There’s a massive outbreak of farmers cows and other livestock having their blood drained in the night. They call for Finn and Jake and tell them they suspect it being Marceline because she’s the only vampire in all of Ooo.
Finn and Jake go to interrogate Marceline because they are convinced she’s draining livestock in her sleep and she thinks so too because she’s begun having weird dreams of this same situation.
Finn and Jake discover that the extracted vampire essence had “gain sentience” (sorta) and begin to chase it down where they find a large cave. Jake finds marceline's freed vampire essence divided into 5 vampires marceline had slain thousands of years before.