Elderly lady Miss Marple is relaxing in a hotel on the Caribbean. Her interlocutor, Major Pelgrave, chatted incessantly about all kinds of trifles. Among other things, he talks about a serial killer who killed his wives. By chance, a major has a photograph of a criminal in his wallet. Looking for a picture, the major accidentally sees something behind Miss Marple, changes in his face and moves on to another topic.

Miss Marple is approached by two couples relaxing at the hotel - Colonel Hillingdon with his wife Evelyn and Greg and Lucky Dysons. Soon, the owners of the hotel, Tim and Molly Kendall, who were sitting nearby and checking account books, were coming up. Taking advantage of the fact that everyone is passionate about the conversation, Miss Marple turns around, but does not see anyone behind her.

In the morning, Miss Marple learns that Major Pelgrave suddenly died at night. He abused alcohol, took medicine for the night, death came from increased pressure. She decides to ask for a photograph of the killer that the major had in her wallet, stating that this is the only picture of her deceased nephew, but the picture disappeared. Miss Marple does not believe in the official version of the death of a major.

Miss Marple ponders what has happened. She realizes that the major saw the same killer, but of the men there were Colonel Hillingdon, Mr. Dinos, and the servant of the resting millionaire Rafael Jackson. She decides to investigate.

Hotel maid Victoria tells Molly Kendall that after the death of Major Pelgrave, a bottle of pressure medicine was discovered in his room that she had not seen before. The girl believes that someone planted it. Tim thinks this is nonsense, but doesn’t want Victoria to talk about it, because this can affect the reputation of the hotel. Nevertheless, the couple consulted about this with Dr. Graham, who ascertained the death of the major. The doctor reassures Kendall: the major could keep the medicine in the drawer, so the maid had not seen her before.

Miss Marple collects information about persons of interest to her. Jackson has been with Mr. Rafiel for about a year. He is single and has a good time. Greg Dyson has been married to Lucky for several years. His first wife, Lucky's cousin, died a few years ago on this island, after Greg cheated on her with Lucky. Colonel Hillingdon is a calm, pleasant man, but he does not have the best relations with his wife.

Dr. Graham is worried about Molly. She is nervous all the time, she has nightmares. A woman takes sleeping pills to calm down, but the pills make her worse. He shares his experiences with Miss Marple, and an elderly lady tells the doctor about her suspicions.

Victoria gives Greg Dyson a pill bottle found in the major’s room. After his death, the girl was told to throw away all the medicines, but Victoria knows that the pills belong to Greg. Surprised Greg does not understand how his tablets could get to the major. Victoria believes that someone planted them.

Mrs. Hillingdon complains to Molly that her husband is partial to Lucky Dyson. A woman is ready to kill her rival out of jealousy. Molly says that she has recently had memory lapses. She can hear some voices, steps. Tim finds out about this and tries to protect her.

Colonel Hilldington confesses to his wife that he wants to leave here as soon as possible, otherwise he will kill Lucky. Once Lucky asked him to buy something at a pharmacy and rewrote a prescription for this. The purchased medicine was taken by Greg's first wife and died. Lucky motivated this by the fact that the woman is seriously ill, and death will relieve her of suffering. Now Colonel Hilldington hates the woman he once loved. Evelyn believes that they should stay and behave as usual.

Evelyn tells Tim that Molly needs to be shown to the doctor. Tim does not agree, he believes that there is no danger, Molly just upset the death of Major Pelgrave. Calling her family does not make sense, since Molly broke up with her family, mother and sisters. Their conversation is interrupted by Molly's cry: she found Victoria killed by a knife in the bushes.

Victoria's husband tells the police that his wife demanded money from someone for silence.

Greg Dyson tells the police that Victoria found his pill bottle. He thinks Victoria knew something.

One of the cooks reports that he saw Molly with a knife in his hand. Tim replies that she checks the appliances on the tables. Perhaps she was carrying a knife that the waiters forgot to put. Molly doesn't remember if there was a knife in her hands. The police cannot find the suspect and is at a standstill.

