Marceline approaches Princess Bubblegum and asks her to cure her of her vampirism, as she no longer wants to be immortal. Bubblegum agrees to perform an experimental procedure which extracts the vampiric essence from Marceline's body, but is unsure of whether it has worked or not. As Marceline recovers, the extracted essence gains a mind of its own and escapes. Meanwhile, Finn and Jake are called out to investigate livestock being attacked by a bloodsucking creature. Suspecting Marceline, Finn and Jake confront of her, only to discover the extracted essence has transformed into the vampire attacking the livestock. However, as Finn and Jake chase the vampire, a mob of angry villagers capture Marceline and leave her to be destroyed by the rising sun.
As the sunlight engulfs Marceline, she begins reliving memories of her past. As a child, she's comforted by her mother's lullaby. She witnesses Simon leaving her to protect her from himself. As a teenager, she hunts vampires and discovers she can absorb their powers. Later she befriends a tribe of Hyoomans, who are constructing a boat to escape the continent and avoid an impending catastrophe they fear will happen. Marceline helps fight off more vampires, and tells the Hyoomans to flee when the Vampire King arrives. Back in the present, everybody is shocked when they see that the sunlight doesn't kill Marceline, meaning that she has been cured of her vampirism. Meanwhile, Jake encounters the vampires Marceline had previously killed, now resurrected from Marceline's freed vampiric essence.
Old foes surface from Marceline's past. The five deadliest vampires on the planet are the Vampire King, strongest of the vampires, The Fool with his flight power, The Empress with her invisibility and her hypnotic eyes, the shape-shifting but old-fashioned Hierophant and the ever healing and nearly unkillable vampire named the Moon. The vampires are surprised that they are alive again, but quickly disagree on what to do next and go off on their separate ways. Witnessing this, Jake warns Finn and Marceline. Upon realizing the Vampire King is alive again, Marceline flashes back to her previous battle with him. In order to preserve the vampire race, the Vampire King sacrificed himself to bite Marceline, making her the last vampire. Marceline manages to track down and confront the resurrected Vampire King, killing the Fool and reabsorbing his power of flight. However, before she can battle the still weak Vampire King, he warns her that the Empress is headed for the Ice Kingdom. Fearing for The Ice King's safety, Marceline leaves immediately.