City of Thieves is the 13th episode of Adventure Time. It first aired on May 24th, 2010.
Finn and Jake see and head toward a strange looking turlte-shell called the City Of Thieves, even though they are warned not to go into it by a Hag. After meeting a little girl named Penny, they go into the city to find Penny's flower basket, but Penny's story about her stolen flower basket is fake.
Finn is trying to walk on four legs like Jake when they come across a city covered by a dome of cracked desert ground. A Hag appears and tells them that this is the City of Thieves, a place that turns anyone who goes there into a thief. Finn and Jake hear the cries of a young girl named Penny outside the city. She tells Finn that thieves have stolen her flower basket, but he is unable to get the basket back because the thieves in the city are endlessly stealing and re-stealing it from each other.
Penny then tells Finn and Jake about a king thief who steals from little girls and takes their belongings to his tower, which is protected by a magical barrier to keep other thieves out. Finn volunteers to enter the tower and get the king's treasure chest, which Penny says might contain her basket. Meanwhile, Jake can't cross the barrier because he couldn't resist stealing a pair of shiny red boots. Finn enters the tower and takes the chest, which he finds in the chest of the king thief's skeleton. When he emerges, Penny grabs the chest, revealing that she has tricked Finn into stealing the treasure for her. The Hag reappears to gloat about Finn now also becoming a thief. Traumatized over becoming impure, Finn decides to embrace his dark side and get revenge on Penny for stealing his purity. Finn and Jake become avengers and run around the city stealing things, including a crossbow, a pair of swords, and bar of soap from a bathing horse. They confront Penny in her tent and wash off the dirt that marks her as a thief, becoming clean in the process as well. Thinking they have become pure again, Finn and Jake celebrate with Penny, who claims to have reformed -- then immediately strips him of his clothes down to his briefs. Finn screams out "Penny!", concluding the episode.