Dr. James Sheppard, a resident of the small village of King’s Abbot, wakes up on Friday morning to learn that Mrs. Ferrars has died. He’s sent to care for her, but he’s too late. He determines that Ferrars has overdosed on a sleeping medication. His sister, Caroline, with whom he lives, tells him that she’s sure Mrs. Ferrars killed herself out of remorse for having killed her husband, Mr. Ashley Ferrars, the previous year.
Dr. Sheppard is friendly with Roger Ackroyd, a successful middle-aged businessman who lives in the biggest house in the village. Roger was married to a Ms. Paton, who already had a child named Ralph Paton by another marriage. After Ms. Paton drank herself to death, Roger was rumored to be involved in an affair with Mrs. Ferrars. While walking through the streets, Sheppard crosses paths with Roger, who says that he needs to speak with Sheppard right away, and invites him to dinner that evening. During the day, Sheppard is visited by a patient, Miss Russell, who works as a housekeeper in Ackroyd’s house. She asks Sheppard if there’s any cure for drug addiction. In the afternoon, Sheppard meets his neighbor, a mysterious foreigner named “Mr. Porrott.” Porrott claims that he’s come to King’s Abbot to retire and grow vegetables, but that he’s been unable to turn his back on his old profession. Sheppard also visits Ralph Paton at the local inn, where Ralph tells Sheppard that he’s been arguing with his father about money, and that he has to “play a lone hand.”
At 7:30, Sheppard arrives at the Ackroyd estate, carrying his black bag in case he’s summoned on medical duty. Also present in the house is Mrs. Ackroyd (Roger’s sister-in-law), Flora Ackroyd (Mrs. Ackroyd’s daughter, and Ralph’s fiancé), Major Hector Blunt (Roger’s good friend), and Geoffrey Raymond (Roger’s secretary). After dinner, Roger asks Dr. Sheppard to speak to him in his office. There, Roger explains that he and Mrs. Ferrars were in love, but that Mrs. Ferrars admitted that she’d murdered her husband, and has now killed herself. She also told Roger that somebody was blackmailing her. Just then, the butler, John Parker, enters the room with the evening mail, including an envelope from Mrs. Ferrars. Roger opens the letter and sees that it must contain the name of the blackmailer. Sheppard asks Roger to read it, but Roger says he’ll do so later.
Sheppard leaves around 8:50. On his way out, he passes by a mysterious, yet oddly familiar, stranger. When he’s home, he gets a call. Shouting to Caroline that Parker has told him Roger’s been murdered, Sheppard races back to the Ackroyd estate. Parker is confused—he claims not to have called Sheppard at all. Nevertheless, the two men break into the study, which was locked, and find Ackroyd stabbed in the neck.
Alone in the room, Sheppard examines the body and determines that Roger has been dead for at least half an hour. Raymond rushes into the study and determines that nothing has been stolen. However, Sheppard notices that Mrs. Ferrars’s letter is gone. The police arrive and take everyone’s testimony. Notably, Flora claims to have seen her uncle alive at 9:50, after which she told Parker that Roger didn’t want to be disturbed, and Raymond claims that he heard Roger talking to someone around 9:30. The murder weapon is a Tunisian dagger which was kept in a silver table in the Ackroyd house. The police are initially suspicious of Parker, who seems very nervous. Meanwhile, Ralph is nowhere to be found.
The next morning, Flora asks Sheppard to help her convince Sheppard’s neighbor, “Mr. Porrott”—who is actually the famous detective Hercule Poirot—to take on the case of Roger’s murder. Poirot agrees to do so, with the condition that he’ll follow it through to the very end, no matter how painful his conclusions. Poirot says that he likes Sheppard, and begins to ask for Sheppard’s help in investigating the case.
The head police inspector, Inspector Raglan, shows Poirot that the killer came in through the open window, wearing unique shoes with rubber-studded soles—shoes which resemble those owned by Ralph. Poirot also learns from Parker that a chair was shifted slightly in the time between Sheppard and Parker’s discovery of the body and the police’s arrival. The police also determine that the call Sheppard received came from the nearby train station. Raglan seems confident that Ralph is the killer, particularly since he’s nowhere to be found, but Poirot isn’t so sure. In a summerhouse outside the estate, Poirot and Sheppard find a scrap of cloth and a goose quill. They also find a woman’s wedding ring in a goldfish pond, bearing the inscription, “From R.”
