At the turn of the 20th century in the fictional village of Clagmoor Heath in Cornwall several locals are dying from what is deemed to be the "Black Death". Harry Spalding (Ray Barrett) inherits his late brother’s cottage and arrives with his new bride, Valerie (Jennifer Daniel) . The inhabitants of the village keep clear of the newly arrived couple, and only the publican, Tom Bailey (Michael Ripper), befriends them. Bailey explains that the hostility exhibited by the townspeople is the result of many mysterious deaths in the community.
The sinister Dr. Franklyn (Noel Willman), the owner of the nearby Well House, is the only resident in the vicinity of the cottage, and he lives with his daughter Anna (Jacqueline Pearce). The Doctor treats his daughter with cruel contempt, and she is attended by a silent Malay servant (Marne Maitland).
Hoping to learn something of the deaths, Harry invites the local eccentric, Mad Peter (John Laurie), home for dinner. After warning them that their lives are in danger, Mad Peter quickly departs only to return later that evening foaming at the mouth, with his face blackened and swollen. He dies within a few minutes. The Spaldings attempt to alert Dr. Franklyn, but Franklyn arrogantly states that Peter's death is not his concern.
In an attempt to help Harry clear up the mystery, Tom Bailey illegally unearths Mad Peter’s corpse and discovers a strange neck wound like a snake bite. Harry and Tom dig up the coffin of Harry's dead brother, Charles, and find that corpse also has those same strange marks. Realizing that they are threatened by something far worse than they had ever imagined, Harry is quick to answer an urgent message from the Well House. There he is bitten by a mysterious reptilian creature, but still he manages to return to his home and recover from the bite.
Meanwhile, at the eerie abode, Valerie witnesses Dr. Franklyn’s attempt to kill his own cursed daughter (who was changed into the Reptile creature after joining a Malay snake cult that included the doctor's own servant). She also witnesses a fierce struggle between the doctor and the deranged Malay servant. During the battle, a lantern is overturned and Dr. Franklyn is bitten by the venomous fangs of his Reptilian daughter. Both the doctor and the Reptile die in the fire and the Well House crumbles in flames with Harry and Valerie fleeing for their lives.Noel Willman – Dr. Franklyn
Ray Barrett – Harry George Spalding
Jennifer Daniel – Valerie Spalding
Jacqueline Pearce – Anna Franklyn
Michael Ripper – Tom Bailey
John Laurie – Mad Peter
Marne Maitland – The Malay
David Baron – Charles Edward Spalding
Charles Lloyd-Pack – The Vicar
Harold Goldblatt – The Solicitor
George Woodbridge – Old Garnsey
The production was filmed back to back with The Plague of the Zombies, and used many of the same sets, including exterior shots in the grounds of Oakley Court (seen burning in the final frames). Pearce and Ripper appeared in both films.
It was well known that actress Jacqueline Pearce, as documented in several books on Hammer's history, disliked wearing the Reptile make-up as she suffers from claustrophobia. After this she vowed never to wear "creature" make-up in her future acting projects.
The film was released in some markets on a double feature with Rasputin, the Mad Monk.
A novelization of the film was written by John Burke as part of his 1967 book The Second Hammer Horror Film Omnibus.
The cottage used in this film was located in Brentmoor Road, West End, Woking, Surrey. The common shots were of West End & Chobham Common.
The Hammer Story: The Authorised History of Hammer Films called the film "classic sixties Hammer"; while Allmovie wrote, "there are some inconsistencies in Anthony Hinds' script, but the film is handsomely mounted and delivers its share of shocks" ; Time Out wrote, "it's slower and moodier than its companion-piece (Plague of the Zombies), but strikingly Conan Doyleish in its stately costume horrors. Jacqueline Pearce is terrific"; and British Horror Films said simply, "it's superb".