Count Dracula (John Carradine) visits Doctor Franz Edelmann (Onslow Stevens) in his castle, seeking a cure for vampirism. Doctor Edelmann believes a series of blood transfusions may be the trick, using his own blood. To rest up afterward, Dracula has his coffin moved into the castle basement.
That night, Lawrence Talbot (Lon Chaney Jr.) arrives, searching for a cure for his little lycanthropy problem. He is told to wait, but the full moon is rising, so he has himself arrested and put in a cell. Inspector Holtz (Lionel Atwill) of the police asks the doctor to see Talbot, and together they witness his transformation into the Wolf Man. The next morning, he is taken to the castle and the doctor suggests it’s not the moon that is the problem, but pressure on the brain. He has a plant that may be made into a cure. But Talbot becomes despondent and jumps into the ocean and is washed into a seaside cave beneath the castle.
Edelmann finds Talbot in the cave, where they also find Frankenstein’s monster (Glenn Strange). The plant, Clavaria Formosa, grows perfectly in the cave and there is a tunnel directly into the castle. Doctor Edelmann takes the Frankenstein monster back to his laboratory but thinks better of reviving it. Meanwhile, the Count is up to his old tricks and tries to seduce the doctor’s assistant Milizia Morelle (Martha O’Driscoll)) and make her a vampire, but she warns him off with a cross.
The doctor believes the Count needs another transfusion. Edelmann’s other assistant Nina (Jane Adams) follows Milizia and warns Edelmann that the Count is dangerous. The doctor decides to create a transfusion that will kill Dracula, but the Count hypnotizes both Edelmann and Nina, then reverses the flow of the transfusion, sending his blood into the doctor’s veins. When Edelmann and Nina awake, Dracula is carrying Milizia away. They revive Talbot and repel Dracula with a cross. Dracula returns to his coffin and Edelmann drags it into the sunlight, killing him.
But the vampire’s blood begins its work in the doctor and he finds he no longer casts a reflection. He has visions of violence and in the middle of treating Talbot he transforms and murders his gardener. The townspeople discover the body and chase Edelmann, believing him to be the Wolf Man. They follow him into the castle, where Talbot and Edelmann are questioned by Inspector Holtz and Steinmuhl (Skelton Kvaggs). The mob prepares to execute the doctor. Talbot is actually cured of his lycanthropy, but Edelmann turns into a monster. He revives Frankenstein’s creature, breaks Nina’s neck and throws her into the cave. The police are subdued by the creature and Talbot shoots Edelmann dead. The townspeople flee screaming as the castle collapses and burns.
Lionel Atwill was terminally ill with lung cancer and died shortly after the movie was made. Glenn Strange, after three hours in makeup, would have to sit in cold water and mud all day, and he got so cold he couldn’t feel his legs. Lon Chaney would pass him whiskey to keep him warm and he would end up drunk every day. Boris Karloff was the monster in the dream sequences, and in fact, at various times four different actors would play the monster. Since Talbot is cured of his affliction, there is no more Wolf Man and Lon Chaney was let go by Universal. There is no explanation why he returned for Abbott and Costello Meet Frankenstein, unless you believe that the latter is a spoof and not canon.