It was directed by John Gilling, and starred Noel Willman, Jacqueline Pearce, Ray Barrett, Jennifer Daniel, and Michael Ripper.

After warning them that their lives are in danger, 'Mad' Peter quickly departs only to return later that evening with his face blackened and swollen before dying.

In an attempt to help Harry clear up the mystery, Tom illegally unearths Peter's corpse and discovers a strange neck wound like a snake bite.

Initially pitched to Universal Pictures three years earlier as The Curse of the Reptiles, the script was written by Anthony Hinds using his pseudonym, John Elder.

[4] Hammer producer Anthony Nelson Keys, in an effort to save money, had decided to shoot four films on the same sets with staggered release dates.

[4] As documented in books on Hammer Film's history, actress Jacqueline Pearce disliked wearing the Reptile make-up as she suffered from claustrophobia.

[11] Time Out wrote, "it's slower and moodier than its companion-piece (Plague of the Zombies), but strikingly Conan Doyle-ish in its stately costume horrors.

[13] The Monthly Film Bulletin wrote that it "has an unusually controlled dignity for a Hammer production; instead of the customary blood-lettings, we are invited to observe with nervous curiosity the slow self-destruction of a proud but superstitious man incapable of rescuing his daughter from the fate half-wished upon her by himself ...