|The following information is from the Adventure Time comic book series. It may not be part of the same canon or continuity as the animated series.|
Adventure Time #38 is the thirty-eighth Adventure Time comic book. This issue features a back-up story titled "Worst Day," which was written by Rachel Edidin and illustrated by Kel McDonald.
Magic Man’s hold on Finn is proving to be stronger than anyone had thought and it’s taking the whole crew to band together to help their friend, only this time, there really might be nothing they can do. It’s up to Finn to stand up to Magic Man, if only he could find the bully.
Finn nearly falls into Jake's stomach acid, but Jake saves him and tells him to chill, as he would have told his body to not digest him. Jake, under a light in Princess Bubblegum's lab, tells the princess where Finn is. She apologizes to Finn, saying that had she known the location of Jake's sandwich magic, she would have shrunk him and had Jake eat him instead of doing a miniaturization teleport. Jake relays the message as PB just not thinking things through. PB tells Jake that they found the spark of food creation ability that is still inside him. Finn needs to retrieve it so she can make a cure from it and give it to the armies outside, who state that they're hangry.
Jake tells her not to worry and goes back inside himself. He and Finn enter a part of the body he isn't familiar with and encounter his immune system. Jake tells his cells not to destroy Finn, but they try to anyway. Finn zaps them with sword lasers, but this also hurts Jake. Not wanting to lose health, Finn runs away and falls off a ledge, landing in a river below. Jake says that Finn found one of his arteries. He pulls Finn out and easily removes the blood. Finn asks Jake if he could make a clear path to the magical sandwich source. Jake is able to do so and they find a Jake-and-sandwich-themed castle.
The two enter the castle, meeting a sandwich person. Jake tells him who he is and proves it by making a mustache appear on the sandwich person's face. More sandwich people apppear, including the sandwich princess, calling Jake "the eater." The sandwich princess explains that they were created by the Crystal of Perfect Sandwich, which Finn thinks is what they're looking for. Jake looks at the crystal, which contains a piece of the best sandwich he ever had, and apparently gave him powers. When Finn muses that the crystal also created the sandwich princess, Jake states that she and Finn would be a perfect couple, getting a little too excited. Finn says he's not interested in princesses now, as he has his laser-shooting sword and doesn't want a broken heart.
Something starts happening outside, shaking the area. Jake goes to check it out while Finn handles the crystal. When Finn tells the sandwich princess that he has to take the crystal, she and the sandwich people turn monstrous and chase him. Meanwhile, Marauders break into PB's lab, threatening the procedure. Finn jumps off a ledge again while shooting the sandwich people, hurting Jake's guts. The Marauders decide to smash the lab equipment to get food out of it, despite PB saying there isn't any food. Finn and the crystal are carried and burped out on some gas bubbles, and he calls for PB to unshrink him. She does so and sees the crystal, remembering how to make food. As she goes to show everyone, Finn and Jake start arguing. Jake argues that Finn's sword is changing him, and Finn argues that Jake just wanted to keep his sandwich powers. PB shows a projection of the crystal to the armies, who remember how to make food again. One person suggests a feast to celebrate, but PB tells everyone to get lost.
PB gives Finn and Jake sandwiches, saying that it's okay to be cranky from not eating. The two calm down and reconcile. In a preview for the next issue, Janice Bizzlethatcher tells the candy people (who are eating) that Arklothac is coming. Also, the sandwich people are crying over the crystal being gone, but cheer up when Jake returns it.
At Marceline's house, the vampire queen puts a sign on her door (that reads "Leave me alone with my sadness") closes the curtains, and starts making a sad song. Finn and Jake see the sign and tell Princess Bubblegum, who calls someone to help. Finn and Jake make a basket of red treats to cheer up Marceline and present it to her. Marceline wants to be left alone and slams the door. She goes back to her collection of sad stuff, saying she has to start over. She looks in a book of orphans, but is interrupted by a scratching sound at the door. Two demons from the Nightosphere bring her fries from her father, Hunson Abadeer, who heard she was sad. Marceline guesses that he didn't come himself because he doesn't really care. The demons also give her a note from him, in which he says he respects her space and still loves her. Marceline tries the fries and says they're perfect, calling today the worst day ever. There is another knock at the door and Marceline angrily answers it. Finn, Jake, and PB say they were worried and wanted to help her. Marceline tells them that she was trying to write a sad song for a contest, but everyone kept ruining her inspiration by being nice, and now her work is ruined. Finn and Jake think that itself is sad, and PB tells Marceline that she can use that. Later, the four friends are eating the red treats in Marceline's living room, now holding the trophy from the contest. Finn asks if they can hear the song, but Marceline said that she only wanted the trophy and just intimidated the other competitors. PB commends her on this.