Finn, Jake, BMO, Princess Bubblegum, and Marceline must combine their musical talents to open the Door of the Door Lord in order to take back their most treasured belongings, which he has stolen from them.
"What Was Missing" contains examples of:
- Beyond the Impossible: The Door of the Door Lord will specifically only open for a song played by a band, yet Marceline seems to very nearly open the door all on her own when belting out "I'm just your problem"
, and likely WOULD have opened the door had she been honest about how much she misses PB. - Broken Bird: Marceline's solitary past as a child was hinted at in the episode "Memory of a Memory" and is further expanded here with two facts in particular — her and Princess Bubblegum's conflicting past (brought out by their snark and Marceline's famous song "I'm Just Your Problem") and the fact Marceline hasn't had friends in several years (showed when it's revealed the Door Lord didn't steal anything from her, she just wanted to hang out with them).
- Call-Back:
- The thing that was stolen from Finn is the lump of Princess Bubblegum's hair that he got from her in "To Cut A Woman's Hair".
- It's established in Go With Me that Marcy and Bubblegum don't really get along. This is the first episode they finally interact with one another and indeed both sides, while not overly hostile, throw some verbal words at each other.
- Captain Obvious: PB explains the nature of Door Lords, and how they received their name.
Princess Bubblegum: It's the door of the Door Lord, Finn. We used to lock them up, but they kept breaking out... 'cause they're door lords.
- Chased Off into the Sunset: Finn realises that Marceline didn't lose anything, but just wanted to hang out with the others. Marceline gets embarrassed and angry at having her soft center revealed, transforms into a large monster, and starts chasing Finn and the others.
- Continuity Nod: The Door Lord looks very similar to the gatekeeper in "The Enchiridion", except with a door on his hat instead of a key.
- Creative Differences: Invoked and parodied with Jake acting as "the band jerk".
- Didn't Think This Through: They used to lock up Door Lords, but they broke out, because they're Door Lords.
- Golden Moment: Hilariously subverted, when the Door Lord points out that his actions made the gang realise how much they mean to each other, we cut to a badly beaten and trussed up Door Lord with the others reclaiming their stolen items.
- Five-Man Band Concert: invoked Justified since the characters have to play in a band together to try to open the Door Lord's lair and retrieve their most precious belongings. The Hero and Heart Finn beatboxes, The Lancer Jake (who parodies Creative Differences as the band jerk) plays the viola, The Smart Girl Princess Bubblegum uses BMO as a makeshift synthesizer, and The Big Girl Marceline is the bassist and vocalist. They start off with "I'm just your problem", a song with strong Homoerotic Subtext. After several failed attempts, they disband. They are brought back together by Finn, who invokes True Companions with the song "My Best Friends in the World". It's first a solo but the others gradually join him — there's no beatboxing this time, though.
- Foreshadowing: The Door Lord is never actually seen taking anything from Marceline, unlike everyone else.
- Homoerotic Subtext: It was finally admitted by Olivia Olson years later that Marceline's song for PB, and the fact that PB wears a T-shirt Marceline gave her as nightwear, were indeed intended to imply a past romance between them.
- I'll Kill You!: The episode ends with Marceline, embarrassed and angry about being forced to admit she just wanted to hang out with Finn and his friends, Hulking Out and chasing the others while yelling "I'll kill you!"
- Memento MacGuffin: Finn's lock of PB's hair, Jake's puppy blanket, BMO's controller, and the T-shirt Marceline gave to PB.
- Musical Episode: This episode mainly consists of the gang playing music together to try to open the door to the Door Lord's realm.
- Portable Hole: The Door Lord can create doors to other dimensions in this way.
- Rewatch Bonus: PB's and Marceline's interactions in this episode become a lot more significant after later episodes explored their prior relationship. Particularly, in hindsight this episode represents a major step in PB and Marceline starting to rekindle their romance, as both realize how much the other misses her (PB hearing Marceline sing about her, and Marceline discovering that PB kept the shirt she gave her.)
- Rockers Smash Guitars: PB's song breaks up when BMO catches fire, and Jake smashes his viola. Fortunately he manages to repair it again.
- Songs in the Key of Lock: Played with, the door doesn't respond to a specific tune, but to music expressing people's true feelings.
- Stylish Sunhats: Marceline dons a wide sunhat as part of her Unlimited Wardrobe, it doubles in functionality to keep the sun off her vampire body.
- Thinking Up Portals: The Door Lords can throw down keys to magically summon doors that let them travel great distances across the land of Ooo.
- Title Drop: Finn says it shortly after "I'm Just Your Problem" fails to open the Door Lord's door.
Finn: I wonder what it liked... or what was missing?
- Technician Versus Performer: Bubblegum is a Technician among a bunch of Performers, and her use of complex musical terms confuses the others and results in her song failing to open the Door Lord's door.
- True Companions: The sentiment of Finn's song which finally opens the door.
- Tsundere: Marceline's song fails to open the door because she gets embarrassed and tries to cover up her true feelings for PB with aggression.
- The Unintelligible: The Door Lord just mumbles unintelligibly.
- Well, Excuse Me, Princess!: The general sentiment of Marceline's song.
- Whole-Plot Reference: Some distinct similarities to the Buffy the Vampire Slayer episode "Once More With Feeling":
- The concept has the characters forced to express their true emotions in song, although in a different way.
- Marceline's song is emotionally reminiscent of Spike's "Let Me Rest In Peace", and comes at a similar point in the episode.
- Finn's final song is emotionally similar to the penultimate one in the Buffy episode, that defeats Sweet.
