This movie adaptation of an Agatha Christie novel is similar in feel to "The Seven Dials", and has a number of the same actors in it, but I found it inferior. The movie is quite long, but it still manages to leave several things unexplained by the end. The traditional "explanation" scene at the end, where the devious plans of the criminal(s) are laid bare, is oddly uninvolving, despite the fact that the heroine is alone in a big house, at the mercy of the killer. It seems to go on forever, and seems to be answering questions that no one really bothered to ask. Besides, there is one outrageous piece of information provided at this moment that the viewer could not possibly have known, and which is thrown in just to make things easier for the villains. I suspect this is the fault of the original book (Christie was not above tossing in a sort of deus ex machina, especially in her early books) but it leaves the viewer feeling cheated.
Francesca Annis is a touch too smug and smooth in her portrayal of Frankie, and I found her "bright young thing" character grating after a while. Eric Porter, as the suspicious Dr. Nicholson, gives the best performance of the movie, both affable and alarming.