This all makes Mary a bit jumpy, and it doesn't help that she's already kind of an oddball for 1962. A healthy, intelligent young woman who doesn't want friends, isn't interested in the advances of the loutish tenant across the hall in her boarding house, plays professionally on a church organ but lacks religious conviction... she doesn't seem to fit in anywhere. Her landlady, her boss at the church and her doctor are all slightly baffled by her. You've already got the plot twist figured out, ten minutes into the movie, but it's fascinating to watch: on so many levels, Mary doesn't belong here. She belongs somewhere else.
It's creepy as hell to watch Mary's destiny close in on her. Gorgeous, too. Why aren't more horror movies set in Utah? The black and white cinematography makes the most of the sweeping, open landscape, especially around the vacant lakeside pavilion that irresistibly draws Mary: white skies against dark water. This would be great to see on a big screen. The soundtrack, almost entirely organ music, is gorgeous too.
As a fairly spacey person myself, with an often-tenuous grip on reality ("Where are my glasses? What's her name again? Wait, what year is this?"), I could powerfully identify with Mary's terror as her world starts to blur and something else starts to come in. It's a universal fear, this fear of losing one's grip altogether. James Thurber puts it this way in his introduction to "My Life and Hard Times":
It is unfortunate, however, that even a well-ordered life can not lead anybody safely around the inevitable doom that waits in the skies. As F. Hopkinson Smith long ago pointed out, the claw of the sea-puss gets us all in the end.
Brr. Let's turn on some more lights and have some eggnog!