Considering the movie is essentially a single set, 4 actors, and a "dead man", it's a heck-uva good little thriller. Credit should go to writer Sangster, director Holt, and 4 excellent performances, especially Strasberg's vulnerable young woman. Okay, the whole story is pretty contrived with a number of unexpected twists, but the measure of a thriller is how well you're glued to your seat, and this one uses epoxy.
So, what's up with Penny's (Strasberg) dad. He keeps turning up in odd places like the bottom of a pool. For a guy who looks dead, he sure gets around. And what's with step-mom Jane (Todd) always fussing around in the background, along with her friend the doctor. If I were Penny, I wouldn't let Christopher Lee's doc anywhere near my neck. Good thing Penny has the handsome chauffeur Bob (Lewis) on her side, because that wheelchair keeps going off- track. For some reason, the movie, especially the rancid pool, kept reminding me of the French classic Les Diabolique (1954).
I'm rather curious, however, about actor Ronald Lewis, a strong presence, and perhaps the only actor of the period to appear in a male bikini. He was a suicide at a relatively young age, suggesting a backstory of some kind—too bad IMDb couldn't provide more. Anyway, the movie manages to prove that sunny new mansions can be just as creepy as old dark ones, and that the ritzy French Riviera may not be a good place to vacation, especially if you're trying to get an ocean view from the cliffs.