Rosemary Jones’s Baby is a romantic psychological comedy horror film set in modern day London, England in 1965. Rosemary Jones a bright but somewhat naive young housewife, and Guy, her husband, a struggling actor, move into the Bramford, an opulent but antiquated New York City apartment building in London, where Rosemary now works as a television producer.
On her 43rd birthday, Rosemary is awoken by Mrs. Gardenia, an elderly woman who had seemingly gone senile who reminds her that her time to have children is running out. Their friend Hutch tries to dissuade them from taking the apartment, informing them of some of the Bramford's rather unseemly history but, undeterred, Rosemary and Guy move into the building.
Three months later,Rosemary attends the funeral of Hutch, who is presumed dead after a plane crash. He leaves Rosemary a book about witchcraft and it is delivered to her at his funeral along with the cryptic message: “The name is an anagram”. While there she sees her ex, Mark Darcy and his new wife. They bump into one another somewhat awkwardly and then go their separate ways.
On the night she plans to conceive, Rosemary accepts Shazza's offer to take her away for the weekend. They go to a music festival where a drunk Rosemary crawls into a yurt she thinks belongs to her and Shazza, but actually belongs to the handsome stranger Jack she met earlier. The two have sex and in the morning Rosemary tries to not be disappointed when she wakes up alone.
Returning home Rosemary goes to the christening of Jude's youngest child where she is the godmother and Mark has been asked to be the godfather. Mark tells her that his wife has thrown herself to her death from the window of the Castevets' seventh-floor apartment and Rosemary has sex with him.
Rosemary suffers severe abdominal pains, loses weight again, becomes unusually pale, and craves raw meat and chicken liver. Shazza suggests that Rosemary might be pregnant, especially when she realises that Rosemary used old, expired condoms when having sex. Dr. Sapirstein insists the pain will subside soon, and assures her she has nothing to worry about.
Rosemary realises that she does not know who the father is and doesn't even have a way of contacting Jack, the handsome stranger, Guy, her husband or Mark Darcy. However, the abdominal pain suddenly disappears. Rosemary's health and appearance also improves quickly, and she, Jack, Guy and Mark are finally happy once again.
During labour Jack, Guy and Mark try to help Rosemary. However during labour she accidentally punches Jack in the nose. After Rosemary comes round from sedation, it is remarked upon that the baby, a boy, has “his father's eyes”, to which Rosemary protests that the baby's eyes are nothing like those Jack's, Guy's or Mark's.
It is then explained to Rosemary that child's real father is actually Satan. Rosemary sheepishly admits that she was so embarrassed about her one-night stand with Beezlebub that she didn't enter it into her diary. As the film was based on her diary entries, the audience finally realises why the film was so full of plot holes and missing explanations.