A title card recounts that Borski Castle in Vandorf is inhabited by a monster and that no one can survive there.

Bruno Heitz is painting a semi-nude portrait of his girlfriend, Sascha. She announces she's going to have his baby, so he goes to talk to her father. She runs after him and through the trees sees Castle Borski, then screams in horror.

At Vandorf Medical Institution, Inspector Kanof arrives to see Dr Namaroff to discuss the mysterious murder. Sascha's body is brought in and a calcified finger breaks off the hand as Carla Hoffman, Dr Namaroff's assistant, looks in horror.

Police with dogs search the forest and find the body of Bruno hanging from a tree.

At the coroner's inquest, Bruno is found guilty of Sascha's murder and his father, Professor Heitz, vows to clear his name. Afterwards, Carla chides Dr Namaroff for not telling the truth.

Professor Heitz visits Namaroff and recounts the myth of the Gorgons, whose ugliness turns those who see them to stone. Heitz is later attacked by angry villagers and wires his other son, Paul, to come to Vandorf.

Professor Heitz hears a female voice and goes outside. The full moon appears and he goes to the castle where he glimpses a green figure. He staggers back to the house and writes a letter to his son before he turns to stone.

Paul arrives but is not allowed to see his father's body. Namaroff has attributed his death to heart failure but Paul does not believe him.

Carla tells Paul that Megaera the Gorgon does exist. At the hospital, Carla tells Namaroff what she has learned about the Gorgon from Professor's Heitz' letter to Paul, which she read at his house. Ratoff, an orderly, comes in and reports that Martha, a mentally disturbed patient, has escaped again. Namaroff tells Carla that Megaera has taken on human form.

Paul hears the haunting female voice and goes out into the courtyard where he sees the reflection of a horrible figure in the pool and collapses.

He wakes up in a hospital bed; his hair has gone grey. When he's discharged, Paul tells Namaroff that he's staying in Vandorf to destroy the creature. As he leaves, Namaroff sees that Paul and Carla are holding hands.

Paul digs up his father's grave and finds his body turned to stone. Carla appears and Paul offers to take her away but she refuses. Back at the house, Professor Meister, Paul's tutor, arrives.

Namaroff performs an autopsy on Martha and removes her brain. Carla asks him if Martha is Megaera but he says no.

Paul and Meister talk over events and deduce that Megaera must not be looked at directly. Inspector Kanof shows them details of all the women who have come to live in Vandorf; Carla is one of them.

Carla demands to know why Namaroff spies on her. She arranges to meet Paul at Castle Borski the next morning. They meet and she agrees to come away with him but says it must be immediately. When he says he must find Megaera first, she says she will never see him again.

Ratoff attacks Paul but Meister saves him. Meister finds a file on Carla which shows that she suffered from amnesia attacks during the full moon; he thinks she is Megaera but Paul refuses to believe it. They find Carla outside struggling with Ratoff. She tells Paul it's too late to go away. The door bell rings and Paul hides Carla. Namaroff and the police search the house but can't find her as. Paul has sent her to catch the train to Leipzig. She never arrives.

Meister locks Paul in his room but he climbs out of the window. The police return to arrest Paul for Carla's abduction. Meister escapes and follows Paul to the castle. There, Paul finds Namaroff, armed with a sword; they fight. Megaera appears and Namaroff tries to kill her but dies in the attempt. Paul looks at her and as he collapses, Meister beheads her from behind. The severed head falls beside Paul and turns back into Carla. Meister tells Paul that she is free now and Paul dies.