The Hammond family has been cursed since the Crusades, with family members dying or committing suicide under mysterious circumstances. When two people, including Oliver Hammond, are attacked by an unknown creature, a Scotland Yard scientist, Robert Curtis, and his sidekick Christy are dispatched to investigate. Although the local townspeople are convinced that the attacks are the result of the Hammond family curse, Curtis seeks a more scientific explanation. Curtis' investigation at the Hammond household reveals a number of unusual circumstances, including slamming doors and clanking chains, a recently entered secret room supposedly locked for years, and a statue of a strange dog-like creature in the Hammond family crypt. During his investigation, one of the initial victims of the attack dies, and the case is sent to an English jury for judgement. Upon hearing testimony from members of the Hammond family and their associates, the jury rules that the victim died at the hand of an unknown person or creature of unknown species. After the ruling, Curtis looks for evidence upon the victim's body. He finds a hair that he later identifies as a wolf's, but the hair disappears mysteriously soon after he analyzes it. The monster attacks again, this time kidnapping Helga Hammond, but Curtis and the police chase him down. When shot, the monster transforms into a human being, and we discover that it is actually Oliver Hammond. Afterwards, Dr. Jeff Colbert, a friend of the Hammonds, reveals that they have been afflicted with lycanthropy for generations — that is, they are werewolves — and he had been attempting to cure them of the disease.
Modern response for The Undying Monster has been mixed to positive. Film critic Leonard Maltin awarded the film two and a half out of four stars, commending the film's atmospheric photography while also stating that it added nothing new to the genre. Dennis Schwartz of Ozus' World Movie Reviews gave the film an A-, praising the film for its atmosphere, cinematography, and suspense, calling it " Superior telling of a werewolf". Schwartz's only criticism was directed towards the film's comedy relief, which he felt was unnecessary. Time Out magazine commended the film's atmosphere, cinematography, performaces, and direction, while also criticizing the film's plot. Chris Coffel from Dread Central offered the film similar praise, while also commending the film's mystery aspects and plot, favorably comparing it to Universal Horror films. Alternately, Stuart Galbraith IV from DVD Talk stated that, while it featured great atmosphere, and was well-made and directed, overall the film was "disappointing" due to its less than stellar cast and illogical plot twist. Michael Barrett of PopMatters rated the film a mixed score of five out of ten stars. In his review, Barrett pointed out that the film's title monster as the film's major fault, stating that special effects used to for the monster were ineffective and of poor quality, and the final explanation of the family "curse" was unsatisfactory. TV Guide gave the film two out of five stars, stating that while the film contained effectively "eerie atmosphere" and great location work, it was still just a knock off of Universal's Wolf Man.
