What Was Missing

What Was Missing is one of the LGBT-themed episodes of Adventure Time. In this episode, Finn, Jake, Princess Bubblegum, and Marceline form a band in order to defeat the Door Lord and recover what the creature had stolen from them.

Sugar noted that while the episode may appear to be about friendship, it is really about the power of telling the truth, as exemplified by the episode's songs. Episode composer Tim Kiefer played all of the conventional instruments himself, and used unique instruments, like a Game Boy, to make some of the loops heard in the episode. The episode was watched by 2.185 million people and caused a major controversy due to the suggestion of an intimate (and possibly romantic) history between Marceline and Bubblegum, a fact that also garnered praise for the episode from several LGBTQ-oriented review sites.

Plot
Jake and BMO decide to give Finn some "alone time" with him and his piece of Bubblegum's hair, which he acquired in the second season episode "To Cut a Woman's Hair". However, a Door Lord suddenly materializes and snatches the lock, along with Jake's blanket, and BMO's controller. Finn and Jake chase through the creature's various doors, stumbling upon Princess Bubblegum and Marceline, who both appear to be victims of the Door Lord's theft. Finally, the Door Lord jumps through a door that the group is unable to get past. They realize that they must sing a song to pass through the door; the group decides to create a band. Jake gets his viola, and Marceline retrieves her bass. Princess Bubblegum decides to play BMO as an 8-bit instrument, and Finn beat-boxes.

Tensions amongst the band members, however, boil over. Marceline sings a song ("I'm Just Your Problem") lamenting her and Bubblegum's estranged relationship, which almost opens the door, due to it being a song based on truth. Marceline and Bubblegum begin arguing, and Jake joins in; the band disbands. Finn, dismayed at his friends' behavior, sings "My Best Friends in the World", bemoaning the fact that his friends are fighting. Bubblegum, Marceline, and Jake join in, and the four are able to open the door. Inside, Finn, Jake, and BMO reacquire their belongings. The group also finds a black rock T-shirt which they mistake to be Marceline's. However, it is revealed that it is Bubblegum's; it was a gift given to her by Marceline in the past. Finally the group turn to Marceline, asking what she had lost. When they realize she only wanted to spend time with the friends, Marceline gets embarrassed and chases Finn, Jake, BMO and Bubblegum from the Door Lord's abode.

Reception
"What Was Missing" first aired on Cartoon Network on September 26, 2011. The episode was viewed by 2.185 million viewers and scored a 0.4 Nielsen rating in the 18–49-year-old demographic. This means it was seen by 0.4 percent of all households aged 18 to 49 years old were watching television at the time of the episodes' airing.

Tyler Foster of DVD Talk called the episode a "highlight" of the Fionna & Cake DVD. Specifically, he praised the song "I'm Just Your Problem", calling it both a "fan favorite" and "significant as the moment I decided I was a fan". Dana Piccoli of AfterEllen.com enjoyed the way the episode "alluded to perhaps, more than platonic feelings between" Princess Bubblegum and Marceline, as well as the fact that "What Was Missing" was underlined by potential lesbian "subtext". The A.V. Club named the episode one of the ten additional installments of the series that illustrates that "emotional complexity" lies "beneath Adventure Time's weirdness".

Controversy
"What Was Missing" became controversial because of an allegedly implied past relationship between Marceline and Princess Bubblegum. The controversy largely began after an accompanying "Mathematical" recap—a behind the scenes video series produced by Frederator Studios that implied that there were lesbian relations between Princess Bubblegum and Marceline and that the writing staff actively seeks input from fans. This incident was addressed by Fred Seibert, the show's executive producer, who said that "in trying to get the show’s audience involved we got wrapped up by both fan conjecture and spicy fanart and went a little too far." Soon after, the video recap and the entire channel was pulled off of YouTube, although "What Was Missing" still airs during reruns. Seibert's decision to remove the video also proved controversial; Bitch magazine later wrote an article about how the episode "handled female desire—female queer desire at that—in a subtle but complex way", but that the removal of the recap and the studio's perceived treatment of the controversy was detrimental towards the acceptance of queer romance in children's television. Ward later addressed the issue and gave a more neutral view, saying that, because there were "so many extreme positions taken on it all over the Internet", he did not "really want to comment on it [because] it was a big hullaballoo."

In August 2014, Marceline's voice actress Olivia Olson told a crowd of fans gathered at a Barnes & Noble book signing from The Adventure Time Encyclopedia, that, according to Ward, Marceline and Princess Bubblegum had dated in the past, but that because the series airs in some areas where homosexual relationships are illegal, the show has not been able to officially make clear the relationship in the series itself.