Miss Marple shares her suspicions with Mr. Raffiel and asks him, as an influential person, for help. The major recognized the serial killer. Since he spoke loudly, the killer could hear and put pills on him. Mr. Rafiel heard from the police inspector that an exhumation had been carried out and poison was discovered in the major’s body. But why would a killer kill a major if he had no evidence? Probably another murder is planned.

About those whom Miss Marp suspects, Mr. Rafiel knows nothing. His servant Jackson also does not look like a killer. The millionaire tells Miss Marple that Jackson receives a good salary, but after his death will not receive a penny. His secretary, Esther Walters, a young widow with a child, will receive nothing. Mr. Rafiel believes that Jackson is not indifferent to Esther. Esther joins their conversation, but she knows nothing about the killer and the death of Major Pelgrave.

Speaking with hotel guests, Miss Marple learns the story of Molly. Once a girl met a man. Her mother found out something unpleasant about him and told her to bring him home, but Molly refused. Miss Marple notes that Lucky and Molly have the same hair color, only Molly is natural, and Lucky dyes her hair.

Suddenly, Miss Marple notices how Jackson secretly sneaks into Mr. Rafiel’s room, although he should now be near his master. Following him, she sees that Jackson is rummaging through the papers.

At night, Tim wakes up Evelyn and asks her to look after Molly, who took a lot of sleeping pills. Evelyn succeeds Miss Marple. An elderly lady worries: Molly complains that she can hear footsteps and voices, maybe she is being followed by a man whom her family did not want? Tim dismisses it a long time ago.

Caring for Molly, Miss Marple finds a book on nervous diseases under her pillow. The book is open on the page that describes the mania of persecution and schizophrenia. Seeing that Molly is sleeping, Miss Marple leaves her and goes to look for Dr. Graham. She sees Jackson entering Molly's room. Miss Marple catches him in the bathroom with a box of cream in her hands. He once worked as a pharmacist and wondered what kind of cosmetics women use. He heard that sometimes drugs are mixed in cosmetics. Miss Marple decides to hand the cream box to Dr. Graham.

Esther Walters believes that Molly is worried about her husband's indifference to her and decided to attract his attention, but she herself still loves the man whom her family rejected.

The next evening, Tim tells Miss Marple that Molly has almost recovered and will get up tomorrow. Miss Marple wakes up at night with noise: Molly has disappeared. Tim finds his wife lying face down in the stream. But approached Miss Marple notices that the dead woman is not Molly, but Lucky. Their hair color is the same, but the roots show that the hair is dyed.

Miss Marple again recalls the day Major Pelgrave spoke of the killer. Suddenly it dawns on her: one of the major's eyes was glass. Miss Marple asks Mr. Rafiel for help: she must prevent the killing. To do this, she needs Jackson.

Molly returns to her room. Shocked by the killing of Lucky, she always repeats that it was not she who killed her.Tim tries to calm his wife and gives her a glass of water. At that moment, Jackson pulls a glass from his hands. Miss Marple and Mr. Rafiel and Esther enter the room.

Miss Marple clarifies the situation: all this time she believed that Major Pelgrave was looking to the right. However, from behind his glass eye, he saw only what was on his left, and Tim Kendall sat there. Hearing a conversation about the killer, Tim tossed him pills. Victoria saw this, began to ask him, and Tim killed her. But most importantly, he planned to kill Molly.

Esther interrupts the story of Miss Marple, rushes to Tim and screams that this is not true.

Tim convinced Esther that Molly loved another man. She expected Tim to divorce Molly and marry her. From his friend Jackson, he learned that Esther will receive a large sum after the death of Mr. Rafjell. Molly, seeing that her family is opposed to her relationship with a man, played parting with Tim, and he appeared in front of her relatives under a different name.

With Molly’s money, they bought a hotel. Here Tim met Esther and decided to get her money. He told Molly that she was ill by slipping her a book about mental illness. Jackson guessed what was happening and decided to look at Molly's cream. A drug has been found in the cream that causes memory lapses. After convincing his wife that they should track down the killer at night, Tim lured her out of the house, but drowned Lucky by mistake.

Having suffered a terrible blow, Molly recovers and maintains the hotel with the help of one of Mr. Rafiel's attorneys.