New York City, 1965: Rosemary and Guy Woodhouse (Mia Farrow and John Cassavetes) are a young married couple who rent an apartment in the gothic and splendorous Bramford building in Manhattan.
At first, their friend and current landlord Edward "Hutch" Hutchins (Maurice Evans) tries to dissuade them from doing so: the building has a rather unsavory past.
It has been occupied by cannibal killers, Satanists and witches, such as the Trench Sisters, Keith Kennedy, Pearl Ames--and the sinister Adrian Marcato, who created a scandal in the late 1890s by claiming to have conjured "The Living Devil."
Rosemary and Guy ignore him and move in. Guy is an actor with a fledging career. He's done plenty of TV plays and commercials, which have made him good money; but he wants great parts. Rosemary is a gentle soul, originally from Omaha, Nebraska, where she had been raised in a Catholic home and had attended convent school.
As it is, Rosemary is estranged from her family, since they don't accept her marriage to Guy, who is not only an actor, but is also of mixed Jewish/Protestant upbringing. So her life in New York is all she has: she is a young housewife dedicated entirely to making a good home for her husband, whom she adores. She has a good circle of friends, but is at core sweetly naïve and lonely.
One day in the laundry room, Rosemary makes the acquaintance of Terry Gionnoffrio (Victoria Vetri), a young former drug addict who was "rescued from the gutter" by an elderly, eccentric couple, Roman and Minnie Castevet (Sidney Blackmer and Ruth Gordon). The Castavets's apartment had formerly been the front part of theirs, but is now separated by a partition.
Her friendship with Terry is short-lived. A few days after Rosemary meets her, Terry plunges to her death from the bay window.
Rosemary and Guy are walking home when they see the police surrounding Terry's corpse. The Castavets happen to be walking home, too.
Presumably distraught, they strike up an acquaintance with the Woodhouses and later invite them to dinner.
Rosemary becomes suspicious about their new elderly friends, particularly the way that Roman insists on speaking with Guy in private.
Other things trouble her, too, such as when Guy's career gets a jump start when his main rival, Donald Baumgart, suddenly goes blind. Now all he cares about is his new play.
But then suddenly, he decides he wants to become a father. Rosemary is thrilled. Evidently having studied her ovulation cycle himself, he announces the ideal "baby night."
That evening, Minnie drops by to give them some chocolate mousse--or mouse as she calls it.
Rosemary complains of a chalky undertaste, but Guy gets angry over her ingratitude. She eats part of it, and then furtively hides the rest in her napkin.
Suddenly feeling disoriented, she passes out and has a bizarre dream.
Rosemary visits the Sistine Chapel and sails on a cruise ship.
The unsettling dream becomes a nightmare when two figures tie her to a bed.
Something that looks and feels inhuman brutally rapes her. "This is no dream!" she cries. "This is really happening!"
Pope Paul VI, then visiting NYC and having mass at Yankee Stadium, comes to offer her absolution. When she wakes up, Rosemary is sore and scratched.