London Inspector Quennell [Peter Cushing] is investigating the deaths of several men found on the heaths. The only witness is said to be insane, raving about a huge creature with wings and horrible eyes, and all of the bodies have been drained of blood and badly scratched. Quennell wonders if the attacker might have been a large eagle and seeks out information from his old friend, Professor Karl Mallinger [Robert Flemyng], an entomologist at the university. Mallinger doubts that an eagle could be responsible for the deaths. What he fails to tell Quennell is that his experiments have transformed his daughter Clare [Wanda Ventham] into a giant, blood-sucking, death's-head moth.
When the moth attacks Frederick Britewell [William Wilde], a naturalist just returned from Africa with numerous specimens of moths he has collected for Dr Mallinger, Quennell stumbles upon Britewell's dying body just in time to hear him mysteriously whisper "death's-head." Quennell also finds some peculiar scales around the body in the bushes. Dr Mallinger, who denies ever seeing Britewell before, agrees to try and identify the scales. Quennell returns home to pick up his daugher Meg [Vanessa Howard]. Upon their return, Quennell learns that Britewell was a guest of Dr Mallinger's. His suspicions arisen, Quennell rushes to Mallinger's house only to find the butler dead, several of the mysterious scales lying around, and the house closed up and vacated.
After interviewing the maids, who tell him of Mallinger's sudden departure, Quennell learns that Mallinger has moved to the country and he and his daughter follow, taking rooms at a local inn. There, he meets Mr Warrender [John Paul] and his bug-collecting son William [David Griffin], both on a fishing holiday. Meg and William take a liking to each other, and Meg accompanies William on his bug-collecting trips, one of which takes him to the old Miles house, where Mallinger and his daughter have taken up residence and Mallinger is continuing his efforts to create another big death's-head moth as a mate for Clare. Unfortunately, galvanism hasn't worked to awaken the moth, so they hypnotize Meg and transfuse some of her blood into it.
On one of his bug-collecting trips, William captures what he thinks is a death's-head moth. When Quennell hears the word "death's-head", he makes the connection to Britewell's dying words. When Warrender and William dredge up the body of Clem Withers [Simon Cain], the gardener at the old Miles house, Quennell puts two and two together. In the meantime, Mallinger has suddenly realized the evilness of his creations and destroyed the male moth. Enraged, Clare turns into mothform and kills her father. William takes his moth to get a confirmed identification from Mallinger, but he is attacked by Clare. Quennell builds a bonfire. Clare flies into it and is burned to a crisp. [Original synopsis by bj_kuehl.]