Dr James Sheppard, the story's narrator, lives with his unmarried sister Caroline in the English country village of King’s Abbot. Telling the story in his own words, Dr Sheppard recounts being called to certify the death of a wealthy widow, Mrs Ferrars, who has committed suicide a year after her abusive husband's demise. Caroline speculates that Mrs Ferrars had poisoned her husband and has committed suicide out of remorse.
Roger Ackroyd, wealthy widower and owner of Fernly Park, tells Dr Sheppard that he needs to talk to him urgently, and invites him to dinner that evening. In addition to Dr Sheppard, the dinner guests include Major Blunt, Ackroyd's sister-in-law Mrs Cecil Ackroyd and her daughter Flora, and Ackroyd's personal secretary Geoffrey Raymond. Flora tells the doctor that she is engaged to be married to Ralph Paton, Ackroyd's stepson, though the engagement is being kept confidential.
In his study after dinner, Ackroyd tells Dr Sheppard that he had been engaged to Mrs Ferrars for several months, and that she admitted yesterday that she had indeed poisoned her husband. She was being blackmailed, and promised to reveal the blackmailer's name within 24 hours. At this point Ackroyd's butler, Parker, enters with a letter that Mrs Ferrars had posted just before she killed herself. Ackroyd apologetically asks Dr Sheppard to leave so that he can read it alone.
Shortly after arriving back home, the doctor receives a phone call and, stopping just long enough to shout to Caroline that the caller is Parker reporting that Ackroyd has been found dead, he rushes straight back to Fernly Park. On his arrival, Parker professes to know nothing about the phone call. Unable to get a response at the study door, Sheppard and Parker break it down and find Ackroyd dead in his chair, killed with his own dagger. The letter is missing, and footprints are found leading in and out of the study through an open window.
Ralph disappears, and becomes the primary suspect when the footprints are found to match shoes that he owns. Flora, convinced that Ralph is innocent, asks the recently-retired detective Hercule Poirot if he will investigate.
Asking Dr Sheppard to accompany him, Poirot visits the scene of the crime. Parker notices that a chair has been moved. Poirot questions the guests and staff including Ursula Bourne, the parlourmaid, who has no alibi. The window of opportunity for the murderer after Dr Sheppard's departure appears to have been quite short as Raymond and Blunt later heard Ackroyd talking in his study, and Flora says she saw him just before going up to bed.
Poirot unravels a complex web of intrigue, and presents the missing Ralph Paton – who was, it transpires, already secretly married to Ursula when Ackroyd had decided that he should marry Flora. The pair had arranged a clandestine meeting in the garden. Flora is forced to admit she never in fact saw her uncle after dinner, leaving Raymond and Blunt as the last people to hear Ackroyd alive. Blunt confesses his love for Flora.
Alone with Dr Sheppard, Poirot reveals he knows that Dr Sheppard himself is the blackmailer and Ackroyd's killer. Realising that Mrs Ferrars's letter would implicate him, Sheppard had stabbed Ackroyd before leaving the study. He left publicly through the front door, then put Ralph's shoes on, ran round to the study window and climbed back in. Locking the door from the inside, he set a desk dictaphone going, playing a recording of Ackroyd's voice, and pulled out a chair to hide it from anyone standing in the doorway. To provide the excuse for returning to Fernly Park, he had asked a patient to call him at home. While Sheppard had the study to himself for a few minutes after the crime was discovered, he slipped the dictaphone into his medical bag, and replaced the chair.
Poirot tells Sheppard that this will be reported to the police in the morning, and he suggests that to spare his sister Caroline the doctor might take his own life. Dr Sheppard finishes writing his report on Poirot's investigation (the novel itself), with his last chapter – Apologia – serving as his suicide note.
