Inspired to surround herself with their younger, energetic circle of friends, Rosemary plans a Saturday night party to entertain "young friends - Minnie and Roman are not invited. Neither is Laura-Louise nor is Dr. Sapirstein. It's gonna be a very special party. You have to be under 60 to get in." When Minnie learns of the party and attempts to invite herself, she is denied - and Rosemary affirms the rejection by dumping Minnie's concoction down the sink.

At the party, guests comment that gaunt-looking Rosemary has a "chalk"-white face and looks "a bit tired." [It was rumored, but probably untrue, that director Polanski's wife Sharon Tate made a small cameo appearance as one of the party-goers in this scene.] Guy continually lurks in the background of scenes, spying on Rosemary's interactions and behaviors. Rosemary confides in her girlfriends about her abdominal pains and her fears: "I'm so afraid the baby's gonna die." They suggest that her ineffectual doctor be replaced since he "sounds like a sadistic nut...Pain like that is a warning that something isn't right. Go see Dr. Hill. Go see anybody besides that...nut. You can't go on suffering like this." Rosemary blurts out an affirmation of her Catholic training: "I won't have an abortion."

After the guests have left, Rosemary rebelliously informs a flabbergasted and angered Guy that she will return to Dr. Hill, her former gynecologist-obstetrician, after receiving advice from her girlfriends ("not very bright bitches" in Guy's opinion):

Dr. Sapirstein is either lying or he's, I don't know, out of his mind. Pain like this is a warning something's wrong...And I'm not drinking Minnie's drink anymore. I want vitamins in pills like everyone else. I haven't drunk it for the last three days. I've thrown it away...I've made my own drink...I'm tired of hearing how great Dr. Sapirstein is.

An argument flares up between the young, expectant couple about her proposed visit to Dr. Hill for "a second opinion." As they fight, the pain unexpectedly stops in Rosemary's womb, and she feels movement for the first time, forgetting about her determined resolve to visit a different doctor and dismissing her fixation that she is carrying something unnatural in her womb: "It's alive. Guy, it's moving. It's alive, it's all right...I feel it kicking. It's alive." She begins to show, and the nursery for the child is prepared with fresh wall-paper and a white crib and rocking cradle.

Three weeks from the expected delivery date, Rosemary packs her "hospital suitcase" for her trip to the hospital. Then, she receives a phone call that Hutch has tragically died. After the burial ceremony at the cemetery, Rosemary is handed a book from one of Hutch's friends, Grace Cardiff (Hanna Landy): "Hutch regained consciousness at the end and he thought it was the next morning. You know, when you had the appointment...I wasn't there but he told the doctor to make sure that you got the book that was on his desk." As she departs, she quickly adds:

Grace: Oh, and I'm to tell you. The name is an anagram.
Rosemary: (confused) The name of the book?
Grace: Apparently. He was delirious, so it's hard to be sure.

Back at her apartment, before Rosemary can even open the brown-paper-wrapped, unmailed book/package from Hutch, Minnie delivers the daily concoction and asks to hold the parcel, mentioning that she knows Grace's return-address location: "Oh, I know that house. The Gilmore's used to live there...I've been there lots of times. Grace - that's one of my favorite names." After a suspenseful few moments with Minnie, Rosemary finds privacy to slit open the twine-tied package. The book is titled All of Them Witches, with chapters on the practices of various witches including THOMAS COLLEY and PRUDENCE DUVERNOY. One section of the book is underlined - it regards the discovery made by Hutch about the tannis-root:

In their rituals, they often use the fungus called Devil's Pepper. This is a spongy matter derived from swampy regions having a strong pungent odor. Devil's Pepper is considered to have special powers. It has been used in rituals and worn on charms.

Another chapter is about ADRIAN MARCATO: "Born in Glasgow in 1846, he was soon after brought to New York. He resided for several years in the United States but spent a great deal of time on the Continent...He was educated in England and during the course of his studies, he became interested in Black Magic. Soon, he left England and devoted himself completely to Satanism. The peak of his popularity was in 1899 when he claimed to have called forth Satan and was attacked by a mob outside the Bramford..." A dog-eared page illustrates a picture of Marcato with his wife and thirteen-year old son Steven (underlined twice). Rosemary contemplates the truth of it all: "There are no witches, not really....The name is an anagram." She retrieves a Scrabble board game and uses the game letters to spell out ALL OF THEM WITCHES, arranging them into: COMES WITH THE FALL, ELF SHOT LAME WITCH, and HOW IS HELL FACT ME (with the letter T unused). Suddenly, she opens the book and sees the word Steven underlined. She spells out STEVEN MARCATO and gradually figures out that an anagram-repositioning spells ROMAN CASTEVET - the son of Adrian Marcato.

