Season synopsis
Contains spoilers for this season and earlier seasons.
Princess Bubblegum’s history catches up with her when Uncle Gumbald returns. She created him, Aunt Lolly and Cousin Chicle centuries earlier, hoping for a family, but Gumbald wanted an empire. His plan to reduce her to an obedient candy person instead transformed the relatives themselves. Lumpy Space Princess’s restoration of Ooo has now revived them. Gumbald builds a rival kingdom and brings back Fern as the Green Knight, who humiliates Finn at his seventeenth birthday. Attempts to prevent the resulting Gum War are undermined by Gumbald’s plans to betray Bubblegum.
Jake interrupts the confrontation with a potion that sends the leaders into a shared dream, where Finn reconciles with Fern. Afterward, Lolly stops Gumbald’s renewed betrayal, and the kingdoms agree to peace. But Betty and King Man summon GOLB, hoping to restore Simon and Margles. Inside GOLB, Ice King becomes Simon again and Betty loses her powers. Outside, Marceline saves Bubblegum and they kiss, while BMO and their friends discover that singing together weakens GOLB. Finn and Simon escape; Betty stays to use the restored crown, wishing to protect Simon at all costs. She merges with GOLB and departs through a portal. Fern dies after asking Finn to plant him, leaving a seed that grows into a new tree bearing the Finn Sword.