Now the proprietress of Pryde's Detective Agency, Cordelia Gray has an amazing success rate when it comes to locating lost family pets, but needs a more substantial case to pay the bills. Fortuitously, Sir George Rathbone comes to her office wanting to hire her to protect his wife, the actress Clarissa Lisle, who has been receiving anonymous notes, and is extremely scared. Clarissa is very highly strung and wants to have the notes intercepted by Cordelia so that they do not distract her from her role in The Duchess of Malfi on the private island of Courcy. Cordelia agrees and readies herself for the trip. One of the preparations is to thumb through The Penguin Dictionary of Quotations, and she quickly realizes that most of the quotes in Clarissa's anonymous letters can be found there. All of the quotes threaten imminent murder and all include a crude doodle of a skull.
Cordelia takes the train to Speymouth where she boards a boat bound for Courcy Island. Aboard she meets two of her fellow guests, Simon Lessing, Clarissa's teenage stepson, and her cousin Roma. They are then joined by theater critic Ivo Whittingjam who is terminally ill and physically jarring to look at because of this.
Cordelia meets Clarissa whilst unpacking as their rooms at Courcy Castle are adjoining. Shortly after arriving the group are taken on a tour of the castle, including owner Ambrose Gorringe's chamber of horrors, a collection of death-related Victoriana that includes items worn by murderers and figurines of infamous serial killers.
A walk on the beach with Ivo Whittingham gives Cordelia a great deal of background knowledge about her fellow guests. She learns that Ivo and Clarissa had an affair whilst Clarissa was still married to Simon's father, who committed suicide by drowning himself shortly after discovering this. Ivo also tells her that Roma looks upon Clarissa as a source of money which she desperately needs to keep her little bookstore from going under. Finally he tells her about Tolly, Clarissa's longtime dresser and assistant, whose daughter was dying in the hospital during a run of a play in which Clarissa was starring. One night in particular the hospital called the theater advising Tolly to come right away as her daughter was duing. Clarissa failed to give her the message because she wanted Tolly to stay at the theater to dress her, and consequently by the time Tolly reached the hospital after the play was over, her daughter had already passed away. From her walk with Ivo Cordelia realizes there are many people with a motive to scare or threaten Clarissa.
The following day the guests take a trip to the church crypt with their host, where they discover row upon row of skulls, and where they visit the Devil's Kettle, an underground passageway with a direct opening to the ocean, which was used by smugglers through generations. Whilst they are in the passageway, Cordelia finds another poison pen note, confirming that the sender is one of the small group of guests staying that weekend.
Everyone returns to the castle to get ready for the performance of the play that evening. Clarissa wants to be left alone but agrees to lock her door and Cordelia arranges to come and wake her with time to get prepared to go to dinner. However when she does so she finds her murdered with her eyes gouged and her face battered. The weekend guests gather to agree upon a broad story for the police then they telephone them to report the murder.
When the police arrive, they begin to interview everyone individually, finding an irritating similarity between every account of events.They also find it hard to take Cordelia seriously as a detective and briefly consider that she and Sir George concocted the entire story of the notes together before murdering Clarissa. Sergeant Buckley is also concerned that the old cliche "the butler did it" will actually turn out to be true since he finds that Ambrose Gorringe's butler, Munster, is rather suspicious and definitely hiding something. Initial interviews give no further clue as to the identity of the murderer. The detectives leave but plan to return the following day. That evening's dinner proves difficult and is interrupted by a wildly intoxicated Munster who blusters aggressively into the room shouting,"Murderer!" at one of the party around the table. He is overpowered and removed by Simon and Ralston but later that night is found floating in the pool, drowned. Simon, a strong swimmer, dives in to pull out the body and also discovers one of Ambrose's Victorian music boxes caught under the water with the butler.
One of the items missing from Clarissa's room is her jewel box and the presa cutting from 1977 that she kept inside it. Cordelia decides to visit the local newspaper in Speymouth to find out why the review was so important to her as her gut feeling tells her the cutting holds the key to the murder. The newspaper staff are helpful and show her the archive but the review Corxwlia is seeking has already been cut out carefully and removed. The elderly gentleman working in the lobby suggests that she visit a lady in town whose sister collected anything and everything to do with the monarchy; the newspaper Coedelia is looking for contained coverage of a royal visit and is likely to be included in the collection. It is an arduous task but eventually she locates the clipping, recognizing a picture of Angus Gorringe in the crowd at the Queen's visit and realizing that at this time he was supposed to be on tax exile overseas; if the authorities were to learn he was on British soil during that time he would be liable for all of the tax he had avoided. Realizing this was a motive to murder Clarissa and prevent her from speaking about this, Cordelia heads back to Courcy to confront Ambrose, catching a ride on a fishing boat as Ambrose had not sent the island's boat to pick her up. To her unease, Cordelia discovers the other guests have left and she is alone with the man she believes to have murdered Clarissa. Ambrose patiently explains how Clarissa had blackmailed him and forced him to put on the play for her to star in; it was he who sent the later notes, hoping to sufficiently scare her that her emotions would unravel and that her performance would be so terrible that she would never be able to work again. Hoarder, he did not murder her; Simon, summoned to her bedroom for sexual favors was so repulsed by her demands that he had struck and accidentally killed her. Ambrose had battered her after death to throw police off the scent and to free himself of his pent up anger towards her. He took a long time telling the story and Cordelia realizes this is to enable Simon to get away. She traces him to the Devols Kettle after noticing a pair of Victorian handcuffs missing from their display case; he has handcuffed himself to the railing and intends to drown himself in the riding tide. Cordelia doves for the key and finds it, unlocking the handcuffs, but hearing the heavy trapdoor above them close and the deadbolts on it fasten. Realizing their only hope is to dive through the small entrance out into the ocean before they drown, Cordelia pushes an exhausted Simon in front of her as they swim for their lives. Popping up into sunlight she is rescued by the same boatman who returned her to the island earlier, on his way to return the thick leather belt she had accidentally left on board.
Simon does not fare so well and drowns. Ambrose denies her story and tells the police that she must have forgotten to prop the trapdoor correctly, causing it to fall. She contacts her secretary, Miss Maudsley, who informs her they have another lost pet case in urgent need of her attention who h pleases her enormously, and she pledges to be back in London by the end of the day.