The narrative is on behalf of Dr. Sheppard. He lives in a small English village with his unmarried sister Carolina, a big news lover. Doctors call Mrs. Ferrar, who lives in the neighborhood, but the woman is already dead, because she took too much of a sleeping pill. Her husband, who abused alcohol, died a year ago from gastritis, but Carolina believes that his wife poisoned him with arsenic.

In the same village lives the manufacturer Mr. Roger Ackroyd. He early widowed and raised, as a native, the son of his wife from his first marriage, Ralph Pentane, a rather frivolous young man. Mr. Ackroyd showed attention to his housekeepers, especially the last, Miss Russell, but the appearance in the village of Mrs. Ferrar changed everything. Recently, the widow of his brother, Mrs. Cessil Ackroyd, came to Mr. Ackroyd with her daughter Flora, who is not interested in marrying her brother-in-law.

Carolina is interested in their new neighbor, who grows pumpkins and hides his profession. Dr. Sheppard gets to know him and learns that, at Mr. Ackroyd’s insistence, Ralph is engaged to Flora.

Carolina tells her brother that Ralph was leaving for London, but when he returned, he stayed in a hotel, which surprised Mr. Ackroyd. She suggests that Ralph has a secret date with some girl. Carolina heard Ralph complaining to some woman that his stepfather would leave him without money.

Miss Russell comes to Dr. Sheppard with a request to examine her sore knee. In conversation, she asks about rare poisons and drugs.

Dr. Sheppard comes to Mr. Ackroyd without a call with his suitcase, as he must go to the sick patient. Mr. Ackroyd is worried: he asked Mrs. Ferrar to marry him. At first, the woman refused, as she observed mourning, but when the period of mourning expired, she admitted that she had poisoned her husband. She hated Mr. Ferrara and killed him because of her love for Mr. Ackroyd. Someone found out about this and extorted large sums from Mrs. Ferrar. Unable to stand, she committed suicide. The conversation is interrupted by the butler Parker, who brought a letter from Mrs. Ferrar. Mr. Ackroyd wants to read it himself, realizing that the letter says the name of the blackmailer.

At ten past nine in the evening, Dr. Sheppard leaves. Leaving Mr. Ackroyd’s office, he sees Parker and gives him the request of the owner not to disturb. It seems to Sheppard that Parker was eavesdropping behind the door. On the way home, Sheppard is stopped by a stranger who wonders where Mr. Ackroyd’s house is.

At a quarter past ten in the evening, Dr. Sheppard's house rings a bell: he is informed that Mr. Ackroyd has been killed. Grabbing his briefcase, the doctor goes to the call.

A surprised Parker opens the door and knows nothing. She and Sheppard are trying to open the door to the office, but the door is locked from the inside. Hacking the door, they see Mr. Ackroyd killed by a dagger in the neck. An examination of the room reveals that nothing of value has been lost, but Sheppard does not find Mrs. Ferrar’s letter.

A police inspector arrives at the scene. He spots footprints on the windowsill. The inspector assumes that the killer climbed through the window, killed and fled. Mr. Ackroyd's secretary, Raymond, reveals that at half past nine he heard the landlord talking to someone. Parker reports that at a quarter to ten Flora left her uncle's room.

In order not to frighten the girl, the inspector tells her that the theft took place. Flora says that she wished her uncle good night, and he asked her to tell her not to be bothered. Upon learning the truth, Flora faints.

The dagger with which Mr. Ackroyd was killed lay in the window in the living room.No one remembered if there was a dagger in the window on the day of the murder.

Flora asks Sheppard to introduce her to their neighbor. The girl explains to the surprised doctor that this is the famous detective Hercule Poirot. Flora was at the hotel where Ralph was staying, but she was informed that at nine o’clock in the evening he left, and no one else saw him. She was also told that Ralph was looking for Dr. Sheppard. The doctor is unhappy - he does not want anyone to know about his arrival. Now the police suspect Ralph of killing his stepfather, and Flora wants to turn to Poirot for help.

The famous detective agrees to take up the investigation, but warns that he will finish the case. Poirot examines the room in which the murder occurred. Taking Ralph's boots, he sees that the tracks on the window coincide with their soles. Poirot also notices a chair standing in a corner and strangely set by the table. Police find that Dr. Sheppard got a call from the train station, and a few minutes later the train to Liverpool left.

The police are interested in the unknown, who asked Dr. Sheppard for the address of Mr. Ackroyd. Raymond reports that his owner wanted to buy a voice recorder, and a representative of the voice recorder company came.

Poirot studies the situation: Flora saw her uncle at a quarter to ten, and according to the doctor’s testimony, the murder was committed at about ten in the evening. But everyone from Mr. Ackroyd’s entourage has a solid alibi for this time. In the gazebo, located near the window, Poirot finds a piece of cambric cloth and a goose feather. In the pond, he finds an engagement ring with the signature "from R.".

The lawyer opens a will. Fixed capital is received by Ralph. Flora also receives a decent amount, and now the girl does not need to ask for money for new outfits.

A large amount disappears from the box where Mr. Ackroyd kept cash. The maids know nothing about this. Mr. Ackroyd, one of the maids, Ursula Bourne, fired the day before because she messed up the papers on his desk. Previously, the girl worked for a certain Mrs. Folliot and provided excellent recommendations. Poirot seems strange that the girl was fired because of such a trifle. Also, the famous detective does not understand why Mr. Ackroyd talked with her about dismissal for so long. Poirot decides to find out everything about Ursula, especially since she does not have an alibi. He shares his observations with Dr. Sheppard and asks her former mistress to ask about the girl.