When Guy returns, she tells him that their neighbor Roman is the son of a famous martyred witch (warlock):

Do you know who Roman really is?...He's Adrian Marcato's son...Roman Castevet is Steven Marcato rearranged...(pointing to the book's picture) There he is when he was thirteen. See the eyes?...In the same house, and look, look here. (reading) Soon after that, in August 1886, his son Steven was born. 1886 - got it? That makes him seventy-nine now. No coincidence....His father was a martyr to it.

Rosemary deduces that the next-door Castevets and their friends have formed a ritualistic congregation (or coven) of witches, and that her unborn baby's blood will be used in their rituals:

The parties with the singing and the flute, and the chanting. Those are S'baths or sabbaths, whatever they're called....They use blood in their rituals and the blood that has the most power is baby's blood. And they don't just use the blood, they use the flesh too...They're not setting foot in this apartment ever again and they're not coming within fifty feet of the baby...We're gonna sub-let and move out.

Guy downplays her fears and suppositions, claiming that the Castevets are only "old people. They have a bunch of old friends." He removes the book from her shaky hands and places it high on a bookshelf atop two volumes of the Kinsey Sex Report on Sexual Behavior in the Human Female and Sexual Behavior in the Human Male.

During Rosemary's next visit to Dr. Sapirstein, he listens to her "fantastic" findings, and obligingly calms the neurotic, hysterical mother-to-be by condescendingly agreeing with her: "Any mother would feel the same way." He prescribes some pills for her final few weeks of pregnancy, and divulges that a terminally-ill Roman (with Minnie) is planning to leave New York to visit a "few of his favorite cities" during the expected time of her baby's birth. As they leave for Kennedy airport for a trip to Europe, Rosemary (with Guy) bids her cheerful, well-wishing neighbors goodbye.

Later, Rosemary discovers that Guy has disposed of Hutch's book without her permission, arguing that he didn't want her to upset herself anymore. She reprimands him: "That's a terrible thing to do." Clues begin to pile up for Rosemary. She tosses her tannis-root charm into a sidewalk sewer drain on her way to a bookstore to purchase and read tomes on witchcraft: WITCHCRAFT, by Stella Lauden, subtitled Book of Ceremonial Magic:

Many people during that time died supposedly natural deaths. Since then it has been determined that the United Mental Force of the whole coven could blind, deafen, paralyze, and ultimately kill the chosen victim. This use of a United Mental Force is sometimes called a coven.

She feels menaced by the neighborhood's coven of witches that she thinks want her baby for their strange cannibalistic rituals. She also reads: "In some cults, it was believed that a personal possession of the victim was necessary and spells cannot be cast without one of the victim's belongings." She telephones Donald Baumgart (the voice of Tony Curtis), the blinded, embittered actor whose disability ("break" or "misfortune") enabled Guy to secure his acting position. Cleverly, she gets him to admit that following drinks with Guy, Baumgart's tie was deliberately traded/exchanged with Guy's - the actor's personal possession was later used to cast the deadly spell.

Rosemary hurriedly leaves her apartment with her "hospital suitcase" and rushes to see Dr. Sapirstein in his office for an unscheduled visit. While in the waiting room, she flips through Time Magazine's controversial, April 8, 1966 cover-story issue titled "Is God Dead?" [This is precisely the question that the film forces the viewer to ask. The title of the article itself was "Toward a Hidden God".] The talkative office receptionist (Marilyn Harvey) off-handedly mentions that Dr. Sapirstein also wears a smelly tannis-root charm - marking him as a member of the witch's coven:

He has the same smell once in a while, whatever it is, and when he does, oh boy.

Now more convinced than ever that her fears are real and that "all of them" are conspiring against her, Rosemary frantically excuses herself and walks to a corner phone booth where she places a call to Dr. Hill. When she reaches the doctor's answering service, she leaves an emergency message to return her call at the phone booth. When the doctor finally returns her call after a few suspenseful moments, she explains how her present doctor "isn't good": "He's been lying to me and giving me unusual kinds of drinks and capsules."

In a paranoid, tense tone with fears of a plotted conspiracy against her and her baby, she requests that he deliver her expected baby in the next few days, and voices her own fear that he may think: "Oh, my God. This poor girl has flipped." While demanding and begging that the doctor see her immediately, an ominous character [producer William Castle in a cameo appearance], resembling the back of Dr. Sapirstein, stands behind her outside the phone booth. After begging Dr. Hill for an appointment, he reluctantly agrees to an 8:00 pm office visit. When Rosemary eventually determines that the waiting man is not Sapirstein, she is relieved and exits the phone booth.