After being brought into the Castevet apartment that evening, the Woodhouses are served and toasted with vodka blush drinks by their congenial hosts. Roman boasts of his world travels: "You name a place and I've been there...I'm seventy-nine, and I've been going one place or another since I was ten. You name a place, and I've been there." During the conversation over a steak dinner, they talk about the Pope's impending visit and fatuous religiosity:

Roman: No Pope ever visits a city where the newspapers are on strike.
Minnie: I heard he's gonna postpone and wait till it's over.
Guy: Well, that's show-biz.
Roman: (chuckling with his wife) That's exactly what it is. All the costumes or rituals, all religions.

Slightly offended and "uncomfortable" by anti-religious references to the Pope, Rosemary mentions her Catholic upbringing: "I was brought up a Catholic. Now I don't know." Roman presses his point about the papacy: "You don't need to have respect for him because he pretends that he's holy...A good picture of the hypocrisy behind organized religion was given I thought in Luther." [Significantly, Luther rebelled against the Catholic church, citing hypocrisy. The Castavets do the same, criticizing the church as hypocritical as well.] The dinner talk then turns toward Guy's acting career and methods (not coincidentally as an understudy for Albert Finney in Luther), that Roman praises and characterizes as authentic:

You have a most interesting inner quality, Guy. It appears in your television work too. It should take you a long way, indeed, provided, of course, that you get those initial breaks.

While assisting Minnie as she washes the dishes, Rosemary describes her sisters and herself as "fertile" mothers: "I've got sixteen nieces and nephews." After returning home, Rosemary mentions that she observed how the Castevets strangely removed their pictures from the wall, leaving hooks and clean spaces. [Guy and Roman are left alone to discuss private matters -- presumably this is when Guy enters an agreement with the Castavets to join the coven and offer Rosemary as the new prospective mother for their group. In exchange, the Castavets will use witchcraft to get him promising stage work. Guy has literally sold his soul for success.]

The next evening while Guy is visiting with Roman next door by himself, Minnie introduces another odd-ball, 12th floor neighbor Laura-Louise (Patsy Kelly) to Rosemary. During their conversation, Rosemary is presented with a noxious tannis-root charm - the one worn by Terry, and according to Laura-Louise: "You'll get used to the smell before you know it." When Guy returns, he also proposes that she wear it, but Rosemary places it inside a jewelry box instead. In an unusual circumstance, Guy soon acquires an acting role - the actor who had been chosen over him for a key role in a play is unexpectedly and suddenly going blind. Guy remarks about his instant success: "That's a hell of a way to get it." [Ambitious, Guy has made a pact "with the devil" in exchange for success as a Broadway actor. The Satanists convinced him of their power through blinding his competition in an acting job.]

According to Rosemary, her husband is "suddenly very hot" but has, like other well-known actors, become "self-centered," "vain," "pre-occupied," and self-absorbed. One day, Rosemary arrives home and finds bouquets of red roses placed throughout the house. Guy apologizes for being a "creep" and humbly tells her: "Even if I'm Mr. Yahama for the rest of my days, I'm gonna stop giving you the short end of the stick." Then, he abruptly proposes, "Let's have a baby, all right?" and suggests circled days on the calendar when they should start having sex - October 4th or 5th, 1965. [The Satanists have determined that these are prime days for Rosemary's ovulation.] Soft music playing on the phonograph, a cozy fire in the fireplace, and a candlelight dinner serve as a prelude for an evening of love-making. Minnie briefly stops by (to Rosemary's relief) and brings a strange dessert ("one of her specialties") for each of them - two tall white cups filled with chocolate mousse topped with whipped cream. After a few bites, Rosemary detects a "chalky undertaste" and quits eating, but Guy insists that she have more to remain neighborly: "The old bat slaved all day. Now eat it!...It's delicious." While he is changing the record, she scoops out the remaining mousse into her napkin to discard it.

Later, Guy watches TV with live coverage of the Pope mobbed by crowds at Yankee Stadium. [Note: The Pope at the time, Pope Paul VI, visited New York's Yankee Stadium (where he gave a Eucharist Mass to 90,000 people) on October 4, 1965. It was the first time a Pope had ever visited the Western Hemisphere.] "Dizzy," woozy and disoriented after eating some of Minnie's chocolate mousse (laced with sleeping powder), Rosemary is put to bed. In an hallucinatory scene that impersonates a Black Mass, she imagines being on a mattress drifting on the ocean, and then as a passenger on a presidential yacht. A John F. Kennedy [the first Catholic US President] look-a-like dons a sailor's cap, and then transforms into an older man who points out to sea. Rosemary asks a passenger later why Hutch, who is left on the dock, isn't allowed on board. She's told it is because he isn't Catholic

Guy completely undresses her until she is shivering and naked, and then abruptly is seen wearing a bathing suit. Images include the Birth of Man paintings on the Sistine Chapel ceiling, a typhoon at sea, and a naked descent into the hold of the ship, where a fire burns and Rosemary lies on a mattress. She is surrounded by many chanting, naked figures, including Guy, Roman, and Minnie. The latter individual assures everyone: "As long as she ate the mousse, she can't see nor hear. She's like dead now." A bloody-red liquid is painted on Rosemary's bare chest. A person resembling Mrs. John F. Kennedy (Patricia Ann Conway) who wears a white diaphanous gown descends a staircase and suggests that her legs are tied down in case of convulsions. Attendants spread her legs apart and bind them.

