The following version of this book was used to create the guide: Prose, Nita. The Mystery Guest. Ballantine Group, 2023.

Prose's novel is divided into 30 chapters, with roughly half taking place in the present day and half taking place 19 years earlier. Molly Gray, the 29-year-old Head Maid at the fictional Regency Grand Hotel, is the novel's first-person narrator and protagonist.

Molly's narration begins with her recalling a parable her grandmother (Gran) once told her about a maid being fired because her employers believe she stole a silver spoon. In reality, the spoon is stolen by a rat, whose mummified body and the missing spoon are eventually discovered under the floorboards. Molly comments that Gran, who died several years ago, taught her everything she knows, personally and professionally. Because of Gran's guidance, Molly has risen to the rank of Head Maid. She takes pride in her hotel cleaning work, which she knows some people would think of as lowly.

When J. D. Grimthorpe, a famous mystery novelist, books the hotel's Grand Tearoom to make a rare public announcement in front of VIP guests and reporters, Molly attends the pre-event signing for hotel staff. She is shocked that he does not remember her. At the podium, Grimthorpe delays his announcement to prepare a cup of tea from a cart that Lily, Molly's shy but capable Maid-in-Training, wheels out for him. After stirring honey into his tea and drinking it, Grimthorpe seems to asphyxiate. He drops to the tearoom's herring bone–patterned floor. His secretary, Serena Sharpe, takes his pulse and declares him dead.

Following the death, Detective Stark interviews Lily in Mr. Snow's office. Molly feels protective of Lily, who will inevitably be blamed for the death. Stark wrongly accused Molly of killing a guest, Mr. Black, several years earlier. When Molly points out to Stark that Grimthorpe had cue cards at the podium, Stark reluctantly lets Molly assist her with the case.

While walking Stark through the morning's events, Molly is not upset to see that Grimthorpe is dead. She conceals the fact that she knew Grimthorpe when she was ten years old because Gran worked as a maid at his mansion. She also conceals the fact that Gran had a reason to despise Grimthorpe. Molly suspects Serena Sharpe of having murdered her boss. She finds it suspicious that Serena checked out of the hotel and that the police did not detain her. Mr. Preston, the hotel doorman who eats dinner with Molly and her boyfriend Juan every Sunday, is eager to tell Molly something. However, she does not pick up on his non-verbal cues and puts off meeting with him until Juan returns from Mexico, where he is visiting family.

In the book's "Before" chapters, Gran pulls Molly out of school because her principal wants to hold her back a year, citing underdeveloped social skills. After polishing the patina off a Fabergé egg (unwittingly diminishing the antique's value), Molly spends half her time at the mansion polishing Mrs. Grimthorpe's silverware and half her time reading Great Expectations in the upstairs library. Grimthorpe's study is in an adjoining room, accessed through a secret door behind a bookshelf. Although she initially fears Grimthorpe, Molly and Grimthorpe strike up an unlikely friendship as he shares details about his writer's block and lingering grumpiness after having gotten sober with Gran's help. Molly mentions a fact she recently learned from Gran—that the lye in silver polish can burn skin. This gives Grimthorpe the idea that a character in his novel should dissolve a murder victim's body in lye.

In the present, Molly is surprised when Serena arrives at the hotel to collect items she left in the hotel's display case to promote the Grimthorpe event. Cheryl (helped by Lily, whom she pressures) pulls the fire alarm; in the confusion, Cheryl steals Serena's box of Grimthorpe ephemera, which includes a first-edition copy of his bestselling book, The Maid in the Mansion, the novel Molly unwittingly helped Grimthorpe with when she was a child. Angela, who runs the hotel's Social Bar and Grill, lies to members of Grimthorpe's fan club, the Ladies Auxiliary Mystery Book Society, claiming that Molly is an undercover detective. Beulah and Gladys speculate that Birdy may have poisoned Grimthorpe because she was jealous of Beulah, who thinks of herself as Grimthorpe's biographer. The guilt of lying to the LAMBS women prompts Molly to go to Stark at the police station. She confesses to having known Grimthorpe as a child. Stark, annoyed that Molly is not there to confess to the murder, says she does not care. Stark tells Molly Grimthorpe's honey pot was laced with antifreeze, which has a sweet taste. Whoever killed him knew that he had weak internal organs from years of alcohol abuse.

