So, Finn heads off to explore the cave and find something red to satiate her until they can be rescued. Finn finds an old truck, and Jake calls Banana Man to help with the repairs.But when BMO stays up all night working on the truck, he hears on a spooky radio talk show that Princess Bubblegum is a reptilian replicate.Due to the Old Tart Toter having gone mad and old, Princess Bubblegum asked Finn and Jake to volunteer to tote the Royal tarts through the Royal Tart Path for Ooo's Annual Back-rubbing Ceremony but instead, Finn tried to create his own brilliant plan that would take them to a very wrong turn.Finn and Jake meet a dying frog-turned gnome knight who is bestowed upon three seeds; two of them are good, one is bad, and he vowed that they will never be planted.If Finn wants to rescue his friend, he's going to need Lady Rainicorn's help...and a videotape that will reveal how to open the portal to Kee-Oth's messed-up dimension.
But when their vessel is attacked by nasty goo monsters, F&J start to think the mission has been sabotaged by Princess Bubblegum's robot helper James.
Prior to the season's airing, Adam Muto said that Marceline's backstory would be explored further, and Jeremy Shada also mentioned that Marceline and Ice King would sing together.
Both took place in the episode "I Remember You." The season featured guest voices by Donald Faison, George Takei, Erik Estrada, Andy Milonakis, Emo Phillips, Bobcat Goldthwait and Susie Essman.
It was also revealed that this season was to contain a one-hour special (with the production codes 1014-125, 1014-126, 1014-127, and 1014-128), but it was pushed off for later, and eventually cancelled.
As of June 30, 2013, storyboarder Jesse Moynihan revealed that the one-hour "movie" was intended to divide season 5 in half, which has now been extended by another 26 episodes.