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 realise she only wanted to spend time with the friends, Marceline feels embarrassed and chases Finn, Jake, BMO and Bubblegum from the Door Lord's abode. Production "What Was Missing" was written and storyboarded by Rebecca Sugar and Adam Muto from a story developed by Mark Banker, Patrick McHale, Osborne, and series creator Pendleton Ward. The episode was directed by Larry Leichliter, with Cole Sanchez serving as creative director and Nick Jennings serving as art director. The original title for the episode was "Door Jam", but this was later changed because, according to Muto, the show tries to avoid pun-based titles. "What Was Missing" was Sugar and Muto's second episode dealing with Marceline (the first being "It Came from the Nightosphere" from season two), and because of this, Sugar "wanted to get it right". Sugar later explained on her Tumblr that, "It might seem like this episode is about friendship, but I wanted it to be about honesty! Marceline almost gets the door open because she drops her guard and tells the truth for a second while she sings this song." She later likened the theme of the episode to a quote by Bob Fosse—“The time to sing is when your emotional level is too high to just speak anymore"—noting that she "really wanted to try for that feeling." When looking over the episode’s plot prior to it being storyboarding, Sugar noticed that the interactions between Marceline and Princess Bubblegum recalled the animosity shared between two ex. She consequently pitched to Adam Muto and Pendleton Ward the idea that the two had been in a relationship, but had since broken up. The production crew of the series approved of this story approach, but due to restrictions at Cartoon Network, the writers were unable to explicitly confirm this information in the episode. All of