Mrs. Folliot welcomes the doctor warmly, but hearing about Ursula Bourne in an icy tone, she declares that she knows nothing about her.

Police are interested in fingerprints on a dagger. Since they do not belong to any of the inhabitants of the house, the inspector decides that the prints are either Ralph or a mysterious stranger. Poirot is much more interested in an incomprehensible call. Having solved it, he will solve the murder. And he advises to take prints from the killed person.

Poirot shares his reasoning with Dr. Sheppard. A mysterious stranger came to Mr. Ackroyd's house, and the maid saw him. Then he went to the hotel bar, where visitors paid attention to his American accent, and he himself admitted that he was American. The goose feather found smells strongly of a drug common in America. The doctor suspects the stranger, although everything turns against Ralph. Poirot believes that Ralph is innocent.

Mrs. Ackroyd is worried and complains to Dr. Sheppard. She draws attention to the strange behavior of Ursula Bourne. She never liked this girl and seemed too educated for the servant. Dr. Sheppard notices that Ursula is worried about Ralph.

Poirot asks Carolina to find out what color the shoes were on the day of the murder on Ralph, which is surprising to the doctor. Using all his connections, Carolina performs the task. The color of the shoes is not the one that the famous detective was thinking about.

Poirot consults with Dr. Sheppard about who could blackmail Mrs. Ferrar.Their suspicion falls on Parker, who overheard conversations.

Carolina with friends discuss the details of what happened. They are sure that Ralph and Flora have nothing to do with each other. Most likely, Flora is not indifferent to Mr. Ackroyd's friend, Major Blent.

Poirot asks Dr. Sheppard to attend Parker's interrogation. The famous detective accuses the butler of blackmail. He inquired about him and found out that Parker was blackmailing his former master. Parker confesses this. Having overheard the conversation between Mr. Ackroyd and the doctor, he wanted to profit, but did not have time.

Poirot and Dr. Sheppard visit the attorney Mrs. Ferrar. The lawyer confirms that recently a woman has paid large sums.

Dr. Sheppard is being called to Liverpool. Police detained a certain Charles Kent, and doctors are asked to identify if he is that mysterious stranger.

Charles admits that he was near Mr. Ackroyd’s house, but refuses to give a reason and denies any involvement in the murder. Poirot believes that he is innocent.

Poirot talks with Flora. He knows that it was she who took the money, and furnished everything as if she were leaving her uncle’s office. Therefore, the girl fainted from fear because of the responsibility for giving false testimonies. Flora confirms everything, Major Brent is trying to protect her, and Poirot advises him to confess to the girl in love.

The police inspector is at a standstill: Charles Kent is not guilty, the police found that the prints on the dagger belong to the murdered man.

Poirot talks to Miss Russell. The woman admits that Charles is her illegitimate son. The guy went astray, became a drug addict and came to her for money. Poirot noticed their similarities, compared Miss Russell's interest in drugs and poisons, and came to the conclusion that there was a related relationship between them.

Poirot places a note in a newspaper stating that Ralph was detained in Liverpool. Ursula Bourne comes to him, who actually turns out to be Ralph’s wife. Being a girl from a poor family, she got a job as a maid. Her sister married Captain Folliot and gave her a recommendation. Ursula and Ralph fell in love, decided to secretly get married, since Ralph was sure that his stepfather would not give consent to this marriage. Mr. Ackroyd insisted on Ralph’s marriage with Flora, and Ralph asked Ursula to wait. It was this conversation that Carolina heard. Ursula told everything to Mr. Ackroyd, then met in a gazebo with Ralph. They quarreled and did not see each other again. Poirot reassures the girl that Ralph is all right.

Dr. Sheppard says he keeps a diary, and Poirot asks to see his records of the murder.

Poirot gathers all the inhabitants of Mr. Ackroyd's house. Starting the investigation, he found a piece of a cambric and a feather in the arbor. Having revealed these riddles, Poirot approached the following: who did Mr. Ackroyd talk to in his office at half past nine? Mr. Ackroyd was alone, he was speaking the text into the recorder.

At that moment, Ralph enters the room, which Dr. Sheppard hid in the hospital, as he was his friend. After reading the doctor’s diary, Poirot understood who the young man would turn to for help and where the doctor could hide him. But who is the killer? Poirot knows who it is and warns that the police will find out about it in the morning. The famous detective is brought a telegram from the board of a ship sailing to America.

Poirot is left alone with Dr. Sheppard. The famous detective sets out a chain of reasoning. From the very beginning, a strange bell haunted him. The killer's accomplice called, wishing that the murder was discovered immediately. The killer had to get into the office at the time the body was discovered. Entering the study, Poirot saw how strange the chair was. Having studied this fact, he came to the conclusion that it hid some object. The killer could not carry him right after the murder and entered the office to pick him up. The subject was a dictaphone, because it was not in the office. At half past nine Mr. Ackroyd was already dead, and his voice was recorded.Also, the killer should have had Ralph’s shoes in order to leave marks on the window with her help, and the ability to take a dagger. The person who did this could only be Dr. Sheppard. He specially came to Mr. Ackroyd with a suitcase to pick up the recorder. Having committed the murder, he left the house, climbed into the open window in Ralph’s shoes, set the recorder’s clock to the right time and locked the door from the inside. As the attending physician, Dr. Sheppard knew why Mr. Ferrar had died and blackmailed the widow. If Mr. Ackroyd found out about this, he would not spare his friend. And the doctor called his patient, who went to Liverpool to sail from there to America.

Poirot gives the killer the opportunity to take a large dose of sleeping pills.

Dr. Sheppard leaves the manuscript. He worries what a blow to Carolina will be his death. He also does not understand why Hercule Poirot settled here to grow pumpkins?