Ten years after Dipper and Mabel Pines left Gravity Falls, they're back-only to encounter a horror they'd never anticipated. Mabel has been transformed into a vampire by one Jerry Alucard, and she's not the only one. Wendy, Candy, and Grenda have all been succumbed to the same evil curse, and even Dipper was not exempt. While a strange stranger rescued him with a sedative, the effect would only be temporary for three days. With time ticking away, Dipper, Pacifica, Stan, Ford, and Soos have to take a perilous journey to Castle Frankenstein in Transylvania. They are assisted by a foul-mouth stranger and a legendary monster hunter called Charlie, the stranger who saved Dipper, whose real identity and history are still a mystery.

A few months have passed since the legendary monster hunter Charlie Van Helsing sacrificed himself to save Dipper, Mabel, Pacifica, Wendy, Candy, and Grenda from eternal vampirism, and stopped the apocalyptic descent of the Flying Castle Frankenstein. Gravity Falls has slowly returned to its strange, peaceful rhythm-but the scars of that battle still haunt the Pines twins. Believed dead, Charlie has been quietly resurrected and reassigned by the Holy Order. His new mission: a bizarre but simple task-watch over a petrified statue deep within the cursed forests of Gravity Falls. But this isn't just any statue. It's Bill Cipher, frozen in stone, his twisted smile as lifeless as the ancient runes etched beneath him. But evil never stays quiet for long.