Thieves is how similar it is in general outline to Memories of Boom Boom Mountain.  Both stories involve Finn being faced with an impossible problem that pushes him to the very edge of his heroism.  In Memories of Boom Boom Mountain, it's the irreconcilable desires of all sentient beings; in City of Thieves, it's the moral erosion caused by the toxic environment of the city itself, resulting in Finn losing his so-called "purity." That's not the only difference.  MBBM was, despite pushing Finn pretty far, a relatively upbeat and positive episode in which all the characters were decent people.  City of Thieves has a grimy, unpleasant demeanor which overwhelm's Finn's desire to do good.  This is evident from the color palette of the episode:
The episode is awash with earth tones and appears to have poor lighting.  The houses are jumbled up and chaotically placed.  This air of confusion and griminess becomes increasingly apparent as Finn and Jake also get dirtier and dirtier throughout the episode to go along with their apparent corruption. The nature of Finn and Jake's corruption is very interesting.  Jake is corrupted much more easily than Finn; ultimately, a pair of bright red boots is all that is needed to tempt Jake into a life of crime, although he appears to be unaware of stealing them. In this way, Jake is brought down by the city much more than Finn (who we'll discuss later) is.  Jake essentially becomes a typical denizen of the city, stealing whatever takes his fancy.  The primary visual focus in this episode is, like in MBMM, Rube Goldberg-like: the chains of theft, making it clear that nobody is innocent in this city.  It also provides a lot of excellent opportunities for more bizarre and grotesque character design than the typical, more positive Adventure Time setting. Finn's corruption is much more complex and nuanced.  It comes out of trusting Penny, a pitiable orphan girl who instructs Finn to take a treasure chest that she claims contains her lost flower basket.  However, it transpires that she tricked Finn into stealing the treasure chest so she could have the gold, thus turning Finn into a thief. This is an interesting ethical principle for Finn to live by.  He judges himself not by his intentions, which were undeniably good, but by the actions he takes.  This is emphasized by his inability to pass through a thief-proof force field, which causes Finn to rethink his own identity as a thief. Finally, Finn and Jake unite in their corruption.  Having been crushed by the nature of the city, they take a surprisingly nihilistic stance in their resolve to take vengeance on Penny, resulting in a spree of thefts and a late-night confrontation. Here is where things veer back into normal Adventure Time territory.  Being the type of show we've established that Adventure Time is in season 1, the show needs to reset the status quo, which it does by having Finn and Jake scrub down Penny with soap in an attempt to wash the city's grime and villainy off of her.  In so doing, they end up washing themselves and declare themselves newly pure. However, the episode has one last interesting twist in that Penny is not actually redeemed by the soap, as she steals Finn's clothes.  The redemption only exists in Finn and Jake's heads, just as their corruption was purely psychological as well. Next time:  the Witch's Garden.