Season synopsis
Contains spoilers for this season and earlier seasons.
Season four opens with Finn pursuing Flame Princess after Jake’s visit to the Fire Kingdom. She mistakes Finn’s attention for taunting and sets out to turn the Goblin Kingdom into her own domain. Finn, Jake and NEPTR build flame-retardant battle robots and try to put out the fires, which hurts Flame Princess physically and emotionally. Finn steps out of his robot and cries; she interprets his tears as proof that he is a water elemental, so their natural elements will hurt one another. The season later follows Finn and Jake on a dangerous rescue in the Nightosphere, where they try to save Marceline from her father’s realm.
Near the end, Marceline helps Ice King write music using the notes he left as Simon Petrikov, who cared for her after the Mushroom War. She remembers their bond, but he cannot recognize that history and treats his apology as lyrics. In the finale, Finn and Jake seek Billy’s help after a nightmare about the Lich. Following Billy’s instructions, they collect magical gems to activate the Enchiridion. Bubblegum warns that Billy is the Lich in disguise; when Finn breaks the book, he accidentally opens a portal. The boys try to stop the Lich and are pulled through with him. The final scene shows an alternate Finn living on a farm with his family and an ordinary dog named Jake.