The story begins two years ago, narrated by Rainy March, a 25-year-old Book Witch. While trying to give flea medicine to her feline familiar, Koshka, she is interrupted by a call on the coven hotline. Her grandfather, Sullivan “Pops” March, also a Book Witch, answers and learns of a new case from their coven leader, Dr. Regina Fanshawe. The mission involves Rainy’s favorite fictional character, a private eye known as the Duke of Chicago, who is in danger within his noir mystery series.

Pops initially forbids Rainy from taking the case, deeming it too dangerous. Rainy argues that she is the most qualified for it, revealing that the Duke of Chicago books comforted her as a teenager after her mother’s death. She knows everything about the series world and the Duke’s past cases. When a fellow Book Witch, Professor Dodsworth, delivers the mission file—a paperback copy of Empty Graves, the relevant Duke book—Rainy convinces Pops to let her go. Dr. Fanshawe reluctantly agrees, but she makes Rainy recite the eight foundational rules for Book Witches, the Eight Black and Whites, which include a strict prohibition against falling in love with a fictional character.

The mission briefing reveals that a page in Empty Graves has gone blank, indicating the Duke has been taken captive and removed from his plot. To blend in and move easily through the era, Rainy disguises herself in a man’s suit. Using her magic umbrella, she opens a portal into the book and, with Koshka, jumps into the world of 1930s Chicago.

Rainy and Koshka arrive in an alley outside a speakeasy called the Bathtub, which Rainy recognizes from the series. After gaining entry to the club, Rainy sends Koshka through the crowd to find the Duke of Chicago. The cat quickly locates a hidden back room. Inside, Rainy finds Duke tied to a chair, apparently unconscious. She is struck by his handsomeness just as he reveals that he is awake.

Duke immediately sees through her disguise as a young man. He is surprised to discover that she knows he hides a knife in his sock, but she makes up an excuse for her knowledge. He refuses to let her retrieve a knife from his sock until she tells him her name. When she says she cannot, he dubs her “darling.” As Rainy cuts his bonds, the Duke describes his kidnapper, a man smelling of smoke whom Rainy recognizes as her old nemesis, a Burner named X.

The Duke flirts with Rainy, asking a barrage of questions. After she frees him, he kisses her hand, prompting an overwhelmed Rainy to break protocol and tell him her real name.

Rainy tries to get the newly freed Duke of Chicago back to the Empty Grave plot, which involves tracking a socialite named Edith King, but he is more focused on seducing her. He calls Rainy the “realest girl” he has ever met. She tells him that she is a Book Witch, and to prove it, she reads the title of the book he was most recently reading from an impression left in his eyes.

As they try to leave, X corners them, holding a pistol. He reveals his plan to use gasoline to burn the story from the inside, which would erase every copy of the book from existence and memory. To torment the Duke, X reveals that he is a fictional character. Rainy is forced to confirm this and explains her role as a Book Witch. X declares the Burners’ intent to destroy all books they deem “garbage,” leaving only the “classics.”

As X prepares to ignite the fuel, Rainy makes a desperate plea for the Duke to escape and finish his story. She confesses how much his books have meant to her. The combination of the Duke’s newfound self-awareness and her emotional appeal causes the story world to shake violently. As the building collapses, X escapes. The Duke grabs Rainy, wishes them to be safe together, and his newfound self-awareness gives him the power to shape the narrative and instantly transport them away.

Rainy, Duke, and Koshka materialize in the Duke’s private detective office. Rainy explains that his self-awareness has given him the power to alter his own story. He is moved upon seeing a burn scar on her arm from a previous mission. As he processes the news of his fictional nature, he finds comfort in the idea that the tragic deaths of his three brothers never actually happened.

The Duke questions the Book Witch rules that forbid a relationship between them. She tells him that when he returns to his book, his memory of their encounter will be erased. He asks for one hour with her, and Rainy reluctantly agrees. Curious about a long-standing fan debate, she asks what he keeps in his wall safe. To show her, he transports them to his penthouse apartment. In exchange for the secret, he asks for her hat. After she gives it to him, he admits that he did it just so that he can see her with her hair down.

Duke opens his safe and removes a hatbox containing mementos of his deceased brothers: a wristwatch, a book of poetry, and a toy horse. He and Rainy bond over their shared experiences of loss and grief. When the Duke questions the purpose of finishing his story if the characters are not real, Rainy reads him the book’s powerful conclusion, where he helps a woman escape an abusive husband. She explains how fictional acts like these inspire real-world revelations and change, convincing him of his story’s importance.

Before they leave, the Duke gives Rainy a gift: His mother’s mourning ring, which features a forget-me-not flower. He places a tiny slip of paper with her own mother’s name inside its locket and slips the ring onto Rainy’s finger. He then drives them back to the alley.

As Rainy prepares to return to her world, he begs her not to perform the spell that would make him forget her. She agrees on the condition that he promises to always complete his stories. He agrees, kisses her, and confidently tells her he will see her soon. Rainy and Koshka return home.

Rainy successfully restores Empty Graves, earning praise from Pops and Fanshawe, but she is miserable and misses Duke intensely. She confesses to Pops that he was right about the danger of meeting one’s heroes, but only because Duke was even better than she had imagined.

Her intense longing continues for eight days. On the eighth morning, she awakens to find Duke sitting on her bed, holding her copy of the first Nancy Drew novel, The Secret of the Old Clock, which her mother left her. He explains that he simply appeared there, and Rainy realizes that her powerful yearning must have pulled him from his book into the real world. Despite her panic over breaking the rules, Duke reframes their situation as a mystery for them to solve together. When he pulls her into a kiss, she gives in, agreeing to try to find a way for them to be together.