The series revolves the adventures of protagonists Finn (voiced by Jeremy Shada), a 16-year-old human boy, and his best friend Jake (voiced by John Di Maggio), a dog with magical powers to change shape and grow and shrink at will.
Finn and Jake live in the post-apocalyptic Land of Ooo, and travel the land while they adventure.
In "Billy's Bucket List", it is revealed that Finn's birth father, Martin, is alive, and is trapped in a dimension known as the Citadel, which is a prison for the most dangerous criminals in the Multiverse.
The episode "Joshua & Margaret Investigations" reveals that when Joshua and Margaret—Finn and Jake's parents—were on an investigation, Joshua was bitten by some sort of shape-shifting, other-worldly entity.
BMO also has the features of other household objects, such as a portable electrical outlet, music player, camera, alarm clock, flashlight, strobe light, and video player.
While BMO is an object used for recreation by Finn and Jake, it is still considered a close friend and treated as an equal by the two.
In the episode "What was Missing," it is implied that she and Marceline may have had some sort of relationship in the past.