Rosemary awakes in bed to see Guy staring back at her. He tells her it's a boy, and when she asks if he's alright, Guy says yes. Rosemary dozes off and wakes to find Laura-Louise reading Reader's Digest with a magnifying glass near her bedside. When Rosemary asks where the baby is, Laura-Louise startles and nervously goes to fetch Dr. Sapirstein and Guy. When they arrive, Dr. Sapirstein tells Rosemary there were "complications, but nothing that can affect future births." Dr. Sapirstein explains the baby died from being birthed in the wrong position (an ectopic pregnancy, like Rosemary predicted), but could have been saved at a hospital, and blames Rosemary for fleeing their care.
As the men try to soothe Rosemary by saying she can always conceive again, she becomes increasingly agitated. she screams "It didn't die! You took it! You're lying! You're witches! You're lying!" before Dr. Sapirstein sedates her once more with the syringe. Rosemary later awakens, now wearing a white nightgown, with a soup and grilled cheese by her side. Guy shows up with a glass of water and a pill and tells her she suffered from the "pre-partum crazies," and that she'll get over it. Meanwhile, he teases her with his potential offers from Paramount and Universal, and the prospect of a new life in Beverly Hills, Los Angeles. Rosemary asks him to show her his left shoulder. He pulls his shirt off and lets her examine both shoulders, seemingly free of any marks.
Rosemary watches a weatherman on TV talk about a heat wave and humidity spike, and then hears a baby crying. She turns off the air conditioning unit and listens for it through the walls. Laura-Louise enters holding a tray with a glass of water, a breast pump, a cup, and a daily pill, which Rosemary stashes in the bed-frame instead of consuming. Guy comes home and tells Rosemary new people moved in "up on eight." Rosemary asks if they have a baby, and when Guy says yes, Rosemary tells him she heard it crying. Laura-Louise rebukes Rosemary when she goes to put a used spoon in the cup of breast milk. We see that several pills are now stashed in Rosemary's bed-frame.
Now in a blue gown, Rosemary puts on slippers and wanders out into the hallway, opening the hall closet originally blocked by the giant secretary in the film's opening scene. Rosemary removes everything from the closet and then dismantles the shelves one by one. Inspecting the closet's back wall, Rosemary discovers it is a panel and pushes her way into a dark secret passage. Through a keyhole in a door in the wall behind the panel, she realizes she can see into the Castevets' apartment. Rosemary retreats into her own kitchen and grabs a knife just as Guy enters through the front door. Rosemary hastily hides in the shadows, rocking a bassinet in the process, which she steadies with her knife. Guy grabs an ice bucket and leaves, and Rosemary ventures back through the secret passage into the Castevets' apartment. She gazes at an oil painting of a cathedral in flames before wandering down the hallway, where the voices of Minnie and others can be heard.
At the end of the hallway is a giant dining room where over a dozen people have gathered, including the Castevets, Laura-Louise, Guy, an unknown Japanese man, and others. A giant black bassinet is visible in one of the room's large window bays, with an upside down crucifix hanging from the top. The coven gradually take notice of Rosemary gripping her knife, and Laura-Louise screams. One woman advises Rosemary to go back to bed. When Roman moves to speak, Rosemary says, "Shut up. You're in Dubrovnik. I don't hear you." Rosemary wanders over to the bassinet and pulls back the taffeta along the sides; when she sees what's inside, her face blanches and she cries, "What have you done to it? What have you done to its eyes?"
Roman says he has his father's eyes, and Rosemary asks pleadingly how that's possible if Guy is his father. Roman tells her that Satan is his father, and his name is Adrian. Roman leads the others in a chorus of "Hail Adrian! Hail Satan!" and Minnie explains to Rosemary that she should feel honored for being selected to be the mother of the devil's only living son. Rosemary denies it, but they urge her to look at the baby again. When she drops her knife, it sticks in the floor and sways, and Roman cries out, "God is dead! Satan lives! The year is one." Rosemary collapses out of shock, and Roman urges her to take on the role of mother, especially given the elderliness of Minnie and Laura-Louise. Rosemary looks up to find the Japanese man grinning back at her while inserting film in his camera, and when she cries out for God, Laura-Louise snaps, "Shut up with your 'Oh God's or we'll kill you."
