The Poster Gives Away The Ending (SPOILERS For The Stepford Wives and Rosemary's Baby)
I'm not sure one can write "SPOILER" when the poster is there for all to see who come to this page but...honestly...when this movie came out in 1974, even if you had not read the book, the poster pretty much gave away the ending.
Which, I suppose was OK...because it doesn't take but 30 minutes of watching The Stepford Wives to figure out what's going on . Its one of those movies where "the audience knows what's going on but the characters don't...and you just wait for the characters to catch up."
Intrigingly, the author of the novel The Stepford Wives also wrote Rosemary's Baby...and thought he poster for THAT movie didn't give away the ending, again, about an hour into THAT movie, you could pretty much figure out what was going on...and waited for Mia Farrow to catch up.
There is an "either/or" aspect to the mysteries in The Stepford Wives and Rosemary's Baby:
The Stepford Wives: Either the wives are reprogrammed to become compliant(brain surgery or hypnosis) OR the real wives are killed and replaced with sexy robots.
Rosemary's Baby: Either Rosemary is paranoid about her pregnancy and the baby is by her husband(John Cassavetes) OR the baby's father is Satan.
Since the right guess is pretty obvious in both films, I suppose the chill of both films(moreso in the classic Rosemary's Baby than in the minor Stepford Wives) is "understanding what's going on as it is happening." In both films, men USE their women and subject them to something horrible(Satan's baby; death) to obtain their own satisfaction(career success in Rosemary's Baby; a compliant sexual housewife in Stepford.)
Thus the films stand as feminist works early in the feminist era, viewable more as satires or social studies than "mystery thrillers."
That said, Rosemary's Baby is great, and The Stepford Wives(70's "straight" version) is pretty good.