After finishing their adventures at the sea, Jake, an old wise dog with magical power, who lives with a young teenager boy named Finn, who together do their best, in order to save the world from evil, through their adventures, face a horrible attack that enters their dreams and tries to destroy them, but they manage to overcome it.
Adventure Time is in full-on mythology mode, taking advantage of this world and its characters' centuries of history to explore loneliness, the need for companionship, and asserting authority when it's challenged.