Molly sees Mr. Preston pawning a first edition of The Maid in the Mansion. She meets with him at Olive Garden. Preston is upset when she accuses him of being involved in the theft or the murder. He explains that he worked as gatekeeper at the Grimthorpe mansion and Grimthorpe gifted him the first edition. Molly realizes she once gave Preston a rose to thank him for lending Gran money when she could not pay their rent, the rent money envelope having been stolen by Molly's estranged mother, who was a drug addict.

In the past, Molly witnesses Grimthorpe sexually harass Gran. When Gran refuses to quit her job, Molly devises a scheme to get her fired (and thus keep her safe from Grimthorpe's abuse). Molly pretends to steal the Fabergé egg. Upon being caught, Molly admits to her crime. Mrs. Grimthorpe fires Gran and bans both of them from returning to the house.

Angela discovers that whoever stole the items from Serena has sold them on a memorabilia website called KultureVulture.com. Briefly suspecting Lily, Molly determines that Cheryl is the one who listed the items, which have all sold. Grimthorpe's cue cards from the podium were also sold, but Cheryl says she did not make note of what was written on them. She has blurred the writing for the listing photos. However, she did not blur the contents of one of Grimthorpe's Moleskine notebooks because the writing was impossible to decipher anyway. Molly realizes that Grimthorpe's notebooks were always illegible, meaning his secretary must have been doing the actual writing for his novels.

Molly and Stark visit the Grimthorpe mansion; Jenkins, the gardener, is packing up the house, and lets them in. Beyond Grimthorpe's study is another secret office belonging to his secretary. In the tidy room are printouts of Grimthorpe's manuscripts. Serena arrives to ask what they are doing in her office. She explains that her mother was the ghostwriter of Grimthorpe's novels—this was the big announcement Grimthorpe intended to make. Following her mother's death, Serena got Grimthorpe to sign a contract agreeing to pay her a fair share of what his books earned and all royalties going forward. He stipulated that he wanted to announce the news of his ghostwriter himself, however. Serena's lawyer has said that she must not reveal the news herself, even though he is dead, because she may violate the contract. Serena says that she would not have killed him because he was no good to her dead. While stirring her tea, which Jenkins brought in, Molly has a revelation: she remembers that the spoon Grimthorpe used was stainless steel, meaning the hotel's silver spoon was switched out of the honey pot.

Back at the hotel, Molly finds the missing poisoned honey pot and a stainless steel spoon from the Social in Beulah's bedside table. She explains that maids never found these objects because it is the hotel's policy to tidy up around a guest's personal possessions, and Beulah still had not checked out. Stark and Molly lure Beulah to the tearoom and test her knowledge about Grimthorpe. As his unofficial biographer, Beulah knows that he liked honey in his tea and that alcoholism had made him susceptible to poisoning. Stark arrests Beulah.

At the end of the novel, Molly has promoted Lily to Maid and has kept Cheryl on staff on the condition that she retrain under Molly's supervision. Stark informs Molly that Beulah pled guilty to the crime. Her motive: Grimthorpe's constant refusals to name her his official biographer and her bitterness over having dedicated much of her life to a fraud. Stark apologizes for underestimating Molly and recommends she become a detective. Molly is unsure about the career change. The maids prepare the Grand Tearoom for a big announcement from Serena Sharpe. Because Grimthorpe's publisher had already begun transferring rights and royalties, Serena is free to name her mother as the true author of Grimthorpe's novels.

In the epilogue, Molly finally has Mr. Preston over at her apartment. He reveals that he is her biological grandfather. As young people, he and Gran were together, though Gran's wealthy parents disapproved. When Gran got pregnant, her parents sent her to a home for unwed mothers. She escaped before giving birth, and her parents disowned her. Poisoned against Preston, Gran decided to raise the child on her own. She moved into Molly's current apartment and supported herself by working as a maid. Eventually she let Preston back into her life, but by then he had married someone and started a family. Molly is delighted to learn that he is her grandfather. When Juan arrives, he hints that he will soon propose to Molly. Molly is thankful for everything she has as she watches her grandfather fill Juan in on everything he missed while away.