Alfred, a young scientist, is the assistant to a cranky professor on an expedition to Transylvania. The professor, a lively old man called Abronsius, has made it his life’s work to prove the existence of vampires and to set mankind free. On the search for the castle of the vampire Count von Krolock, they got lost in deep snow in the wilderness. Half-frozen, they finally reach a lonely country village and stumble upon an inn.
There, the farmers and lumberjacks sing the praises of garlic (Knoblauch), which makes Professor Abronsius suspect that they have finally reached their destination but the village inhabitants deny that such a castle exists. In the room next door to the guestroom, to which the Jewish landlord Chagal shows the guests, is a bath in which Chagal’s beautiful daughter Sarah is bathing. For Alfred, it is love at first sight and Sarah also likes the young man.
As Sarah’s father, Chagal fears for his daughter’s innocence. To be on the safe side, he boards up her bedroom door (Eine schöne Tochter ist ein Segen). While Sarah and Alfred dream separately of each other (Nie geseh’n), Chagal creeps through the dark inn to his maid, Magda, with whom he is having an affair. Rebecca, Chagal’s resolute wife, notices that her husband has crept out of their bed. She arms herself with a salami and tries to take her revenge, but by mistake she knocks down the professor.
Finally, when everyone is asleep, a threatening shadow falls over the inn. Count von Krolock makes it clear that he has his eye on Sarah (Gott ist tot).
The next day, Professor Abronsius tries to find out what the landlord and landlady know about vampires. His investigation meets with silence. When a hunchback appears in order to get some candles, everyone becomes nervous and this makes Abronsius certain that the cripple acts as a servant for the vampire. He is determined to find out the truth. (Wahrheit).
A fierce flirtation develops between Alfred and Sarah. Sarah is against letting anyone else use the bathroom in which she passionately likes to bathe. Alfred observes her through the keyhole and witnesses Count von Krolock coming through a skylight and inviting Sarah to the annual midnight ball in his castle (Einladung zum Ball). Alfred raises the alarm so Abronsius and Chagal come quickly, but the vampire has vanished.
Later, Alfred sings an ardent song under Sarah’s window (Draußen ist Freiheit). But Sarah had left her room a while ago in order to collect a present from the Count, which the hunchback has left in the snowy garden. When she discovers Alfred, he thinks she has come down because of him. She sends Alfred inside under a pretext and opens the present. Inside is a pair of red boots. Sarah puts them on and dreams about being the admired centre of the ball (Die Roten Stiefel).
Before Alfred returns, Sarah runs away. She wants to go to the castle. Alfred’s cries wake Chagal in the house. He hurries after his daughter and leaves the desperate Rebecca behind.
The next day the lumberjacks bring the stiff, frozen corpse of Chagal into the inn. Professor Abronsius professionally establishes that the landlord may become a vampire. He suggests putting a stake through the dead heart so that Chagal does not become a member of the undead. Rebecca prevents this and chases the vampire researcher away.
In the night, Magda comes to the room in order to see Chagal and sees how thoroughly death has changed him (Tot su sein ist komisch). As she turns away, the body comes to life. Chagal seizes his maid and bites her throat. Then he lays the lifeless Magda under the cloth that had covered him. He has just enough time to hide under the table before Abronsius and Alfred return. They are about to drill through the heart under the cloth. At the last moment they notice what has happened. They find and overpower Chagal. He asks for a favour and suggests that he will lead the strangers to the castle, if they spare him.
They set off immediately. On the way, Chagal runs off, but Alfred and Professor Abronsius find the castle. They are just about to look at the splendid portal when Count von Krolock appears. He welcomes the strangers and invites them to be his guests and, to Alfred’s dismay, Abronsius accepts. The Count’s son, the gay Herbert, also appears to greet them. He is very happy to have found a playmate in Alfred.
Von Krolock sees to it that he can speak to Alfred in private and tries to lure him away from Abronsius. He calls the professor an old fool who has no idea about the wishes and needs of a young man like Alfred. Instead, von Krolock offers himself as a paternal friend (Finale erster Akt). He hints that he can lead Alfred to Sarah.