Guy tries easing Rosemary by saying she hasn't really been injured and compares the situation to a mere miscarriage, but she spits in his face. Roman then introduces Guy to a foreign, dignified-looking man named Argyrov Stavropoulos. When the baby begins to cry, Rosemary wanders near the bassinet and criticizes Laura-Louise for rocking it too fast. Roman orders a belligerent Laura-Louise to let Rosemary rock the baby instead. Rosemary then asks, "Are you trying to get me to be his mother?" to which Roman responds, "Aren't you his mother?" Rosemary approaches the bassinet and begins to rock it gently as the various coven members gather around her. Everyone looks on approvingly and the Japanese man takes several photographs. Rosemary gazes ambiguously down at the baby's face. As the lullaby from the film's opening scene begins to play, the image of her face fades to an exterior view of The Bramford, and then to credits.
Upon awakening, Rosemary's premeditated, inhumane captivity clashes with Laura-Louise's oafish, bumbling demeanor. Laura-Louise perusing Reader's Digest emphasizes that the coven is ordinary and modern, rather than weird and atavistic. Dr. Sapirstein ironically tells Rosemary that the baby died due to an ectopic pregnancy, which Rosemary had guessed early on, and he and Guy continue to limit Rosemary's agency over her own body by keeping her confined to the bed and instructing her to take daily pills, which she begins hiding instead of consuming.
Rosemary's suspicion that Guy wears pajamas at night to hide a mark is her only incorrect one, and the film's one major red herring. In this film, witches have no marks, implying that there is nothing separating the coven from the rest of society. They hold prestigious jobs, wield enormous influence, and move freely around the world. Rather than reduce witchcraft to an essential identity or mark, Polanski instead emphasizes the theme that witchcraft is theatrical and performative, with the coven demanding that Rosemary play her "role." The Bramford itself symbolizes this theme—at once a coven of witches, and a hotbed for the theater industry.
Wondering how Guy and the coven gained entry into the apartment after she chained the lock, Rosemary correctly guesses that the hall closet contains a secret passage, referencing the film's very first scene. In Gothic narratives, hidden spaces and passageways are metaphors for family secrets. Rosemary's discovery of the secret passage and the shot of her looking through the keyhole into the Castevets' apartment symbolize her proximity to discovering the truth about her baby.
When a panic-stricken Rosemary escapes and finds the coven members gathered around the Castevets' large drawing room, they for the first time do not know how to act in her presence, and are at first merely silent. The black bassinet—a massive crib draped in taffeta and built into the crevice of the bay window—ominously contrasts with Rosemary's modest, white one. Polanski emphasizes psychological perception over objective reality in the scene, not showing the viewer the baby, instead showing Rosemary's shocked face. Her line "What have you done to its eyes?" foregrounds the theme of perception as well, referencing the shot of Guy's eyes morphing into Satan's yellow eyes on top of her during the rape scene.
Roman then dramatically discloses his family secret, which the Gothic energy of the narrative has worked tirelessly to conceal: Guy is not the baby's father— Satan is. The baby's name will be "Adrian," not "Andy," like Rosemary desires. The Japanese man taking photographs indicates that the coven is a thoroughly modern, global conspiracy, as does the arrival of a foreign man named Argyrov Stavropoulos. The intrusion of strangers suggests that the baby's birth is a historic event, which Roman underlines when he repeats a modified version of his New Year's toast, "God is dead! Satan lives! The year is one!"
The final beat of the film implies that Rosemary may acquiesce to being the child's mother after all. In keeping with the theme of religion as performance, the coven eagerly encourages Rosemary to play her part in this family drama they have cast her in—to be the mother of the child of Satan. Rosemary gently rocking the baby's bassinet lends a new layer of meaning to the child's lullaby sung by Mia Farrow over the opening credits, which re-emerges here. The last image we see before the credits returns the viewer to the primary symbol of the film: The Bramford, the Gothic castle that has shrouded all these events in secrecy from the eyes of the outside world.