Finn and Jake come over to Marceline's cave for their usual musical jam session, only to find a vacant house with a threatening note warning them not to go inside without her under any circumstances. A game of hide and seek to pass the time gets Finn and Jake into the house and serious trouble when Marceline comes back. Stuck inside and desperate to escape, Finn and Jake secretly peer into Marceline's personal life at home.
Season three of Adventure Time is well-known by now for being particularly shocking. From creepy horror episodes to unexpected plot twists to last week's surprisingly dark character development, this season has been upping the ante when it comes to viewer expectations. That's why it's a bit ironic to see that an episode with the potential to reveal shocking things about a fan-favorite character is a pretty normal escapade, with the writers just focusing on fun and amusing gags rather than any attempt to surprise the viewer.
Most of the jokes in this episode work at a leisurely pace, like a paper airplane gag or a bit with a spider. It's light, shallow fun, but it doesn't really feel like it adds up to anything substantial in comparison to the season's other episodes. Instead, it just sets up an interesting idea - what does an outrageous and attention-grabbing character like Marceline do when she's alone? - and never really follows through with much past the staid answer of "not much, I guess." Marceline's musical number is the most egregious example of this: a meandering mess of a song that completely pales in comparison to the show's other Marceline songs in terms of emotional depth and catchiness. This scene is representative of "Marceline's Closet" as a whole: fun by first season standards, but flat and noticeably lacking by third season standards.
