The novel is narrated by Dr. James Sheppard, a physician in the town of King’s Abbott. The story begins with the death of Mrs. Ferrars, who overdosed on Veronal, a sleeping medication. Much of the town (including Dr. Sheppard’s gossip-loving sister, Caroline) believes Mrs. Ferrars poisoned her husband a year before. Caroline now speculates that Mrs. Ferrars has committed suicide with Veronal over the guilt.
Meanwhile, Roger Ackroyd, the town’s wealthiest citizen who was expected to marry Mrs. Ferrars, invites Dr. Sheppard to dinner, claiming he has something important to tell him. Also present at dinner are Mrs. Cecil Ackroyd, Roger’s sister-in-law, Flora Ackroyd, his niece (who recently got engaged to Ackroyd’s stepson Ralph Paton), Geoffrey Raymond, Ackroyd’s young secretary, and Major Blunt, Ackroyd’s friend. Distraught throughout the meal, Ackroyd finally has a private discussion with Dr. Sheppard after dinner. In his study, he reveals to Dr. Sheppard that not only did Mrs. Ferrars actually kill her husband, she was being blackmailed for it. She revealed all of this to Ackroyd before taking her life because of the emotional and financial strain she was under.
Although she didn’t tell him who the blackmailer was, he had a feeling she left a message for him before her death. At that moment, Ackroyd’s secretary Parker arrives with the evening mail, which contains a letter from the late Mrs. Ferrars. Ackroyd begins to read it out loud. In it, Mrs. Ferrars announces she will name her blackmailer and asks Ackroyd to seek revenge on her blackmailer. Ackroyd refuses to read the name of the blackmailer with Sheppard present.
Dr. Sheppard leaves Fernly Park (Ackroyd’s home), but on his walk back bumps into a stranger who asks for directions to the estate. At home, Dr. Sheppard and Caroline are about to go to bed when the phone rings. He explains to Caroline that Parker has called from Fernly Park to tell him that Ackroyd has been murdered. Dr. Sheppard rushes over, but when he gets there Parker denies making the phone call. Nonetheless, they break into Ackroyd’s locked study and find him murdered in his chair.
The next day, Dr. Sheppard is approached by Flora Ackroyd to help recruit Dr. Sheppard’s new neighbor, the retired Belgian detective Hercule Poirot, to help investigate the murder. Flora is worried that the police are going to blame Ralph Paton for killing her uncle, especially since Ralph, who had been spotted approaching Fernly Park that night, is now nowhere to be found. The police have found footprints matching a pair of shoes that Ralph owned outside the window to Ackroyd’s study, and at 9:30 pm (after Dr. Sheppard had left, but before the body was found at 10:30), Major Blunt and Geoffrey Raymond both overheard Ackroyd speaking to someone in his office.
Because Flora claims to have wished her uncle goodnight at 9:45 pm and Dr. Sheppard is convinced that Ackroyd had been dead at least a half hour when he found the body at 10:30, the police are convinced that the murder was committed between 9:45 and 10:00 pm. Although all members of Ackroyd’s household stood to gain financially from his death, Ralph especially inherited the bulk of his uncle’s fortune. After they have determined that Ralph recently was in a great deal of debt, they suspect him all the more.
Although the police are convinced it is Ralph, Poirot’s thorough investigation leaves him unconvinced. He is fixated on the phone call to Dr. Sheppard (which the police trace, and determine came from the King’s Abbot train station, not Fernly Park), and the position of a grandfather chair in Ackroyd’s office, which had been moved into the center of the room when Dr. Sheppard and Parker found the body. In the course of his investigation, Poirot discovers a goose quill and a piece of cambric in the summerhouse on the Fernly Park grounds, as well as a wedding ring inscribed “From R” in a goldfish pond on the grounds.
Poirot gathers Mrs. Cecil Ackroyd, Flora Ackroyd, Geoffrey Raymond, Major Blunt, and Dr. Sheppard together and accuses all of them of hiding something from him which is relevant to the case. In the meantime, Poirot notes that the parlormaid, Ursula Bourne, is the only person in the household who doesn’t have a clear alibi for the theoretical time of the murder and so identifies her as another suspect. Mrs. Cecil Ackroyd and Geoffrey Raymond quickly come forward to Dr. Sheppard and Poirot to admit their secrets – both separately acknowledge that they were in debt, and Ackroyd’s death resolves most of their money issues.
Poirot also determines that Parker, who he suspected as Mrs. Ferrars’ blackmailer, is guilty of nothing more than snooping on Roger Ackroyd the night of the murder when he heard the word “blackmail.” Parker successfully blackmailed his previous employer, and was hoping to blackmail Ackroyd, as well. The police, meanwhile, have tracked down the stranger who approached Fernly Park the night of the murder. They arrest a man named Charles Kent, who Dr. Sheppard confirms was the stranger.
Meanwhile, Poirot determines that Flora lied about wishing her uncle goodnight at 9:45 on the night of the murder – instead, she had snuck up to his bedroom to steal money to settle some debts of her own, and had to pretend she was merely wishing Ackroyd goodnight to avoid suspicion. In the course of her confession, Major Blunt’s secret love for Flora is revealed when he tries to take the blame for her crime. Poirot convinces Major Blunt to be honest with Flora about his love for her – he assures him that Flora and Ralph Paton are not really in love, but merely engaged for convenience’s sake.
Poirot asks for a meeting with Miss Russell, Ackroyd’s housekeeper. He gets her to admit that Charles Kent is the son she bore out of wedlock many years before, and that he owns the goose quill (for snorting heroin) that they found in the summerhouse. Miss Russell met him there before dinner on the night of the murder after he contacted her; she insists he cannot be the murderer.
Soon, Flora and Major Blunt are engaged, and Poirot traces the owner of the gold ring to Ursula Bourne. After placing a fake notice in the newspaper that the police captured Ralph Paton, Bourne seeks out Poirot and confesses to having married Ralph in secret months before. They met around 9:30 in the summerhouse and fought about his announced engagement to Flora Ackroyd (Ralph agreed to the engagement hoping it would mean his uncle would discharge his debts.)
Poirot summons all the suspects to his home once again and reveals all these discoveries. He further reveals that he has determined Ackroyd purchased a dictaphone the week before and that is what Raymond and Blunt heard when they assumed Ackroyd was talking to someone at 9:30. He then reveals Ralph Paton, who Dr. Sheppard had been hiding by pretending he was a patient in a local asylum. Once Poirot determined what Sheppard had done, he got Ralph out. Ralph admits that he fought with Ursula in the summerhouse, then walked around, frustrated. He does not have an alibi for the time of the murder. Poirot announces that it’s simple: for Ralph to be exonerated, the real murderer must come forward. He claims to know the identity of the real murderer, and explains he will bring the truth to the police the next morning.
He discharges the group, but asks Dr. Sheppard to stay behind. In a stunning plot twist, Poirot reveals that it is Dr. Sheppard who is the murderer. Sheppard stabbed Ackroyd before leaving him that night, programmed the Dictaphone to go off at 9:30 and provide him with an alibi, then snuck around the side of the house, crawled into Ackroyd’s study, locked it from the inside, and planted the footprints with Ralph’s shoes in the mud. He hid Ralph so that the police would find him more suspicious. He murdered Ackroyd because it was he, Dr. Sheppard, who was Mrs. Ferrars’ blackmailer, and he knew if Ackroyd found out he would be ruined.
Faced with the knowledge that Poirot will go to the police in the morning, and hoping that Poirot can keep the truth from his sister Caroline, Dr. Sheppard goes home and prepares to kill himself with an overdose of Veronal.