In her dream-like sleep, Guy begins making love to her, but then his appearance changes into a grotesque beast-like figure resembling the Devil, with yellowish eyes and clawed, scaly hands. He strokes the length of her body with his hairy claw. While being 'raped' during this horrific copulation scene, she realizes:

This is no dream, this is really happening.

Rosemary senses and suspects that something is strangely wrong with her impregnation.

[Although it has been widely claimed that the Church of Satan's founder and author of 'The Satanic Bible' Anton LaVey played the uncredited role of the Devil during this impregnation scene, and also served as technical advisor for the film, it's highly unlikely that these claims are true. There has always been an attempt to link LaVey to the film and to the subsequent Manson murders, because one of LaVey's ex-followers (Susan Atkins) turned out to be a member of the Manson Family, who ultimately murdered Polanski's young wife, Sharon Tate.]

The Pope (Michael Shillo) appears in her vision with a red suitcase, remarking: "They tell me you have been bitten by a mouse." [The 'mouse' statement is a deliberately-corrupted reference to the 'mousse' she was given to make her fertile. "Bitten" presumably refers to becoming pregnant.] Apologetic for not attending the ceremony in Yankee Stadium and guiltily acting like a lapsed Catholic, Rosemary is allowed to kiss the Pope's ring finger (a tannis-root charm!) to receive forgiveness.

The next morning, Rosemary questions mysterious scratches she finds on the side of her body. She is appalled that Guy admits making love to her while she was passed out - supposedly from mixing alcohol - "It was kind of fun in a necrophile sort of way." She remembers something quite different from Guy's recollection - a demonic, inhuman rape:

Rosemary: I dreamed someone was raping me, I think from someone inhuman.
Guy: Thanks a lot. Whatsa matter?
Rosemary: Nothing.
Guy: I didn't want to miss the night.
Rosemary: We could have done it this morning or tonight. Last night wasn't the only split-second.
Guy: I was a little bit loaded myself, you know.

Guy's behavior reverts again, and he is distant and preoccupied. As "Fur Elise" plays in the background during breakfast a few weeks later, Guy remarks that her expected period "was due on Friday." Shortly afterwards, Rosemary visits her gynecologist/obstetrician Dr. Hill (a young Charles Grodin) for a blood/pregnancy test and is informed that she is definitely pregnant, with a due date of June 28th. Rosemary loses a quarter bet with her husband about the prognosis, and then proposes: "Let's make this a new beginning, OK, a new openness in talking to each other, because we haven't been open." Overjoyed by the expectation of a child, Guy runs next door to inform the Castevets. They burst into the apartment with a wine bottle to congratulate Rosemary on the "good news." Minnie insists that she see their recommended, upper-class obstetrician, Dr. Abe Sapirstein (Ralph Bellamy), instead of continuing to see Dr. Hill. They toast "to a fine, healthy baby." That night, Rosemary lies awake, contemplating a name for her baby, and she begins wearing the tannis-root brooch. [Tannis-root is a fictional herb.]

In the office of Dr. Sapirstein, he advises her against reading books or listening to friends due to the fact that each pregnancy is unique: "No two pregnancies are ever alike." To keep her further isolated from traditional sources of sustenance and commercially-sold vitamins, he also suggests that she drink a daily, herbal mixture concocted by Minnie Castevet that "will be fresher, safer, and more vitamin-rich than any pills on the market." According to Minnie, its ingredients are "a raw egg, gelatin, herbs...along with some other things." At this point, about half-way through the film, Rosemary suddenly appears with a shorn hairstyle: "I've been to Vidal Sassoon."

The film plays upon the fears and anxieties of most pregnant young women as they approach childbirth and experience hormonal changes. She suffers sharp pains and wonders if she has an ectopic pregnancy. Sapirstein explains the pains as a "natural expansion of the pelvis. You can fight it with ordinary aspirin." More symptoms appear - she becomes pale, gets blotches under her eyes, has difficulty sleeping and loses weight - confused and tormented by her pregnancy. She develops an appetite for very rare steak meat. One day when Hutch visits, he believes she looks "terrible," asking: "You're not on one of those Zen diets, are you?" He worries about her declining health and weight loss: "Pregnant women gain weight, they don't lose it." Rosemary explains her belief in natural ingredients for pre-natal care: "I like the idea of having everything fresh and natural. I'll bet expectant mothers chew bits of tannis-root when nobody'd even heard of vitamin pills," and she shows a skeptical Hutch her tannis-root good-luck charm around her neck. He believes the awful-smelling substance in the charm is more like "mold or fungus of some kind" and vows to do some research on the root.

After Hutch is introduced briefly to Rosemary's strange neighbor Roman Castevet, both notice his "pierced ears" and "piercing eyes." Rosemary explains how the Castevets are close to Guy and have "become parent figures" for him. As Hutch leaves, he can't locate one of his gloves that he put in their closet when he arrived. Later, he calls and sets up a rendezvous with Rosemary at the Time-Life Building in downtown Manhattan. At the appointed time the next morning (about two weeks before Christmas), it is unusual that Hutch doesn't appear. A phone call to his apartment reveals that Hutch was "taken ill...and in a deep coma at St. Vincent's Hospital."

At a New Year's Eve party held in the Castevet's apartment to bring in the new year of 1966, Rosemary discusses her physical condition with her doctor: "It's like a wire inside me getting tighter and tighter." She is introduced to one of the many elderly guests - Dr. Shand (Phil Leeds), a famous dentist who made the chain for her charm. As the New Year is toasted at midnight, Roman exclaims: "To 1966, the Year One." Rosemary is paranoid about what is happening to her - while ravenously eating chicken livers in her kitchen, she spies her reflection in a shiny toaster and vomits into the kitchen sink.