That evening in Dr. Hill's office, Rosemary explains how her husband Guy is integrally involved in the coven's plot. She tells him how he lied to Dr. Hill about them leaving the city for Hollywood, and now sleeps in pajamas to hide a suspicious mark of coven membership. The tormented mother rambles incoherently about her fantastic suspicions that her next-door neighbors hold rituals, and that her husband participates with them:

You can hear them singing through the wall...Guy said it was Dr. Shand, one of these people playing a recorder. Now, how did he know it was Dr. Shand unless he was there with them. They're very clever people. They planned everything right from the beginning. They probably made some sort of deal with Guy. They gave him success and he promised them our baby to use in their rituals. I know this sounds crazy, but I've got books here. Look. There was another actor like him, Donald Baumgart and they put a spell on him. They cast a spell on him and made him blind so that Guy could get his part. Look, here. (She shows Dr. Hill the Witchcraft book excerpt) I had this friend, Edward Hutchins. Maybe you heard of him, a writer. He wrote stories for boys. Anyway, he was my good friend since I first came to New York...Anyway, once Mr. Hutchins came to visit me...It was the time I was having this pain, Doctor. I was suffering severe - you can't imagine how much I was suffering. And they wouldn't help me, nobody would. They were giving me a drink with tannis-root in it, also witches' stuff, tannis-root. Hutch came and he immediately saw something was wrong. He knew about witches, you see. Suddenly, Guy rushed home with his make-up still on, which he never did. They probably called him to come home and steal one of Hutch's belongings - which he did. Took his glove. And they put a spell on him too. Put him in a coma. Three months later, he died. Now, maybe all of this is coincidence, but one thing is for sure, they have a coven and they want my baby.

Rosemary sighs when Dr. Hill apparently concurs with her assessments and suggests admitting her into Mt. Sinai Hospital that evening. While she lies down to rest in an adjoining office, he purportedly makes calls to arrange for her hospital admission. Rosemary assures her unborn child: "God bless Dr. Hill. Everything's gonna be okay now, Andy or Jenny. We're gonna be in a nice clean hospital with no visitors." Instead, Dr. Hill contacts Dr. Sapirstein and Guy, explaining that she is experiencing a mental breakdown - and normal pregnancy hysteria. While deliriously dreaming about holding her baby amidst admiring onlookers, the bright lights are turned on as Sapirstein and Guy are brought into the room:

Come with us quietly, Rosemary. Don't argue or make a scene. Because if you say anything more about witches or witchcraft, we're gonna be forced to take you to a mental hospital. You don't want that, do you? So just put your shoes on.

She is assured that no one will hurt her or the baby, and then taken home to her apartment. In the lobby, she deliberately empties the contents of her purse onto the floor, and escapes up the elevator to the seventh floor. Already in a frenzied state and feeling betrayed, she immediately goes into labor in the hallway before locking herself securely, she believes, inside her apartment. She phones a friend named Elise Dunstan but only reaches the babysitter. Turning around, she notices that the doctor, Guy, and others have gained entrance into her bedroom (through the closet). As she resists and screams, "Help me! Somebody help me!" she is wrestled on the bed and given a sedative shot to calm her down.

Following labor and the birth of a boy, she gains consciousness and is soon told by Sapirstein that the baby died at birth because of delivery complications: "It was in the wrong position. In a hospital, I might have been able to do something about it, but you wouldn't listen." Reacting hysterically, she doesn't believe it and accuses everyone of being witches who have stolen her baby:

You're lying. It didn't die. You took it. You're lying. You witches. You're lying! You're lying! You're lying! You're lying!

Diabolical and still ambitious, Guy attempts to convince Rosemary of her temporary insanity and their promising future together in Beverly Hills, California:

I know where you got the idea that Minnie and Roman were witches but how come you thought Abe and I joined the party, huh? Let's face it, darling, you had the pre-partum crazies. And now you're gonna rest and you're gonna get over them....From now on, everything's gonna be roses. Paramount's within an inch of where we want 'em, and suddenly Universal's interested too. And we're gonna blow this town. We're gonna be in the beautiful hills of Beverly with a pool and a spice garden - the whole schmeer. And the kids too, Ro. Scouts honor, you heard what Abe said? I've gotta run now and get famous.

She suspects the truth about her kidnapped baby later that night when she hears the distant sounds of a squawling infant through the wall in the Castevet's next-door apartment (they are supposedly away in Europe). Ironically, she is expected to express breast milk and continue taking sedative pills, while closely watched by Laura-Louise and Mrs. Gilmore (Hope Summers). Late one night, Rosemary sneaks into the Castevet's apartment through the closet passageway - with a kitchen knife upraised in her hand. There, she finds a coven of witches (including Guy), surrounding a black-draped baby cradle to pay their respects - it is a macabre presentation of the Adoration of the 'Satanic' Magi, with visitors coming to view the babe from all parts of the globe. A framed portrait of Adrian Marcato hangs above the fireplace. She approaches the black bassinet expecting to see her own human child. In a frightened, angry tone, she asks about her baby cradled there with an inverted cross hanging above - fully expecting the child to have "Guy's eyes":

Mrs. Gilmore: Rosemary! Go back to bed. You know you're not supposed to be up and around.
Japanese man: Is the mother?
Roman: Rosemary -
Rosemary: Shut up.
Roman: Rosemary -
Rosemary: Shut up! You're in Dubrovnik. I don't hear you. (She slowly walks over to the cradle, sees her child in the bassinet - her eyes widen in terror) What have you done to it? What have you done to its eyes?
Roman: He has his father's eyes.
Rosemary: What are you talking about?! Guy's eyes are normal! What have you done to him? You maniacs!

