"Adventure Time's" overarching narrative is one of hidden depths and slow-burn revelations. The whimsical Land of Ooo is, in fact, post-apocalyptic Earth, ravaged a thousand years prior by a nuclear event known as the Great Mushroom War. This context reframes the entire series from pure fantasy to a poignant story of a world recovering from trauma. The magic in Ooo is a direct result of this cataclysm.
The series' main antagonist, The Lich, is a primordial being of death and the embodiment of nuclear annihilation, having been born from the bomb that ended the old world. His goal is the extinction of all life. The bumbling Ice King is the tragic alter ego of Simon Petrikov, a kind-hearted human scientist who survived the war thanks to a cursed crown that granted him ice powers but erased his memory and sanity. He was a loving father figure to a young Marceline, the half-demon Vampire Queen, immediately after the war, a fact neither of them can fully remember until late in the series.
Finn is not the last human. He eventually finds his parents: his father, Martin Mertens, a selfish space criminal who abandons him, and his mother, Minerva Campbell, a doctor who uploaded her consciousness to a network of helper robots to protect the last bastion of humanity on a chain of islands. Finn's destiny to lose his right arm is a recurring motif across his past lives and alternate realities, a loss that finally occurs when he tries to stop Martin from fleeing. The series finale sees the ultimate confrontation not against a single villain, but against the abstract chaos entity GOLB. In the climax, Betty Grof, Simon's long-lost fiancée who traveled through time to save him, sacrifices herself by merging with GOLB to protect Simon and all of Ooo. This act of cosmic sacrifice finally breaks the crown's curse, restoring the Ice King to Simon Petrikov permanently. The finale montage confirms that Princess Bubblegum and Marceline enter a loving relationship, and life in Ooo continues its cycles of change, adventure, and song long after Finn and Jake are gone.