Frieda and Maria are identical twin sisters sent to live with their uncle Gustave (Peter Cushing).
Gustave is the leader of The Brotherhood, a secret group dedicated to killing vampires. The local nobleman, Count Karnstein, is suspected of being a vampire, but because of his noble title and connections to the emperor, they do not dare go after him. Maria is the good twin, but Frieda is intrigued by Karnstein. She sneaks out at night to Karnstein's castle and becomes his vampire mistress. With Frieda turned into a vampire, they have their proof and decide to go after Karnstein.
Karnstein kidnaps Maria and locks her in his castle. Frieda dresses like Maria, and takes her place in the house. Karnstein wants to trick Gustave into killing Maria by mistake. However, the local school teacher (who is in love with Maria) realizes the switch and saves Maria from being burned at the stake.
As the Brotherhood storms the castle, Karnstein and Frieda try to escape out a secret tunnel. Gustave is there waiting, and decapitates Frieda. Karnstein kills Gustave. The teacher drives a spear through Karnstein's heart, eliminating the last of the vampires.
Thanks Joseph C!