And then she is told the truth - she realizes that she was impregnated by the Devil and the baby is the offspring of Satan and Rose-Mary (a variant on the name Mary in the Biblical story). Her baby's eyes are NOT human:

Roman: Satan is his father, not Guy. He came up from hell and begat a son of mortal woman. (Coven members cheer 'Hail, Satan!') Satan is his father and his name is Adrian. He shall overthrow the mighty and lay waste their temples. He shall redeem the despised and wreak vengeance in the name of the burned and the tortured. Hail, Adrian! Hail, Satan! (Others in the room repeat the incantation.) Hail, Satan!
Minnie: He chose you out of all the world - out of all the women in the whole world, he chose you. He arranged things, because he wanted you to be the mother of his only living son.
Roman: His power is stronger than stronger! His might shall last longer than longer.
Japanese man: Hail, Satan!
Rosemary: No! It can't be! No!
Minnie: Go look at his hands.
Laura-Louise: And his feet.
Rosemary: (distraught) (She remembers her impregnation - conveyed through a super-imposed image of the devil's yellow eyes that appears) Oh, God! (She drops her knife - it sticks point-first into the Castevets' wooden floor.)
Roman: God is dead! Satan lives! The year is One, the year is One! God is dead! (A chorus of deafening 'Hail, Satan!' is heard) (Roman walks over to Rosemary) Why don't you help us out, Rosemary? Be a real mother to Adrian. You don't have to join if you don't want to. Just be a mother to your baby. Minnie and Laura-Louise are too old. It's not right. Think about it, Rosemary.
Rosemary: (crying) Oh, God!
Laura-Louise: Aw, shut up with your Oh, Gods or we'll kill ya - milk or no milk!
Mrs. Gilmore: You shut up. Rosemary's his mother, so you show some respect.
Roman: (to Argyron Stavropolous, one of the guests who has just arrived) Come my friend. Come see him. Come see the child.
Guy: (kneeling by Rosemary) They promised me you wouldn't be hurt and you haven't been...really. I mean, supposing you had the baby and you lost it? Wouldn't that be the same? And we're getting so much in return, Ro. (She spits in his face.)
Roman: Oh, Guy, let me introduce you to Argyron Stavropolous.
Argyron: How proud you must be. Is this the mother?
Minnie: (She gives Rosemary a cup and saucer.) Here, drink this. You'll feel a little better.
Rosemary: What's in it, tannis root?
Minnie: Nothing's in it, just plain ordinary Lipton's tea. You drink it. (The baby starts to cry. Rosemary watches as Laura-Louise roughly rocks the bassinet, and then slowly walks over.)
Laurie-Louise: (To Rosemary) Get away from here! Roman!
Rosemary: You're rocking him too fast.
Laurie-Louise: Sit down. (To Roman) Get her out of here. Put her where she belongs.
Rosemary: You're rocking him too fast. That's why he's crying.
Laura-Louise: Oh, mind your own business.
Roman: Let Rosemary rock him. Go on, sit down with the others. Let Rosemary rock him.
Laura-Louise: Well, she's liable to -
Roman: Sit down with the others, Laura-Louise. (She joins the others, but sticks her tongue out at Rosemary as she passes by.) (To Rosemary) Rock him.
Rosemary: Are you trying to get me to be his mother?
Roman: Aren't you his mother?

[Note: The birth of Rosemary's baby, the Anti-Christ, is in "Year One." It is purely coincidental, yet accurate by Church of Satan standards, to call 1966 Year One. This is a self-conscious, parallel reference to the year of Jesus Christ's birth, also in Year One, A.D. - Anno Domini, the year of Our Lord.]

Although Rosemary rejects the devil-worshipping coven of witches, she accepts the reality of the situation and shows an instinctive mothering role and maternal instinct toward the baby in the final scene - she gently rocks the child to sleep. Others gather around to watch, as Dr. Sapirstein arrives, and the Japanese man with a camera [a stereotypical portrayal] takes photographs. She smiles hesitantly toward her offspring.

The innocent lullaby from the film's opening is repeated, to lull the Anti-Christ, Satan's child, to sleep and hush its crying. The camera moves to the curtained window and then to the outside of the apartment - ending the film with the same, slow pan across the urban rooftops that opened the film. The horror within is camouflaged.