I cannot tell. But this motion picture seems like a film adaptation of a stage play, although with very few licenses to leave the main setting: that is, the house with a cellar that contains a beast... or contained, it seems, because right from the beginning whatever was inside is killing soldiers around the countryside, leaving them all bloody (yes, there is enough blood for an early 1970s movie), and more scratched than a phone card.
I enjoyed the proceedings and the chitchat between the two main characters, the Ballantyne sisters, in the first two thirds. But in the third act, Ellie (Beryl Reed) never stops talking about how she loved her father and how her sister Joyce (Flora Robson, by now bedridden) loved their brother Stephen. She confirms what we know since the beginning, that the two sisters locked brother Stephen in the cellar for 30 years. In the meantime, the police, now being alerted by Ellie, looks for the man-beast.
Blond John Hamill (perhaps Britain's most popular male nude model of the 1960-70s) plays a very amiable young corporal who visits the two old ladies with the latest news at least three times a day, and Tessa Wyatt appears too late in the plot as a nurse, but she anyway becomes Hamill's love interest. By then poor Stephen has no hope.
It does not have any right to be so, but somehow the film is enjoyable.