Ackroyd’s will is opened: he’s left some money to Miss Russell, Flora, and Mrs. Ackroyd, but most of his fortune to Ralph. Raymond discovers that some money is missing from Roger’s unlocked desk. Poirot investigates the missing money by interviewing two maids, Ursula Bourne and Elsie Dale. Ursula had been dismissed from her job earlier on Friday. Poirot assembles his suspects—Blunt, Flora, Mrs. Ackroyd, Raymond, and Sheppard—and tells them, “Every one of you in this room is concealing something from me.”
Dr. Sheppard tells Poirot his theory that someone entered Roger’s study through the window, leaving shoeprints behind—and yet this person couldn’t have been the killer, based on Flora’s testimony. Perhaps Ralph left the window open, allowing the killer to come in afterwards. Poirot says that he admires Sheppard’s thinking, but that he’s convinced of Ralph’s innocence. Sheppard begins to see that Poirot is keeping a lot of information secret from him.
The next day, Mrs. Ackroyd, who Roger supported after her husband’s death, confesses to Sheppard that she was stealing silverware from the house, and that she was deep in debt. When Sheppard speaks with Ursula again, she tells him that Ralph “ought to come back.” Poirot asks for Flora and Parker’s help in a “little experiment,” to reenact the events of the night of the murder. Based on Flora’s behavior, Poirot deduces that Flora was lying about saying goodnight to Roger at 9:50—she was just trying to conceal the fact that she stole money from her uncle’s desk. Flora tearfully confesses, but Major Blunt claims that he took the money. Poirot tells Blunt that, quite obviously, Blunt loves Flora. He advises Blunt to share his feelings with Flora.
Raglan takes Sheppard and Poirot to meet a man the police have arrested named Charles Kent. Sheppard realizes that this is the mysterious stranger he saw on the night of the murder. Poirot confronts Miss Russell about the stranger, and Russell admits that Charles is her illegitimate, drug-addicted son, who she met with in the summerhouse on the night of the murder. Poirot later confronts Ursula and reveals that she was Ralph Paton’s secret wife. Ursula admits that Poirot is right—she cast off her wedding ring after Ralph informed her that he was going to marry Flora in order to please Roger and ensure his inheritance.
That evening, Poirot assembles the suspects, including Ursula, Blunt, Flora, Mrs. Ackroyd, Raymond, Parker, and Sheppard, in his home. He then produces Ralph Paton—who, unbeknownst to Poirot until recently, has been in hiding with the help of Dr. Sheppard. Sheppard admits that he’s been protecting Ralph, knowing that he’d be the prime suspect in the murder. Poirot explains that he now knows who the killer is, and that the killer must come forward before he goes to speak with Inspector Raglan the next morning. Nobody comes forward, and the guests leave. Poirot asks Sheppard to stay behind, however. Poirot then explains that he’s deduced that the killer is none other than—Dr. Sheppard himself.
Poirot explains that he’s been suspicious of the phone call that Sheppard claims to have received on the night of the murder. He deduced that the purpose of this phone call was to ensure that Sheppard would be in the room when Roger’s body was first discovered. Sheppard obtained a dictaphone featuring a recording of Roger’s voice, and placed the dictaphone in the office, shielded by the chair, so that it would play Roger’s voice at exactly 9:30, confusing Raymond into believing that Roger was still alive. In fact, Sheppard killed Roger around 8:45, much earlier than the police thought, and then left incriminating tracks on the windowsill. He’d arranged for one of his American patients to call him from the train station, giving himself a pretext for rushing back to the estate and removing the incriminating dictaphone by placing it in his black bag. The reason that Sheppard killer Roger, Poirot has deduced, is that Sheppard was Mrs. Ferrars’s blackmailer: he didn’t want to be caught by Roger.
Poirot calmly tells Sheppard that he can either go to the police or kill himself. Sheppard spends all night writing his confession. He plans to kill himself with an overdose of sleeping medication. He trusts that Poirot and Raglan will keep his secret, so that Caroline won’t have to go through the pain of learning that her beloved brother was a murderer.