Finn and Jake hunt a shadowy predator and Marceline asks Princess Bubblegum for a favor.
While sunlight engulfs her, Marceline experiences flashbacks. She is comforted by her mother's lullaby as a child; witnesses Simon leaving her to protect her from himself; hunts vampires as a teenager and discovers she can absorb their powers. She befriends a tribe of Hyoomans, who are preparing to escape an impending catastrophe they fear will happen. Marceline helps fight more vampires, and tells the Hyoomans to flee when the Vampire King arrives. In the present, Marceline survives the sunlight, successfully cured of vampirism. Meanwhile, Jake encounters the vampires Marceline had previously killed, resurrected by her freed vampiric essence.
The five deadliest vampires resurface: the Vampire King, The Fool, The Empress, the Hierophant and the Moon. Surprised to be alive again, they disagree on what to do and go their separate ways. Witnessing this, Jake warns Finn and Marceline. Learning the Vampire King is alive, Marceline has a flashback of their previous battle—when the Vampire King sacrificed himself to bite Marceline, making her the last vampire. Marceline hunts and confronts the resurrected Vampire King, killing the Fool and reabsorbing his power of flight. However, the Vampire King warns her that the Empress is headed for the Ice Kingdom. Fearing for the Ice King's safety, Marceline leaves immediately.
The Empress arrives at the Ice Kingdom and uses her hypnotism power on the Ice King, who captures Finn for her. Marceline attacks and escapes with Finn, pursued by the Ice King. Marceline tries to jog the Ice King's memory, but it turns out he was never hypnotized and this is how he always acts around women. Marceline then attacks the Empress, who has the upper hand until she is paralyzed by a special gun Bubblegum developed. Marceline kills the Empress and reabsorbs her power of invisibility. She apologizes to her friends for trying to fight the vampires alone and tells them that they will hunt the remaining vampires together.
While on the trail of the vampire known as the Moon, the group run into trouble when the shape-shifting Hierophant finds them. He proves to be too powerful to fight, so on learning that he cannot enter houses without permission, Jake shapeshifts into a house and the group take shelter inside. The Hierophant proposes joining forces against the Vampire King, and Marceline agrees on the condition that the Hierophant stops drinking blood. However, he attacks her and during the struggle, he is accidentally knocked into Jake and dies due to entering a house uninvited. Marceline reabsorbs his shapeshifting powers but then falls ill, having been poisoned by the Hierophant's stinger.
Princess Bubblegum brings Marceline to the Candy Kingdom to work on a cure for the Hierophant's poison. Meanwhile, Finn and Jake find the Moon whose healing power might aid Marceline, but cannot kill the Moon because her healing powers are so great. The Moon awakens at sunset and Finn and Jake flee, luring the Moon to the Candy Kingdom. The Moon uses her powers to incapacitate Finn and Jake as she attempts to kill Marceline, but Peppermint Butler stabs the Moon in the back, her only weak spot. Marceline reabsorbs the Moon's healing power and fully recovers from the poison.
The group prepare to confront the Vampire King, who unexpectedly appears and says he no longer wishes to be a vampire. He maintains this, despite Marceline's attempts to attack him, and the group reluctantly agree to remove his vampiric essence. Bubblegum uses the same contraption she'd earlier used on Marceline, which turns the Vampire King into a relatively harmless lion, and puts his vampiric essence in a bucket. Bubblegum leaves Peppermint Butler to properly dispose of the essence but he trips, causing the essence to detonate into a new monstrosity: The Dark Cloud.
The Dark Cloud moves towards the Candy Kingdom, pursued by Finn, Jake, and Bubblegum. Seeing Bubblegum fighting the Dark Cloud, the people of Candy Kingdom are inspired to overthrow the King of Ooo and join the battle. Marceline resigns herself to defeat until the Ice King instills in her a sense of purpose, convincing her it is her destiny to stop the Dark Cloud. Marceline eventually uses her demonic soul-sucking abilities to drain the Dark Cloud from the inside out. However, in doing so she once more becomes the Vampire Queen. In the aftermath, Marceline comes to terms with her vampiric nature, telling Bubblegum that her time as a mortal has made her more mature. Marceline sings a rendition of "Everything Stays", with short sequences of Ooo's return to normalcy.