This is a terrific thriller, made just the way that thrillers should be done, with nicely crafted tension, atmospheric setting, and plenty of strange and mysterious characters. Adding in the excellent leading performance by Dorothy McGuire makes it a top-notch picture.
From the very beginning, the suspense begins to build, while at the same time McGuire's character Helen immediately attracts and holds the audience's concern and sympathy. The combination works very well, and while you're never sure what else is going on, or which of the other characters can be trusted, you know that Helen is in danger and that you care about her.
The rest of the cast are all nicely matched with their characters, and do a fine job of bringing them to life. Some of the plot developments probably would prove somewhat implausible if analyzed carefully, but the story and atmosphere are crafted so effectively that you never notice while watching.
Anyone who enjoys classic thrillers should make sure to see "The Spiral Staircase".