Two of the great British TV screenwriters of the Swinging Sixties were responsible for the storyline of And Soon the Darkness, a film released in 1970. They were Brian Clemens, famed in particular for The Avengers, and Terry Nation, a name associated above all with Doctor Who thanks to his creation of the Daleks and Davros. They were first-rate storytellers, but their CVs suggest that their gifts were better suited to the small rather than the large screen.
This story begins with two young English women cycling through a remote area of France. The parts of Jane and Cathy are played by Pamela Franklin and Michele Dotrice. Franklin first came to prominence as a child actor in that superb movie The Innocents, though her film career didn't last long. Dotrice remains best known as Frank Spencer's wife in Some Mothers Do 'Ave 'Em, although she continues to act, and appeared in one of my favourite TV shows of 2018 (along with The Bodyguard), A Very British Scandal.
Alas, and all too predictably, the area they are exploring (the film was shot in the Loire valley) turns out to have a sinister reputation. Things go from bad to worse when the friends fall out. Jane cycles on without Cathy, before beginning to worry about her friend. When she returns to the spot where she left Cathy sunbathing, her pal is nowhere to be seen. She has a series of encounters with spooky locals, as well as a handsome but suspicious-seeming chap whose eye Cathy had caught. Jane enlists the aid of a police officer (John Nettleton, best known as a lofty civil servant in Yes, Minister, is surprisingly good in this role) and in due course a rather inevitable discovery is made.
I thought the film watchable but protracted, and that the plot seemed disappointingly slender. In terms of characterisation, the protagonists are a bit two-dimensional, and I'm afraid Pamela Franklin's performance didn't really engage me. The atmospherics are inevitably rather dated, and even allowing for the passage of time, I was rather surprised to learn that the film was deemed worthy of remaking in 2010, under the same title; the remake seems not to have made much impact. I very much enjoy Laurie Johnson's music (he was responsible for the theme from The Avengers, for instance) but felt that his soundtrack was a little intrusive at times. However, Sergio Angelini, whose opinions on films are always of interest, is rather more positive than I am, and you can read his review here.